Subject: Transitional Fossils FAQ pt 1

Transitional Vertebrate Fossils FAQ v6.0 Jan 13, 1994 

Welcome to the transitional fossils FAQ!
PART I (this file) has FISHES TO FIRST MAMMALS & BIRDS: 1. Introduction:
a. Types of transitions
b. Why are there gaps?
c. Predictions of creationism & evolution d. What's in this FAQ
e. Timescale
2. Transitions from primitive fish to sharks, skates, rays
3. Transitions from primitive fish to bony fish
4. Transition from fishes to first amphibians
5. Transitions among amphibians
6. Transition from amphibians to first reptiles
7. Transitions among reptiles
8. Transition from reptiles to first mammals (long)
9. Transition from reptiles to first birds 

PART 2 (separate file) has transitions among mammals (starting with primates),
including numerous species-to-species transitions, discussion, and references.
If you're particularly interested in humans, skip to the primate section of
part 2, and also look up the fossil hominid FAQ.

I wrote this FAQ as a reference for answering the "there aren't any
transitional fossils" statement that pops up on talk.origins several times each
year. I've tried to make it an accurate, though highly condensed, summary of
known vertebrate fossil history in those lineages that led to familiar modern
forms, with the known transitions *and* with the known major gaps both clearly
mentioned. Version 6.0 of the FAQ has been almost entirely rewritten, with:
1) A completely rewritten introduction & conclusion, discussing what
"transitional" means, why gaps occur, and what the fossil record shows.
2) A greatly expanded list of "chains of genera" for most groups, especially
mammals.
3) References for documented species-to-species fossil transitions, mostly for
mammals.
4) Explicit mention of the notable remaining gaps in the fossil record. 

I'll be adding to this FAQ from time to time. Please send your suggestions,
comments, and any fossil information to me at jespah@u.washington.edu.

1 INTRODUCTION

A. WHAT IS A TRANSITIONAL FOSSIL?
The term "transitional fossil" is used at least two different ways on t.o.,
often leading to muddled and stalemated arguments. I call these two meanings
the "general lineage" and the "species-to-species transition":

i) "General lineage":
This is a SEQUENCE OF SIMILAR GENERA OR FAMILIES, linking an older group to a
very different younger group. Each step in the sequence consists of some
fossils that represent a certain genus or family, and the whole sequence often
covers a span of tens of millions of years. A lineage like this shows obvious
morphological intermediates for every major structural change, and the fossils
occur roughly (but often not exactly) in the expected order. Usually there are
still gaps between each of the groups -- few or none of the speciation events
are preserved. Sometimes the individual specimens are not thought to be
*directly* ancestral to the next-youngest fossils (i.e., they may be "cousins"
or "uncles" rather than "parents"). However, they are assumed to be closely 
related to the actual ancestor, since they have intermediate morphology
compared to the next-oldest and next-youngest "links". The major point of these
general lineages is that animals with intermediate morphology existed at the
appropriate times, and thus that the transitions from the proposed ancestors
are fully plausible. General lineages are known for almost all modern groups of
vertebrates, and make up the bulk of this FAQ.

ii) "Species-to-species transition":
This is a set of NUMEROUS INDIVIDUAL FOSSILS THAT SHOW A CHANGE BETWEEN ONE
SPECIES AND ANOTHER. It's a very fine-grained sequence documenting the actual
speciation event, usually covering less than a million years. These
species-to-species transitions are unmistakable when they are found. Throughout

successive strata you see the population averages of teeth, feet, vertebrae,
etc., changing from what is typical of the first species to what is typical of
the next species. Sometimes, these sequences occur only in a limited geographic
area (the place where the speciation actually occurred), with analyses from any
other area showing an apparently "sudden" change. Other times, though, the
transition can be seen over a very wide geological area. Many
"species-to-species transitions" are known, mostly for marine invertebrates and
recent mammals (both those groups tend to have good fossil records), though
they are not as abundant as the general lineages (see below for why this is
so). Part 2 lists numerous species-to-species transitions from the mammals.

iii) Transitions to New Higher Taxa
As you'll see throughout this FAQ, both types of transitions often result in a
new "higher taxon" (a new genus, family, order, etc.) from a species belonging
to a different, older taxon. There is nothing magical about this. The first
members of the new group are not bizarre, chimeric animals; they are simply a
new, slightly different species, barely different from the parent species.
Eventually they give rise to a more different species, which in turn gives rise
to a still more different species, and so on, until the descendents are
radically different from the original parent stock. For example, the Order
Perissodactyla (horses, etc.) and the Order Cetacea (whales) can both be traced
back to early Eocene animals that looked only marginally different from each
other, and didn't look AT ALL like horses or whales. (They looked rather like
small, dumb foxes with raccoon-like feet and simple teeth.) But over the
following tens of millions of years, the descendents of those animals became
more and more different, and now we call them two different orders.

There are now several known cases of species-to-species transitions that
resulted in the first members of new higher taxa. See part 2 for details.

B. WHY DO GAPS EXIST? (or *seem* to exist) Ideally, of course, we would like to
know each lineage right down to the species level, AND have detailed
species-to-species transitions linking every species in the lineage. But in
practice, we get an uneven mix of the two, with only a few species-to-species
transitions, and occasionally long time breaks in the lineage. Many laypeople
even have the (incorrect) impression that the situation is even worse, and that
there are no known transitions at all. Why are there still gaps? And why do
many people think that there are even more gaps than there really are?

i) Stratigraphic gaps
The first and most major reason for gaps is "stratigraphic discontinuities",
meaning that fossil-bearing strata are not *at all* continuous. There are often
large time breaks from one stratum to the next, and there are even some times
for which no fossil strata have been found. For instance, the Aalenian
(mid-Jurassic) has shown no known tetrapod fossils anywhere in the world, and
other stratigraphic stages in the Carboniferous, Jurassic, and Cretaceous have
produced only a few mangled tetrapods. Most other strata have produced at least
one fossil from between 50% and 100% of the vertebrate families that we know
had already arisen by then (Benton, 1989) -- so the vertebrate record at the
family level is only about 75% complete, and *much* less complete at the genus
or species level. (One study estimated that we may have fossils from as little
as 3% of the species that existed in the Eocene!) This, obviously, is the major
reason for a break in a general lineage. To further complicate the picture,
certain types of animals tend not to get fossilized -- terrestrial animals,
small animals, fragile animals, and forest-dwellers are worst. And finally,
fossils from very early times just don't survive the passage of eons very well,
what with all the folding, crushing, and melting that goes on. Due to these
facts of life and death, there will always be some major breaks in the fossil
record. 

Species-to-species transitions are even harder to document. To demonstrate
*anything* about how a species arose, whether it arose gradually or suddenly,
you need exceptionally complete strata, with many dead animals buried under
constant, rapid sedimentation. This is rare for terrestrial animals. Even the
famous Clark's Fork (Wyoming) site, known for its fine Eocene mammal
transitions, only has about one fossil per lineage about every 27,000 years.
Luckily, this is enough to record most episodes of evolutionary change
(provided that they occurred at Clark's Fork Basin and not somewhere else),
though it misses the rapidest evolutionary bursts. In general, in order to
document transitions between species, you specimens separated by only tens of
thousands of years (e.g. every 20,000-80,000 years). If you have only one
specimen for hundreds of thousands of years (e.g. every 500,000 years), you can
usually determine the order of species, but not the transitions between
species. If you have a specimen every million years, you can get the order of
genera, but not which species were involved. And so on. These are rough
estimates (from Gingerich, 1976, 1980) but should give an idea of the
completeness required.

Note that fossils separated by more than about a hundred thousand years
*cannot* show anything about how a species arose. Think about it: there could
have been a smooth transition, or the species could have appeared suddenly, but
either way, if there aren't enough fossils, we can't tell which way it happened.

ii) Discovery of the fossils
The second reason for gaps is that most fossils undoubtedly have not been
found. Only two continents, Europe and North America, have been adequately
surveyed for fossil-bearing strata. As the other continents are slowly
surveyed, many formerly mysterious gaps are being filled (e.g., the
long-missing rodent/lagomorph ancestors were recently found in Asia). Of
course, even in known strata, the fossils may not be uncovered unless a roadcut
or quarry is built (this is how we got most of our North American Devonian fish
fossils), and may not be collected unless some truly dedicated researcher
spends a long, nasty chunk of time out in the sun, and an even longer time in
the lab sorting and analyzing the fossils. Here's one description of the work
involved in finding early mammal fossils: "To be a successful sorter demands a
rare combination of attributes: acute observation allied with the anatomical
knowledge to recognise the mammalian teeth, even if they are broken or abraded,
has to be combined with the enthusiasm and intellectual drive to keep at the
boring and soul-destroying task of examining tens of thousands of unwanted fish
teeth to eventually pick out the rare mammalian tooth. On an average one
mammalian tooth is found per 200 kg of bone-bed." (Kermack, 1984.)

Documenting a species-to-species transition is particularly grueling, as it
requires collection and analysis of *hundreds* of specimens. Typically we must
wait for some paleontologist to take it on the job of studying a certain taxon
in a certain site in detail. Almost nobody did this sort of work before the
mid-1970's, and even now only a small subset of researchers do it. For example,
Phillip Gingerich was one of the first scientists to study species-species
transitions, and it took him *ten years* to produce the first detailed studies
of just two lineages (see part 2, primates and condylarths). In a (later) 1980
paper he said: "the detailed species level evolutionary patterns discussed here
represent only six genera in an early Wasatchian fauna containing approximately
50 or more mammalian genera, MOST OF WHICH REMAIN TO BE ANALYZED." [emphasis
mine]

iii) Getting the word out
There's a third, unexpected reason that transitions seem so little known. It's
that even when they *are* found, they're not popularized. The only times a
transitional fossil is noticed much is if it connects two noticably different
groups (such as the "walking whale" fossil reported in 1993), or if illustrates
something about the tempo and mode of evolution (such as Gingerich's work).
Most transitional fossils are only mentioned in the primary literature, often
buried in incredibly dense and tedious "skull & bones" papers utterly
inaccessible to the general public. Later references to those papers usually
collapse the known species-to-species sequences to the genus or family level.
The two major college-level textbooks of vertebrate paleontology (Carroll 1988,
and Colbert & Morales 1991) often don't even describe anything below the family
level! And finally, many of the species-to-species transitions were described
too recently to have made it into the books yet.

Why don't paleontologists bother to popularize the detailed lineages and
species-to-species transitions? Because it is thought to be unnecessary detail.
For instance, it takes an entire book to describe the horse fossils even
partially (e.g. MacFadden's "Fossil Horses"), so most authors just collapse the
horse sequence to a series of genera. Paleontologists clearly consider the
occurrence of evolution to be a settled question, so obvious as to be beyond
rational dispute, so, they think, why waste valuable textbook space on such
tedious detail? 

iv) Misunderstanding of quotes about punctuated equilibrium
What paleontologists *do* get excited about are topics like the average rate of
evolution. When exceptionally complete fossil sites *are* studied, usually a
mix of patterns are seen: some species still seem to appear suddenly, while
others clearly appear gradually. Once they arise, some species stay mostly the
same, while others continue to change gradually. Paleontologists usually
attribute these differences to a mix of slow evolution and rapid evolution (or
"punctuated equilibrium": sudden bursts of evolution followed by stasis), in
combination with the immigration of new species from the as-yet- undiscovered
places where they first arose.

There's been a heated debate about which of these modes of evolution is most
common, and this debate has been largely misquoted by laypeople, particularly
creationists. Virtually all of the quotes of paleontologists saying things like
"the gaps in the fossil record are real" are taken out of context from this
ongoing debate about punctuated equilibrium. Actually, no paleontologist that I
know of doubts that evolution has occurred, and most agree that *at least
sometimes* it occurs gradually. The fossil evidence that contributed to that
consensus is summarized in the rest of this FAQ. What they're arguing about is
how *often* it occurs gradually. You can make up your own mind about that. (As
a starting point, check out Gingerich, 1980, who found 24 gradual speciations
and 14 sudden appearances in early Eocene mammals; MacFadden, 1985, who found 5
cases of gradual anagenesis, 5 cases of probable cladogenesis, and 6 sudden
appearances in fossil horses; and the numerous papers in Chaline, 1983. Most
studies that I've read find between 1/4-2/3 of the speciations occurring fairly
gradually. )


C. PREDICTIONS OF CREATIONISM AND OF EVOLUTION:
Before launching into the transitional fossils, I'd like to run through the two
of the major models of life's origins, biblical creationism and modern
evolutionary theory, and see what they predict about the fossil record.

1. Most forms of creationism hold that all "kinds" were created separately, as
described in Genesis. Unfortunately there is no biological definition of
"kind"; it appears to be a vague term referring to our psychological perception
of types of organisms such as "dog", "tree", or "ant". Creationists used to
equate "kind" to species. Recently I have seen creationists bumping "kind"
further up to higher taxonomic levels, such as "genus", or "family", though
this lumps a large variety of animals in the same "kind".

Predictions of creationism:
Creationists usually don't state the predictions of creationism, but I'll take
a stab at it here. If "kind" means "species", creationism apparently predicts
that there should be *no* species-to-species transitions whatsoever in the
fossil record. If "kind" means "genus" or "family" or "order", there should be
*no* species-to-species transitions that cross genus, family, or order lines.
Furthermore, creationism apparently predicts that since life did not originate
by descent from a common ancestor, fossils should not appear in a temporal
progression, and it should not be possible to link modern taxa to much older,
very different taxa through a "general lineage" of similar and progressively
older fossils. Strictly literal creationism also predicts that fossils should
appear in the order listed in Genesis: seed-bearing trees first, then all
aquatic animals and flying animals, then all terrestrial animals, then humans.
(These predictions assume that God did not create fossils to appear as if
evolution occurred.) 

2. Modern evolutionary theory holds that the living vertebrates arose from a
common ancestor that lived hundreds of millions of years ago (via "descent with
modification"; variety is introduced by mutation, genetic drift, and
recombination, and is acted on by natural selection). Various proposed
mechanisms of evolution differ in the expected rate and tempo of evolutionary
change.

Predictions:
Evolutionary theory predicts that fossils *should* appear in a temporal
progression, in a nested hierarchy of lineages, and that it *should* be
possible to link modern animals to older, very different animals. In addition,
the "punctuated equilibrium" model also predicts that new species should often
appear "suddenly" (within 500,000 years or less) and then experience long
periods of stasis. Where the record is exceptionally good, we should find a few
local, rapid transitions between species. The "phyletic gradualism" model
predicts that most species should change gradually throughout time, and that
where the record is good, there should be many slow, smooth species-to-species
transitions. These two models are not mutually exclusive -- in fact they are
often viewed as two extremes of a continuum -- and both agree that at least
*some* species-to-species transitions should be found.

D. WHAT'S IN THIS FAQ
This FAQ mostly consists of a PARTIAL list of known transitions from the
vertebrate fossil record. The transitions in part 1 are mostly general
lineages, while in part 2 there are both general lineages and
species-to-species transitions. In a hopeless attempt to save space, I
concentrated almost exclusively on groups that left living descendants,
ignoring all the hundreds of other groups and side-branches that have died out.
I also skipped entire groups of vertebrates (most notably the dinosaurs and
modern fish) in order to emphasize mammals, the group t.o.'ers are most
interested in. Note that the general lineages sometimes include "cousin"
fossils. These are fossils that are thought to be very similar and closely
related to the actual ancestor, but for various reasons are suspected *not* to
be that ancestor. I have labelled them clearly in the text. I've also pointed
out some of the significant remaining gaps in the vertebrate fossil record.

I got most of the information from Colbert & Morales' _Evolution of the
Vertebrates_ (1991), Carroll's _Vertebrate Paleontology and Evolution_ (1988),
Benton's _The Phylogeny and Classification of the Tetrapods_ (1988), and from
various recent papers from the scientific literature. These sources are all
listed in the reference section at the end of part 2.

The time of first known appearance of each fossil is given in parentheses after
the fossil name, including absolute dates when I could find them. The only
exceptions are a few cases where my source didn't mention a date and it wasn't
listed in Carroll's text. All of these fossils were dated by *independent*
means, typically by using several different methods of radiometric dating on
the strata around the fossil, and/or by cross-correlating to dated strata at
other sites (e.g. MacFadden et al., 1991). See the dating FAQ if you have
questions about these methods.

Some terminology:
"Anagenesis" = "phyletic evolution" = evolution in which an older species, as a
whole, changes into a new descendent species, such that the ancestor is
transformed into the descendant. "Cladogenesis" = evolution in which a daughter
species splits off from a population of the older species, after which both the
old and the young species coexist together. NOTICE THAT THIS ALLOWS A
DESCENDANT TO COEXIST WITH ITS ANCESTOR.
"Chronocline" = gradual change in one lineage over time "Ma" = millions of
years ago (a date)
"my" = millions of years (a duration)

E. TIMESCALE
Cenozoic:
(See part 2) 65-0 Ma	Mammals & birds & teleost fish dominant.
Mesozoic:
Cretaceous 144-65 Ma Dinos dominant. Small mammals, birds.
Jurassic 213-144 Ma Dinos dom'nt. 1st mammals, then 1st birds.
Triassic 248-23 Ma Mammal-repts dominant. First dinos.
Paleozoic:
Permian 286-248 Ma Amphibs dominant. First mammal-like repts.
Pennsylvanian 320-286 Ma Amphibs dominant. First reptiles.
Mississippian 360-320 Ma Big terrestrial amphibs, fishes.
Devonian 408-360 Ma Fish dominant. First amphibians.
Silurian 438-408 Ma First ray-finned & lobe-finned fish.
Ordovician 505-438 Ma More jawless fishes.
Cambrian	590-505 Ma First jawless fishes.

****************************************************************** 
SUMMARY OF THE KNOWN VERTEBRATE FOSSIL RECORD (We start off with primitive
jawless fish.) 

2. TRANSITION FROM PRIMITIVE JAWLESS FISH TO SHARKS, SKATES, AND RAYS:
Late Silurian -- first little simple shark-like denticles.
Early Devonian -- first recognizable shark teeth, clearly derived from
scales.
GAP: Note that these first, very very old traces of shark-like animals are so
fragmentary that we can't get much detailed information. So, we don't know
which jawless fish was the actual ancestor of early sharks.
_Cladoselache_ (late Devonian)
Magnificent early shark fossils, found in Cleveland roadcuts during the
construction of the U.S. interstate highways. Probably *not* directly ancestral
to sharks, but gives a remarkable picture of general early shark anatomy, down
to the muscle fibers! _Tristychius_ & similar hybodonts (early Mississippian) 
Primitive proto-sharks with broad-based but otherwise shark-like fins.
_Ctenacanthus_ & similar ctenacanthids (late Devonian) 
Primitive, slow sharks with broad-based shark-like fins & fin spines. Probably
ancestral to all modern sharks, skates, and rays. Fragmentary fin spines
(Triassic) -- from more advanced sharks. _Paleospinax_ (early Jurassic)
More advanced features such as detached upper jaw, but retains primitive
ctenacanthid features such as two dorsal spines, primitive teeth, etc.
_Spathobatis_ (late Jurassic)
First proto-ray.
_Protospinax_ (late Jurassic)
A very early shark/skate. After this, first heterodonts, hexanchids, & nurse
sharks appear (late Jurassic). Other shark groups date from the Cretaceous or
Eocene. First true skates known from Upper Cretaceous.

A separate lineage leads from the ctenacanthids through _Echinochimaera_
(late Mississippian) and _Similihari_ (late Pennsylvanian) to the modern
ratfish.

3. TRANSITION FROM FROM PRIMITIVE JAWLESS FISH TO BONY FISH:
Upper Silurian -- first little scales found. GAP: Once again, the first traces
are so fragmentary that the actual 
ancestor can't be identified.
Acanthodians?? (Silurian)
A puzzling group of spiny fish with similarities to early bony fish.
Palaeoniscoids (e.g. _Cheirolepis_, _Mimia_; early Devonian)
Primitive bony ray-finned fishes that gave rise to the vast majority of living
fish. Heavy acanthodian-type scales, acanthodian-like skull, and big notochord.
_Canobius_, _Aeduella_ (Carboniferous)
Later paleoniscoids with smaller, more advanced jaws. _Parasemionotus_
(early Triassic)
"Holostean" fish with modified cheeks but still many primitive features. Almost
exactly intermediate between the late paleoniscoids & first teleosts. Note:
most of these fish lived in seasonal rivers and had lungs. Repeat: lungs first
evolved in FISH. _Oreochima_ & similar pholidophorids (late Triassic)
The most primitive teleosts, with lighter scales (almost cycloid), partially
ossified vertebrae, more advanced cheeks & jaws. _Leptolepis_ & similar
leptolepids (Jurassic) 
More advanced with fully ossified vertebrae & cycloid scales. The Jurassic
leptolepids radiated into the modern teleosts (the massive, successful group of
fishes that are almost totally dominant today). Lung transformed into swim
bladder.
Eels & sardines date from the late Jurassic, salmonids from the Paleocene &
Eocene, carp from the Cretaceous, and the great group of spiny teleosts from
the Eocene. The first members of many of these families are known and are in
the leptolepid family (note the inherent classification problem!).

4. TRANSITION FROM PRIMITIVE BONY FISH TO AMPHIBIANS:
Few people realize that the fish-amphibian transition was *not* a transition
from water to land. It was a transition from *fins to feet* that took place *in
the water*. The very first amphibians seem to have developed legs and feet to
scud around on the bottom in the water, as some modern fish do, not to walk on
land (see Edwards, 1989). This aquatic-feet stage meant the fins didn't have to
change very quickly, the weight-bearing limb musculature didn't have to be very
well developed, and the axial musculature didn't have to change at all.
Recently found fragmented fossils from the middle Upper Devonian, and new
discoveries of late Upper Devonian feet (see below), support this idea of an
"aquatic feet" stage. Eventually, of course, amphibians *did* move onto the
land. This involved attaching the pelvis more firmly to the spine, and
separating the shoulder from the skull. Lungs were not a problem, since lungs
are an ancient fish trait and were present already.

Paleoniscoids again (e.g. _Cheirolepis_)
These ancient bony fish probably gave rise both to modern ray-finned fish
(mentioned above), and also to the lobe-finned fish. _Osteolepis_ (mid-Devonian)
One of the earliest crossopterygian lobe-finned fishes, still sharing some
characters with the lungfish (the other lobe-finned fishes). Had paired fins
with a leg-like arrangement of major limb bones, capable of flexing at the
"elbow", and had an early-amphibian-like skull and teeth.
_Eusthenopteron_, _Sterropterygion_ (mid-late Devonian)
Early rhipidistian lobe-finned fish roughly intermediate between early
crossopterygian fish and the earliest amphibians. _Eusthenopteron_ is best
known, from an unusually complete fossil first found in 1881. Skull very
amphibian-like. Strong amphibian- like backbone. Fins very like early amphibian
feet in the overall layout of the major bones, muscle attachments, and bone
processes, with tetrapod-like tetrahedral humerus, and tetrapod-like elbow and
knee joints. But there are no perceptible "toes", just a set of identical fin
rays. Body & skull proportions rather fishlike. _Panderichthys_, _Elpistostege_
(mid-late Devonian, about 370 Ma)
These "panderichthyids" are *very* tetrapod-like lobe-finned fish. Unlike
_Eusthenopteron_, these fish actually look like tetrapods in overall
proportions (flattened bodies, dorsally placed orbits, frontal bones! in the
skull, straight tails, etc.) and have remarkably foot-like fins.
Fragmented limbs and teeth from the middle Late Devonian (about 370 Ma),
possibly belonging to _Obruchevichthys_.
Discovered in 1991 in Scotland, these are the earliest known tetrapod remains.
The humerus is mostly tetrapod-like but retains some fish features. The
discoverer, Ahlberg (1991), said: "It [the humerus] is more tetrapod-like than
any fish humerus, but lacks the characteristic early tetrapod 'L-shape'...this
seems to be a primitive, fish-like character....although the tibia clearly
belongs to a leg, the humerus differs enough from the early tetrapod pattern to
make it uncertain whether the appendage carried digits or a fin. At first sight
the combination of two such extremities in the same animal seems highly
unlikely on functional grounds. If, however, tetrapod limbs evolved for aquatic
rather than terrestrial locomotion, as recently suggested, such a morphology
might be perfectly workable."
GAP: Ideally, of course, we want an *entire* skeleton from the middle
Late Devonian, not just limb fragments. Nobody's found one yet. _Hynerpeton_,
_Acanthostega_, and _Ichthyostega_
(late Devonian)
A little later, the fin-to-foot transition was almost complete, and we have a
set of early tetrapod fossils that clearly did have feet. The most complete are
_Ichthyostega_, _Acanthostega gunnari_, and the newly described _Hynerpeton
bassetti_ (Daeschler et al., 1994). (There are also other genera known from
more fragmentary fossils.) _Hynerpeton_ is the earliest of these three genera
(365 Ma), but is more advanced in some ways; the other two genera retained more
fish- like characters longer than the _Hynerpeton_ lineage did. Labyrinthodonts
(eg _Pholidogaster_, _Pteroplax_) (late Dev./early Miss.)
These larger amphibians still have some icthyostegid fish features, such as
skull bone patterns, labyrinthine tooth dentine, presence & pattern of large
palatal tusks, the fish skull hinge, pieces of gill structure between cheek &
shoulder, and the vertebral structure. But they have lost several other fish
features: the fin rays in the tail are gone, the vertebrae are stronger and
interlocking, the nasal passage for air intake is well defined, etc.

More info on those first known Late Devonian amphibians: _Acanthostega gunnari_
was very fish-like, and recently Coates & Clack (1991) found that it still had
internal gills! They said: "_Acanthostega_ seems to have retained fish-like
internal gills and an open opercular chamber for use in aquatic respiration,
implying that the earliest tetrapods were not fully terrestrial....Retention of
fish-like internal gills by a Devonian tetrapod blurs the traditional
distinction between tetrapods and fishes...this adds further support to the
suggestion that unique tetrapod characters such as limbs with digits evolved
first for use in water rather than for walking on land." _Acanthostega_ also
had a remarkably fish-like shoulder and forelimb. _Ichthyostega_ was also very
fishlike, retaining a fish-like finned tail, permanent lateral line system, and
notochord. Neither of these two animals could have survived long on land.

Coates & Clack (1990) also recently found the first really well- preserved
feet, from _Acanthostega_ (front foot found) and _Ichthyostega_ (hind foot
found). (_Hynerpeton_'s feet are unknown.) The feet were much more fin-like
than anyone expected. It had been assumed that they had five toes on each foot,
as do all modern tetrapods. This was a puzzle since the fins of lobe-finned
fishes don't seem to be built on a five-toed plan. It turns out that
_Acanthostega_'s front foot had *eight* toes, and _Ichthyostega_'s hind foot
had seven toes, giving both feet the look of a short, stout flipper with many
"toe rays" similar to fin rays. All you have to do to a lobe- fin to make it
into a many-toed foot like this is curl it, wrapping the fin rays forward
around the end of the limb. In fact, this is exactly how feet develop in larval
amphibians, from a curled limb bud. (Also see Gould's essay on this subject,
"Eight Little Piggies".) Said the discoverers (Coates & Clack, 1990): "The
morphology of the limbs of _Acanthostega_ and _Ichthyostega_ suggest an aquatic
mode of life, compatible with a recent assessment of the fish-tetrapod
transition. The dorsoventrally compressed lower leg bones of _Ichthyostega_
strongly resemble those of a cetacean [whale] pectoral flipper. A peculiar,
poorly ossified mass lies anteriorly adjacent to the digits, and appears to be
reinforcement for the leading edge of this paddle-like limb." Coates & Clack
also found that _Acanthostega_'s front foot couldn't bend forward at the elbow,
and thus couldn't be brought into a weight-bearing position. In other words
this "foot" still functioned as a horizontal fin. _Ichthyostega_'s hind foot
may have functioned this way too, though its *front* feet could take weight.
Functionally, these two animals were not fully amphibian; they lived in an
in-between fish/amphibian niche, with their feet still partly functioning as
fins. Though they are probably not ancestral to later tetrapods, _Acanthostega_
& _Ichthyostega_ certainly show that the transition from fish to amphibian is
feasible!

_Hynerpeton_, in contrast, probably did not have internal gills and already had
a well-developed shoulder girdle; it could elevate and retract its forelimb
strongly, and it had strong muscles that attached the shoulder to the rest of
the body (Daeschler et al., 1994). _Hynerpeton_'s discoverers think that since
it had the strongest limbs earliest on, it may be the actual ancestor of all
subsequent terrestrial tetrapods, while _Acanthostega_ and _Ichthyostega_ may
have been a side branch that stayed happily in a mostly-aquatic niche.

In summary, the *very* first amphibians (presently known only from fragments)
were probably almost totally aquatic, had both lungs *and* internal gills
throughout life, and scudded around underwater with flipper-like, many-toed
feet that didn't carry much weight. Different lineages of amphibians began to
bend either the hind feet or front feet forward so that the feet carried
weight. One line (_Hynerpeton_) bore weight on all four feet, developed strong
limb girdles and muscles, and quickly became more terrestrial.


5. TRANSITIONS AMONG AMPHIBIANS
Temnospondyls, e.g _Pholidogaster_ (Mississippian, about 330 Ma)
A group of large labrinthodont amphibians, transitional between the early
amphibians (the ichthyostegids, described above) and later amphibians such as
rhachitomes and anthracosaurs. Probably also gave rise to modern amphibians
(the Lissamphibia) via this chain of six temnospondyl genera , showing
progressive modification of the palate, dentition, ear, and pectoral girdle,
with steady reduction in body size (Milner, in Benton 1988). Notice, though,
that the times are out of order, though they are all from the Pennsylvanian and
early Permian. Either some of the "Permian" genera arose earlier, in the
Pennsylvanian (quite likely), and/or some of these genera are "cousins", not
direct ancestors (also quite likely). _Dendrerpeton acadianum_ (early Penn.)
4-toed hand, ribs straight, etc. _Archegosaurus decheni_ (early Permian)
Intertemporals lost, etc. _Eryops megacephalus_ (late Penn.) Occipital condyle
splitting in 2, etc.
_Trematops_ spp. (late Permian) Eardrum like modern amphibians, etc.
_Amphibamus lyelli_ (mid-Penn.) Double occipital condyles, ribs very
small, etc.
_Doleserpeton annectens_ or perhaps _Schoenfelderpeton_ (both early
Permian) First pedicellate teeth! (a classic trait of modern amphibians) etc.

>From there we jump to the Mesozoic:
_Triadobatrachus_ (early Triassic) -- a proto-frog, with a longer trunk
and much less specialized hipbone, and a tail still present (but very short).
_Vieraella_ (early Jurassic) -- first known true frog. _Karaurus_
(early Jurassic) -- first known salamander.

Finally, here's a recently found fossil: Unnamed proto-anthracosaur --
described by Bolt et al., 1988. This animal combines primitive features of
palaeostegalians (e.g. temnospondyl-like vertebrae) with new anthracosaur-like
features. Anthracosaurs were the group of large amphibians that are thought to
have led, eventually, to the reptiles. Found in a new Lower Carboniferous site
in Iowa, from about 320 Ma. 

6. TRANSITION FROM AMPHIBIANS TO AMNIOTES (FIRST REPTILES): The major
functional difference between the ancient, large amphibians and the first
little reptiles is the amniotic egg. Additional differences include stronger
legs and girdles, different vertebrae, and stronger jaw muscles. For more
info, see Carroll (1988) and Gauthier et al. (in Benton, 1988)

_Proterogyrinus_ or another early anthracosaur (late Mississippian) 
Classic labyrinthodont-amphibian skull and teeth, but with reptilian 
vertebrae, pelvis, humerus, and digits. Still has fish skull hinge. Amphibian
ankle. 5-toed hand and a 2-3-4-5-3 (almost reptilian) phalangeal count.
_Limnoscelis_, _Tseajaia_ (late Carboniferous)
Amphibians apparently derived from the early anthracosaurs, but with
additional reptilian features: structure of braincase, reptilian jaw muscle,
expanded neural arches.
_Solenodonsaurus_ -- (mid-Pennsylvanian)
An incomplete fossil, apparently between the anthracosaurs and the
cotylosaurs. Loss of palatal fangs, loss of lateral line on head, etc. Still
just a single sacral vertebra, though. _Hylonomus_, _Paleothyris_ (early
Pennsylvanian) 
These are protorothyrids, very early cotylosaurs (primitive reptiles). They
were quite little, lizard-sized animals with amphibian-like skulls (amphibian
pineal opening, dermal bone, etc.), shoulder, pelvis, & limbs, and
intermediate teeth and vertebrae. Rest of skeleton reptilian, with reptilian
jaw muscle, no palatal fangs, and spool-shaped vertebral centra. Probably no
eardrum yet. Many of these new "reptilian" features are also seen in *little*
amphibians (which also sometimes have direct-developing eggs laid on land), so
perhaps these features just came along with the small body size of the first
reptiles.

The ancestral amphibians had a rather weak skull and paired "aortas" (systemic
arches). The first reptiles immediately split into two major lines which
modified these traits in different ways. One line developed an aorta on the
right side and strengthened the skull by swinging the quadrate bone down and
forward, resulting in an enormous otic notch (and allowed the later
development of good hearing without much further modification). This group
further split into three major groups, easily recognizable by the number of
holes or "fenestrae" in the side of the skull: the anapsids (no fenestrae),
which produced the turtles; the diapsids (two fenestrae), which produced the
dinosaurs and birds; and an offshoot group, the eurapsids (two fenestrae fused
into one), which produced the ichthyosaurs.

The other major line of reptiles developed an aorta on left side only, and
strengthened the skull by moving the quadrate bone up and back, obliterating
the otic notch (making involvement of the jaw essential in the later
development of good hearing). They developed a single fenestra per side. This
group was the synapsid reptiles. They took a radically different path than the
other reptiles, involving homeothermy, a larger brain, better hearing and more
efficient teeth. One group of synapsids called the "therapsids" took these
changes particularly far, and apparently produced the mammals.

7. SOME TRANSITIONS AMONG REPTILES:
I will review just a couple of the reptile phylogenies, since there are so
many....

Early reptiles to turtles: (Also see Gaffney & Meylan, in Benton 1988)
_Captorhinus_ (early-mid Permain)
Immediate descendent of the protorothryids. Here we come to a controversy;
there are two related groups of early anapsids, both descended from the
captorhinids, that could have been ancestral to turtles. Reisz & Laurin (1991,
1993) believe the turtles descended from procolophonids, late Permian anapsids
that had various turtle-like skull features. Others, particularly Lee (1993)
think the turtle ancestors are pareiasaurs: _Scutosaurus_ and other
pareiasaurs (mid-Permian)
Large bulky herbivorous reptiles with turtle-like skull features. Several
genera had bony plates in the skin, possibly the first signs of a turtle shell.
_Deltavjatia vjatkensis_ (Permian)
A recently discovered pareiasaur with numerous turtle-like skull features
(e.g., a very high palate), limbs, and girdles, and lateral projections
flaring out some of the vertebrae in a very shell-like way. (Lee, 1993)
_Proganochelys_ (late Triassic) -- a primitive turtle, with a fully 
turtle-like skull, beak, and shell, but with some primitive traits such as
rows of little palatal teeth, a still-recognizable clavicle, a simple
captorhinid-type jaw musculature, a primitive captorhinid- type ear, a
non-retractable neck, etc..
Recently discovered turtles from the early Jurassic, not yet described.
Mid-Jurassic turtles had already divided into the two main groups of
modern turtles, the side-necked turtles and the arch-necked turtles. Obviously
these two groups developed neck retraction separately, and came up with
totally different solutions. In fact the first known arch-necked turtles, from
the Late Jurassic, could not retract their necks, and only later did their
descendents develop the archable neck.

Early reptiles to diapsids: (see Evans, in Benton 1988, for more info)
_Hylonomus_, _Paleothyris_ (early Penn.)
The primitive amniotes described above
_Petrolacosaurus_, _Araeoscelis_ (late Pennsylvanian)
First known diapsids. Both temporal fenestra now present. No significant
change in jaw muscles. Have Hylonomus-style teeth, with many small marginal
teeth & two slightly larger canines. Still no eardrum.
_Apsisaurus_ (early Permian)
A more typical diapsid. Lost canines. (Laurin, 1991) GAP: no diapsid fossils
from the mid-Permian. _Claudiosaurus_ (late Permian)
An early diapsid with several neodiapsid traits, but still had primitive
cervical vertebrae & unossified sternum. probably close to the ancestry of all
diapsides (the lizards & snakes & crocs & birds). _Planocephalosaurus_
(early Triassic)
Further along the line that produced the lizards and snakes. Loss of some
skull bones, teeth, toe bones.
_Protorosaurus_, _Prolacerta_ (early Triassic) 
Possibly among the very first archosaurs, the line that produced dinos, crocs,
and birds. May be "cousins" to the archosaurs, though. _Proterosuchus_
(early Triassic) First known archosaur. _Hyperodapedon, _Trilophosaurus_
(late Triassic) Early archosaurs.

Some species-to-species transitions:
De Ricqles (in Chaline, 1983) documents several possible cases of
gradual evolution (also well as some lineages that showed abrupt appearance or
stasis) among the early Permian reptile genera _Captorhinus_,
_Protocaptorhinus_, _Eocaptorhinus_, and _Romeria_. Horner et al. (1992)
recently found many excellent transitional dinosaur
fossils from a site in Montana that was a coastal plain in the late
Cretaceous. They include:
1) Many transitional ceratopsids between Styracosaurus and Pachyrhinosaurus
2) Many transitional lambeosaurids (50! specimens) between Lambeosaurus and
Hypacrosaurus.
3) A transitional pachycephalosaurid between Stegoceras and Pachycephalosaurus
4) A transitional tyrannosaurid between Tyrannosaurus and Daspletosaurus.
All of these transitional animals lived during the same brief 500,000 years.
Before this site was studied, these dinosaur groups were known from the much
larger Judith River Formation, where the fossils showed 5 million years of
evolutionary stasis, following by the apparently abrupt appearance of the new
forms. It turns out that the sea level rose during that 500,000 years,
temporarily burying the Judith River Formation under water, and forcing the
dinosaur populations into smaller areas such as the site in Montana. While the
populations were isolated in this smaller area, they underwent rapid
evolution. When sea level fell again, the new forms spread out to the
re-exposed Judith River landscape, thus appearing "suddenly" in the Judith
River fossils, with the transitional fossils only existing in the Montana
site. This is an excellent example of punctuated equilibrium (yes, 500,000
years is very brief and counts as a "punctuation"), and is a good example of
why transitional fossils may only exist in a small area, with the new species
appearing "suddenly" in other areas. (Horner et al., 1992) Also note the
discovery of _Ianthosaurus_, a genus that links the two synapsid families
Ophiacodontidae and Edaphosauridae. (see Carroll, 1988, p. 367)

8. TRANSITION FROM SYNAPSID REPTILES TO MAMMALS (LONG)
This is the best-documented transition between vertebrate classes. So far this
series is known only as a series of genera or families; the transitions from
species to species are *not* known. But the family sequence is quite complete.
Each group is clearly related to both the group that came before, and the
group that came after, and yet the sequence is so long that the fossils at the
end are astoundingly different from those at the beginning. As Rowe recently
said about this transition (in Szalay et al., 1993), "When sampling artifact
is removed and all available character data analyzed [with computer phylogeny
programs that do not assume anything about evolution], a highly corroborated,
stable phylogeny remains, which is largely consistent with the temporal
distributions of taxa recorded in the fossil record." Similarly, Gingerich has
stated (1977) "While living mammals are well separated from other groups of
animals today, the fossil record clearly shows their origin from a reptilian
stock and permits one to trace the origin and radiation of mammals in
considerable detail." For more details, see Kermack's superb and readable
little book (1984), Kemp's more detailed but older book (1982), and read
Szalay et al.'s recent collection of review articles (1993, vol. 1).

This list starts with pelycosaurs (early synapsid reptiles) and continues with
therapsids and cynodonts up to the first unarguable "mammal". Most of the
changes in this transition involved elaborate repackaging of an expanded brain
and special sense organs, remodeling of the jaws & teeth for more efficient
eating, and changes in the limbs & vertebrae related to active,
legs-under-the-body locomotion. Here are some differences to keep an eye on:

Early Reptiles	Mammals
1 No fenestrae in skull	Massive fenestra exposes all of
braincase
2 Braincase attached loosely Braincase attached firmly to skull
3 No secondary palate	Complete bony secondary palate
4 Undifferentiated dentition Incisors, canines, premolars, molars
5 Cheek teeth uncrowned points Cheek teeth (PM & M) crowned & cusped
6 Teeth replaced continuously Teeth replaced once at most 7 Teeth with single
root. Molars double-rooted
8 Jaw joint quadrate-articular *Jaw joint dentary-squamosal
9 Lower jaw of several bones Lower jaw of dentary bone only
10 Single ear bone (stapes) Three ear bones (stapes, incus, malleus)
11 Joined external nares Separate external nares
12 Single occipital condyle Double occipital condyle
13 Long cervical ribs Cervical ribs tiny, fused to vertebrae
14 Lumbar region with ribs Lumbar region rib-free
15 No diaphragm	Diaphragm
16 Limbs sprawled out from body Limbs under body
17 Scapula simple	Scapula with big spine for muscles
18 Pelvic bones unfused	Pelvis fused
19 Two sacral (hip) vertebrae Three or more sacral vertebrae
20 Toe bone #'s 2-3-4-5-4	Toe bones 2-3-3-3-3
21 Body temperature variable	Body temperature constant

*The presence of a dentary-squamosal jaw joint has been arbitrarily selected
as the defining trait of a mammal. 


_Paleothyris_ (early Pennsylvanian)
An early captorhinomorph reptile, with no temporal fenestrae at all.
_Protoclepsydrops haplous_ (early Pennsylvanian) 
The earliest known synapsid reptile. Little temporal fenestra, with all
surrounding bones intact. Fragmentary. Had amphibian-type vertebrae with tiny
neural processes. (reptiles had only just separated from the amphibians)
_Clepsydrops_ (early Pennsylvanian)
The second earliest known synapsid. These early, very primitive synapsids are
a primitive group of pelycosaurs collectively called "ophiacodonts".
_Archaeothyris_ (early-mid Pennsylvanian) 
A slightly later ophiacodont. Small temporal fenestra, now with some reduced
bones (supratemporal). Braincase still just loosely attached to skull. Slight
hint of different tooth types. Still has some extremely primitive,
amphibian/captorhinid features in the jaw, foot, and skull. Limbs, posture,
etc. typically reptilian, though the ilium (major hip bone) was slightly
enlarged. _Varanops_. (early Permian)
Temporal fenestra further enlarged. Braincase floor shows first mammalian
tendencies & first signs of stronger attachment to rest of skull (occiput more
strongly attached). Lower jaw shows first changes in jaw musculature (slight
coronoid eminence). Body narrower, deeper: vertebral column more strongly
constructed. Ilium further enlarged, lower-limb musculature starts to change
(prominent fourth trochanter on femur). This animal was more mobile and
active. Too late to be a true ancestor, and must be a "cousin". _Haptodus_
(late Pennsylvanian)
One of the first known sphenacodonts, showing the initiation of sphenacodont
features while retaining many primitive features of the ophiacodonts. Occiput
still more strongly attached to the braincase. Teeth become
size-differentiated, with biggest teeth in canine region and fewer teeth
overall. Stronger jaw muscles. Vertebrae parts & joints more mammalian. Neural
spines on vertebrae longer. Hip strengthened by fusing to three sacral
vertebrae instead of just two. Limbs very well developed.
_Dimetrodon_, _Sphenacodon_ or a similar sphenacodont (late Pennsylvanian to
early Permian, 270 Ma)
More advanced pelycosaurs, clearly closely related to the first therapsids
(next). _Dimetrodon_ is almost definitely a "cousin" and not a direct
ancestor, but as it is known from very complete fossils, it's a good model for
sphenacodont anatomy. Medium-sized fenestra. Teeth further differentiated,
with small incisors, two huge deep- rooted upper canines on each side,
followed by smaller cheek teeth, all replaced continuously. Fully reptilian
jaw hinge. Lower jaw bone made of multiple bones & with first signs of a bony
prong later involved in the eardrum, but there was no eardrum yet, so these
reptiles could only hear ground-borne vibrations (they did have a reptilian
middle ear). Vertebrae had still longer neural spines (spectacularly so in
_Dimetrodon_, which had a sail), and longer transverse spines for stronger
locomotion muscles. _Biarmosuchia_ (late Permian)
A therocephalian -- one of the earliest, most primitive therapsids. Several
primitive, sphenacodontid features retained: jaw muscles inside the skull,
platelike occiput, palatal teeth. New features: Temporal fenestra further
enlarged, occupying virtually all of the cheek, with the supratemporal bone
completely gone. Occipital plate slanted slightly backwards rather than
forwards as in pelycosaurs, and attached still more strongly to the braincase.
Upper jaw bone (maxillary) expanded to separate lacrymal from nasal bones,
intermediate between early reptiles and later mammals. Still no secondary
palate, *but* the vomer bones of the palate developed a backward extension
below the palatine bones. This is the first step toward a secondary palate,
and with exactly the same pattern seen in cynodonts. Canine teeth larger,
dominating the dentition. Variable tooth replacement: some therocephalians
(e.g _Scylacosaurus_) had just one canine, like mammals, and stopped replacing
the canine after reaching adult size. Jaw hinge more mammalian in position and
shape, jaw musculature stronger (especially the mammalian jaw muscle). The
amphibian-like hinged upper jaw finally became immovable. Vertebrae still
sphenacodontid-like. Radical alteration in the method of locomotion, with a
much more mobile forelimb, more upright hindlimb, & more mammalian femur &
pelvis. Primitive sphenacodontid humerus. The toes were approaching equal
length, as in mammals, with #toe bones varying from reptilian to mammalian.
The neck & tail vertebrae became distinctly different from trunk vertebrae.
Probably had an eardrum in the lower jaw, by the jaw hinge. _Procynosuchus_
(latest Permian)
The first known cynodont -- a famous group of very mammal-like therapsid
reptiles, sometimes considered to be the first mammals. Probably arose from
the therocephalians, judging from the distinctive secondary palate and
numerous other skull characters. Enormous temporal fossae for very strong jaw
muscles, formed by just one of the reptilian jaw muscles, which has now become
the mammalian masseter. The large fossae is now bounded only by the thin
zygomatic arch (cheekbone to you & me). Secondary palate now composed mainly
of palatine bones (mammalian), rather than vomers and maxilla as in older
forms; it's still only a partial bony palate (completed in life with soft
tissue). Lower incisor teeth was reduced to four (per side), instead of the
previous six (early mammals had three). Dentary now is 3/4 of lower jaw; the
other bones are now a small complex near the jaw hinge. Jaw hinge still
reptilian. Vertebral column starts to look mammalian: first two vertebrae
modified for head movements, and lumbar vertebrae start to lose ribs, the
first sign of functional division into thoracic and lumbar regions. Scapula
beginning to change shape. Further enlargement of the ilium and reduction of
the pubis in the hip. A diaphragm may have been present.
_Dvinia_ [also "Permocynodon"] (latest Permian)
Another early cynodont. First signs of teeth that are more than simple
stabbing points -- cheek teeth develop a tiny cusp. The temporal fenestra
increased still further. Various changes in the floor of the braincase;
enlarged brain. The dentary bone was now the major bone of the lower jaw. The
other jaw bones that had been present in early reptiles were reduced to a
complex of smaller bones near the jaw hinge. Single occipital condyle
splitting into two surfaces. The postcranial skeleton of Dvinia is virtually
unknown and it is not therefore certain whether the typical features found at
the next level had already evolved by this one. Metabolic rate was probably
increased, at least approaching homeothermy. _Thrinaxodon_ (early Triassic)
A more advanced "galesaurid" cynodont. Further development of several of the
cynodont features seen already. Temporal fenestra still larger, larger jaw
muscle attachments. Bony secondary palate almost complete. Functional division
of teeth: incisors (four uppers and three lowers), canines, and then 7-9 cheek
teeth with cusps for chewing. The cheek teeth were all alike, though (no
premolars & molars), did not occlude together, were all single- rooted, and
were replaced throughout life in alternate waves. Dentary still larger, with
the little quadrate and articular bones were loosely attached. The stapes now
touched the inner side of the quadrate. First sign of the mammalian jaw hinge,
a ligamentous connection between the lower jaw and the squamosal bone of the
skull. The occipital condyle is now two slightly separated surfaces, though
not separated as far as the mammalian double condyles. Vertebral connections
more mammalian, and lumbar ribs reduced. Scapula shows development of a new
mammalian shoulder muscle. Ilium increased again, and all four legs fully
upright, not sprawling. Tail short, as is necessary for agile quadrupedal
locomotion. The whole locomotion was more agile. Number of toe bones is
2.3.4.4.3, intermediate between reptile number (2.3.4.5.4.) and mammalian
(2.3.3.3.3), and the "extra" toe bones were tiny. Nearly complete skeletons of
these animals have been found curled up - a possible reaction to conserve
heat, indicating possible endothermy? Adults and juveniles have been found
together, possibly a sign of parental care. The specialization of the lumbar
area (e.g. reduction of ribs) is indicative of the presence of a diaphragm,
needed for higher O2 intake and homeothermy.
NOTE on hearing: The eardrum had developed in the only place available for it-
- the *lower* jaw, right near the jaw hinge, supported by a wide prong
(reflected lamina) of the angular bone. These animals could now hear airborne
sound, transmitted through the eardrum to two small lower jaw bones, the
articular and the quadrate, which contacted the stapes in the skull, which
contacted the cochlea. Rather a roundabout system and sensitive to
low-frequency sound only, but better than no eardrum at all! Cynodonts
developed quite loose 

From early bears (or possibly early weasels?). _Pachycynodon_(early Oligocene)
A bearlike terrestrial carnivore with several sea-lion
traits._Enaliarctos_(late Oligocene, California)
Still had many features of bear-like terrestrial carnivores: bear- like
tympanic bulla, carnassials, etc. But, had flippers instead of toes (though
could still walk and run on the flippers) and somewhat simplified dentition.
Gave rise to several more advanced families, including:
Odobenidae -- the walrus family. Started with _Neotherium_ 14 my, then
_Imagotaria_, which is probably ancestral to modern species. Otariidae -- the
sea lion family. First was _Pithanotaria_ (mid-Miocene, 11 Ma), small and
primitive in many respects, then _Thalassoleon_ (late Miocene) and finally
modern sea lions (Pleistocene, about 2 Ma).
Phocidae -- the seal family. First known are the primitive and somewhat
weasel-like mid-Miocene seals _Leptophoca_ and _Montherium_. Modern seals
first appear in the Pliocene, about 4 Ma.

Now, on to the second major group of carnivores, the cat/civet/hyena line.
Civets (viverrids):
_Stenoplesictis_ (early Oligocene)
An early civet-like animal related to the miacids. Might not be directly
ancestral (has some puzzling non-civet-like traits). _Palaeoprionodon_
(late Oligocene, 30-24 Ma) 
An aeluroid (undifferentiated cat/civet/hyena) with a civet-like skull floor.
Probably had split off from the cat line and was on the way to modern
viverrids.
_Herpestides_ (early Miocene, 22 Ma, France)
Had a distinctly civet-like skull floor, more advanced than _Palaeoprionodon_.
More advanced modern civets appeared in the Miocene.

Cats:
_Haplogale_ (late Oligocene, 30 Ma)
A slightly cat-like aeluroid (cat/civet/hyena). _"Proailurus" julieni_,
(early Miocene)
An aeluroid with a viverrid-ish skull floor that also showed the first
cat-like traits. The genus name is in quotes because, though it was first
thought to be in _Proailurus_, it's now clear that it was a slightly different
genus, probably ancestral to _Proailurus_. _Proailurus lemanensis_
(early Miocene, 24 Ma) 
Considered the first true cat; had the first really cat-like skull floor, with
an ossified bulla.
_Pseudaelurus_ (early-mid Miocene, 20 Ma)
A slightly later, more advanced cat.
_Dinictis_ (early Oligocene)
Transitional from early cats such as _Proailurus_ to modern "feline" cats
_Hoplophoneus_ (early Oligocene)
Transitional from early cats to "saber-tooth" cats

Hyaenids:
Though there are only four species now, hyaenids were once *very* common
andhave an abundant fossil record. There is a main stem of generally small to
medium-sized civet-like forms, showing a general trend toward an increase in
size (Werdelin & Solounias, 1991): _Herpestes antiquus_ (early Miocene)
A viverrid thought to be the ancestor of the hyenid family. _Protictitherium
crassum_ (& 5 closely related species) (early Miocene, 17-18 Ma)
Fox-sized, civet-like animals with hyena-like teeth. Transitional between the
early civet-like viverrids and all the hyenids. Split into three lines, one of
which led to the aardwolf. Another line eventually led to modern hyenas:
_Plioviverrops orbignyi_ (& 3 closely related species) _Tungurictis spocki_, a
mid-Miocene fox-sized hyenid. Truly hyena-like 
ear capsule.
_Ictitherium viverrinum_ (& 6 closely related species) _Thalassictis robusta_
(& 5 other spp.)
_Hyaenotherium wongii_
_Miohyaenotherium bessarabicum_
_Hyaenictitherium hyaenoides_ (& 3 other spp.) _Palinhyaena reperta_
_Ikelohyaena abronia_
_Belbus beaumonti_
_Leecyaena lycyaenoides_ (& 1 other) We're now in the Pliocene. _Parahyaena
brunnea_
_Hyaena hyaena_. _Pliocrocuta_ (below) split off from _Hyaena_ via
cladogenesis. _Hyaena_ itself continued on mostly unchanged as the modern
striped hyena, with one more recent offshoot, the brown hyena, _Hyaena
brunnea_.
_Pliocrocuta perrieri_
_Pachycrocuta brevirostris_ (& 1 other)
_Adcrocuta eximia_, which split into:
_Crocuta crocuta_ (the modern spotted hyena), _C. sivalensis_, and _C.
dietrichi_.

Species-species transitions among carnivores: Ginsburg (in Chaline, 1983)
describes gradual change in the early cats, from
_Haplogale media_ to _Proailurus lemansis_, to (in Europe) _Pseudaelurus
transitorius_ to _Ps. lorteti_ to _Ps. rmoieviensis_ to _Ps. quadridentatus_.
These European lineages gave rise to the modern _Lynx_, _Panthera_, etc.
Different lineages of _Pseudaelurus_ evolved in North American, Africa, and
Asia. Hecht (in Chaline, 1983) describes polar bear evolution; the first
"polar bear" subspecies, _Ursus maritimus tyrannus_, was a essentially a brown
bear subspecies, with brown bear dimensions and brown bear teeth. Over the
next 20,000 years, body size reduced and the skull elongated. As late as
10,000 years ago, polar bears still had a high frequency of brown-bear-type
molars. Only recently have they developed polar-bear-type teeth.
Kurten (1976) describes bear transitions: "From the early Ursus minimus
of 5 million years ago to the late Pleistocene cave bear, there is a perfectly
complete evolutionary sequence without any real gaps. The transition is slow
and gradual throughout, and it is quite difficult to say where one species
ends and the next begins. Where should we draw the boundary between U. minimus
and U. etruscus, or between U. savini and U. spelaeus? The history of the cave
bear becomes a demonstration of evolution, not as a hypothesis or theory but
as a simple fact of record." He adds, "In this respect the cave bear's history
is far from unique."
Kurten (1968) also described the following known species-species
transitions:
_Felis issiodorensis_ to _Felis pardina_ (leopards) _Gulo schlosseri_ to _Gulo
gulo_ (wolverines) _Cuon majori_ to _Cuon alpinus_ (dholes, a type of
short-faced wolf) Lundelius et al. (1987) describe a study by Schultz in 1978
that showed an increase in canine length leading from the dirk-tooth cat
_Megantereon hesperus_ to _Megantereon/Smilodon gracilis_, then to _Smilodon
fatalis_ (a saber-toothed cat), and then to _Smilodon californicus_. Note the
genus transition and the accompanying striking change in morphology.
Werdelin & Solounias (1991) wrote an extensive monograph on hyenids.
They discuss over one hundred (!) named species, with extensive discussion of
the eighteen best-known species, and cladistic analysis of *hundreds* of
specimens from the *SIXTY-ONE* "reasonably well known" hyaenid fossil species.
They concluded: 
"We view the evolution of hyaenids as overwhelmingly gradual. The species,
when studied with regard to their total variability, often grade insensibly
into each other, as do the genera. Large specimens of Hyaenotherium wongii
are, for example, difficult to distinguish from small specimens of
Hyaenictitherium hyaenoides, a distinct genus. Viewed over the entire family,
the evolution of hyaenids from small, fox-like forms to large, scavenging,
"typical" hyenas can be followed step by step, and the assembly of features
defining the most derived forms has taken place piecemeal since the Miocene.
Nowhere is there any indication of major breaks identifying macroevolutionary
steps."

5. RODENTS
Lagomorphs and rodents are two modern orders that look superficially similar
but have long been thought to be unrelated. Until recently, the origins of
both groups were a mystery. They popped into the late Paleocene fossil record
fully formed -- in North America & Europe, that is. New discoveries of earlier
fossils from previously unstudied deposits in *Asia* have finally revealed the
probable ancestors of both rodents and lagomorphs -- surprise, they're related
after all. (see Chuankuei-Li et al., 1987)
_Anagale_, _Barunlestes_, or a similar anagalid (mid-late Paleocene) 
A recently discovered order of primitive rodent/lagomorph ancestors from Asia.
Rabbit-like lower cheek teeth, with cusps in a pattern that finally explains
where the rabbits' central cusp came from (it's the old anagalid protocone).
Primitive skeleton not yet specialized for leaping, with unfused leg bones,
but has a rabbit-like heel. No gap yet in the teeth. These fossils have just
been found in the last decade, and are still being described and analyzed.
_Barunlestes_ in particular (known so far from just one specimen) has both
rodent-like *and* rabbit-like features, and may be ancestral to both the
rodents and the lagomorphs. This lineage then apparently split into two
groups, a eurymyloid/rodent-like group and a mymotonid/rabbit-like group.
_Heomys_ (mid-late Paleocene, China)
An early rodent-like eurymyloid. Similar overall to _Barunlestes_ but with
added rodent/lagomorph features (enamel only on front of incisors, loss of
canines and some premolars, long tooth gap) plus various rodent-like facial
features and rodent-like cheek teeth. Probably a "cousin" to the rodents,
though Chuankuei-Li et al (1987, and in Szalay et al. 1993) think it is "very
close to the ancestral stem of the order Rodentia."
*News flash* _Tribosphenomys minutus_ (late Paleocene, 55 Ma)
A just-announced discovery; it's a small Asian anagalid known from a single
jaw found in some fossilized dung (well, we all have to die somehow). It still
had rabbit-like cheek teeth, but had fully rodent-like ever-growing first
incisors. This probably *is* the "ancestral stem" of the rodents. (see
Discover, Feb. 1995, p. 22). _Acritoparamys (was "Paramys") atavus_ (late
Paleocene) 
First known primitive rodent.
_Paramys_ & its ischyromyid friends (late Paleocene)
Generalized early rodents; a mostly squirrel-like skeleton but without the
arboreal adaptations. Had a primitive jaw musculature (which modern squirrels
still retain). Rodent-like gnawing incisors, but cheek teeth still rooted
(unlike modern rodents) and primitive rodent dental formula.

Squirrels:
_Paramys_ (see above)
_Protosciurus_ (early Oligocene)
An early squirrel with very primitive dentition and jaw muscles, but with the
unique ear structure of modern squirrels. Fully arboreal. _Sciurus_ -- the
modern squirrel genus. Arose in the Miocene and has not changed since then.
Among the rodents, squirrels may be considered "living fossils".

Beavers:
_Paramys_ (see above)
_Paleocastor_ (Oligocene)
Early beaver. A burrower, not yet aquatic. From here the beaver lineage became
increasingly aquatic. Modern beavers appear in the Pleistocene.

Rats/mice/voles:
_Paramys_ (see above)
Eomyids -- later Eocene rodents with a few tooth and eyesocket features 
that show they had branched off from the squirrel line. Geomyoids -- primitive
rodents that have those same tooth & eyesocket features, and still have
squirrel-like jaws; Known to have given rise to the mouse family only because
we have intermediate fossil forms. In the Oligocene these early mice started
to split into modern families such as kangaroo rats and pocket gophers. The
first really mouse- like rodent, _Antemus_, first appeared in the Miocene
(16 Ma) in Asia. In the Plio-Pleistocene, modern mice, hamsters, and voles
appeared and started speciating all over the place. Carroll (1988, p. 493) has
a nightmarish diagram of vole speciation which I will not try to describe
here! The fossil record is very good for these recent rodents, and many
examples of species-species transitions are known, very often crossing genus
lines (see below). 

Cavies:
GAP: No cavy fossils are known between _Paramys_ and the late
Oligocene, when cavies suddenly appear in modern form in both Africa and South
America. However, there are possible cavy ancestors (franimorphs) in the early
Oligocene of Texas, from which they could have rafted to South America and
Africa. 

Known species-species transitions in rodents: Chaline & Laurin (1986) show
gradual change in Plio-Pleistocene water voles, with gradual speciations
documented in *every step* in the following lineage: _Mimomys occitanus_ to
_M. stehlini_ to _M. polonicus_ to _M. pliocaenicus_ to _M. ostramosensis_.
The most important change was the development of high-crowned teeth, which
allows grass-eating. They say: "The evolution of the lineage appears to
involve continuous morphological drift involving functional adaptation
processes. It presumably results from changes in diet when Pretiglian steppes
were replaced in Europe by a period with forest...In our opinion phyletic
gradualism [in this lineage] seems well characterized. It lasts for 1.9 my and
leads to very important morphological changes, and the transitional stages in
the chronomorphocline are sufficiently easily recognizable that they have been
described as morphospecies..."
In a previous paper, Chaline (1983, p. 83) surveyed speciation in the
known arvicolid rodents. About 25% of the species have fossil records complete
enough to study the mode of appearance. Of those 25%, a wide variety of modes
was seen, ranging sudden appearances (taken to mean punctuated equilibrium),
to quick but smooth transitions, to very slow smooth transitions. Both
cladogenesis and anagenesis occurred. Overall, smooth species-to-species
transitions were seen for 53% of the studied species, but no single mode of
evolution was dominant.
Chevret et al. (1993) describe the transition from mouse teeth to vole
teeth (6-4.5 Ma).
Fahlbusch (1983) documents gradual change in various Miocene rodent
transitions.
Goodwin (in Martin, 1993) describes gradual transitions in prairie dogs,
with _Cinomys niobrarius_ increasing in size and splitting into two
descendants, _C. leucurus_ and _C. parvidens_. Jaeger (in Chaline, 1983)
describes gradual shifts in tooth size and shape two genera of early mice,
related to the development of grazing.
Kurten (1968) describes a transition in voles, from _Lagurus pannonicus_
to _L. lagurus_.
Lundelius et al. (1987) summarizes and reviews species-species
transitions in numerous voles, grasshopper mice, jumping mice, etc., from at
least 11 different studies. Ex: _Sigmodon medius_ to _Sigmodon minor_, and
_Zapus sandersi_ to _Zapus hudsonius_. The authors point out that some
promising, well-fossilized groups have not even been studied yet for
species-to-species transitions (e.g. the packrats, _Neotoma_).
Martin (1993) summarizes and reviews the numerous known Pleistocene
rodent species-to-species transitions in muskrats, water voles, grasshopper
mice, prairie voles, pocket gophers, and cotton rats. Michaux (in Chaline,
1983) summarized speciations in mice. He found a wide variety of modes of
speciation, ranging from sudden appearance to gradual change.
Rensberger (1981) describes a likely lineage in the development of
hypsodonty (high-crowned teeth for eating grass), among seven species of
meniscomyine rodents in the genus _Niglarodon_. Stuart (1982, described by
Barnosky, 1987) showed smooth transitions in water voles, including a genus
transition. _Mimomys savini_ gradually lost its distinctive tooth characters,
including rooted cheek teeth, as it changed into a new genus, _Arvicola
cantiana_, which in turn smoothly changed into the modern _A. terrestris_.
Vianey-Liaud (1972) showed gradual change in two independent lineages of
the mid-Oligocene rodent genus _Theridomys_. For example, the molars become
gradually more hypsodont over time from species to species. Vianey-Liaud &
Hartenberger (in Chaline, 1983) also describe gradual shifts in size and shape
in Eocene rodents (mainly theridomyids), concluding that gradual evolution
explains their data better than punctuated equilibrium.

6. LAGOMORPHS
_Barunlestes_ (see above) The possible Asian rodent/lagomorph ancestor.
_Mimotoma_ (Paleocene)
A rabbit-like animal, similar to _Barunlestes_, but with a *rabbit* dental
formula, changes in the facial bones, and only one layer of enamel on the
incisors (unlike the rodents). Like rabbits, it had two upper incisors, but
the second incisor is still large and functional, while in modern rabbits it
is tiny. Chuankuei-Li et al. (1987; also see Szalay et al., 1993) think this
is the actual ancestor of _Mimolagus_, next.
_Mimolagus_ (late Eocene)
Possesses several more lagomorph-like characters, such as a special enamel
layer, possible double upper incisors, and large premolars. _Lushilagus_
(mid-late Eocene)
First true lagomorph. Teeth very similar to Mimotoma, and modern rabbit & hare
teeth could easily have been derived from these teeth. After this, the first
modern rabbits appeared in the Oligocene.

Known species-to-species transitions in lagomorphs: The mid-Tertiary lagomorph
_Prolagus_ shows a very nice "chronocline"
(gradual change over time), grading from one species to the next. Gingerich
(1977) says: "In _Prolagus_ a very complete fossil record shows a remarkable
but continuous and gradual reorganization of the premolar crown morphology in
a single lineage." Lundelius et al. (1987) mention transitions in Pleistocene
rabbits, particularly from _Nekrolagus_ to _Sylvilagus_, and from _Pratilepus_
to _Aluralagus_. Note that both these transitions cross genus lines. Also see
the lagomorph paper in Chaline (1983). Some of these transitions were
considered to be "sudden appearances" until the intervening fossils were
studied, revealing numerous transitional individuals.

7. CONDYLARTHS, THE FIRST HOOFED ANIMALS _Protungulatum_ (latest Cretaceous)
Transitional between earliest placental mammals and the condylarths
(primitive, small hoofed animals). These early, simple insectivore- like small
mammals had one new development: their cheek teeth had grinding surfaces
instead of simple, pointed cusps. They were the first mammal herbivores. All
their other features are generalized and primitive -- simple plantigrade
five-toed clawed feet, all teeth present (3:1:4:3) with no gaps, all limb
closed. 

Within a few million years the condylarths split into several slightly
different lineages with slightly different teeth, such as oxyclaenids (the
most primitive), triisodontines, and phenacodonts (described in other
sections). Those first differences amplified over time as the lineages drifted
further and further apart, resulting ultimately in such different animals as
whales, anteaters, and horses. It's interesting to see how similar the early
condylarth lineages were to each other, in contrast to how different their
descendants eventually, slowly, became. Paleontologists believe this is a
classic example of how 'higher taxa" such as families and orders arise.

Says Carroll (1988, p.505): "In the case of the cetaceans [whales] and the
perissodactyls [horses etc.], their origin among the condylarths has been
clearly documented....If, as seems likely, it may eventually be possible to
trace the ancestry of most of the placental mammals back to the early
Paleocene, or even the latest Cretaceous, the differences between the earliest
ancestral forms will be very small -- potentially no more than those that
distinguish species or even populations within species. The origin of orders
will become synonymous with the origin of species or geographical subspecies.
In fact, this pattern is what one would expect from our understanding of
evolution going back to Darwin. The selective forces related to the origin of
major groups would be seen as no different than those leading to adaptation to
very slightly differing enviromments and ways of life. On the basis of a
better understanding of the anatomy and relationships of the earliest
ungulates, we can see that the origin of the Cetacea and the perissodactyls
resulted not from major differences in their anatomy and ways of life but from
slight differences in their diet and mode of locomotion, as reflected in the
pattern of the tooth cusps and details of the bones of the carpus and tarsus."
(p. 505) 

Species-to-species transitions among the condylarths: The most common fossil
mammal from the lower Eocene is a little primitive weasel-looking condylarth
called _Hyopsodus_. It was previously known that many very different species
of _Hyopsodus_ were found at different sites, with (for example) very
different tooth size. In 1976, Gingerich analyzed the tooth size of all the
known fossils of _Hyopsodus_ that could be dated reliably and independently.
He found that "the pattern of change in tooth size that emerges is one of
continuous gradual change between lineages, with gradual divergence following
the separation of new sister lineages." When tooth size is charted against
time, it shows the single lineage smoothly splitting into four descendant
lineages. (This was one of the first detailed & extensive studies of
speciation.)
By 1985, Gingerich had many more specimens of _Hyopsodus_ and of several
other Eocene condylarth lineages as well, such as _Haplomylus_. For example:
"_Haplomylus speirianus_ ...gradually became larger over time, ultimately
giving rise to a new species _Haplomylus scottianus_... _Hyopsodus latidens_
also became larger and then smaller, ultimately giving rise to a still smaller
species, _Hyopsodus simplex_." These analyses were based on *hundreds* of new
specimens (505 for _Haplomylus_, and 869 for _Hyposodus_).
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