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Transitional Vertebrate Fossils FAQ
Part 1A
Copyright © 1994-1997 by [3]Kathleen Hunt
[Last Update: March 17, 1997]

[4]
Contents [5]
Part 1B

PART 1

1. Introduction

What is a transitional fossil?

The term "transitional fossil" is used at least two different ways on
talk.origins, often leading to muddled and stalemated arguments. I
call these two meanings the "general lineage" and the
"species-to-species transition":

"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.

"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.

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.

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?

_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.

_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]

_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?

_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.)

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.
* 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". In previous centuries,
creationists equated "kind" to species. With the discovery of more
and more evidence for derivation of one species from another,
creationists bumped "kind" further up to mean higher taxonomic
levels, such as "genus", or "family", though this lumps a large
variety of animals in the same "kind". Some creationists say that
"kind" cannot be defined in biological terms.
_Predictions of creationism:_ Creationists usually don't state the
predictions of creationism, but I'll take a stab at it here.
First, though there are several different sorts of creationism,
all of them agree that there should be no transitional fossils at
all between "kinds". For example, 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.
Other predictions vary with the model of creationism. For
instance, an older model of creationism states that fossils were
created during six metaphorical "days" that may each have taken
millenia to pass. This form of creationism predicts that fossils
should be found in the same order outlined in Genesis:
seed-bearing trees first, then all aquatic animals and flying
animals, then all terrestrial animals, then humans.
In contrast, many modern U.S. creationists believe the "Flood
Theory" of the origin of fossils. The "Flood Theory" is derived
from a strictly literal reading of the Bible, and states that all
geological strata, and the fossils imbedded in them, were formed
during the forty-day flood of Noah's time. Predictions of the
Flood Theory apparently include the following:
+ most rock should be sedimentary and indicative of cataclysmic
flooding. There should be no rock formations that indicate
the passing of millenia of gradual accumulation of
undisturbed sediment, such as multi-layered riverbed
formations. There should be no large lava flows layered on
top of each other, and definitely not with successively older
radiometric dates in the lower levels.
+ terrestrial animal fossils should either not be sorted at
all, or should be sorted by some "hydrodynamic" aspect such
as body size, with, for instance, extinct elephants and large
dinosaurs in the lowest layers, and small primitive dinosaurs
in the upper layers. Terrestrial animal fossils should not be
sorted by subtle anatomical details (such as, say, the number
of cusps on the fourth premolar).
+ marine animals are a puzzle, since it is unclear that a Flood
would cause any extinctions of aquatic animals. If such
extinctions did occur, aquatic fossils would perhaps be
"sorted" by body size or ecological niche (bottom-feeder vs.
surface swimmer). For instance, plesiosaurs, primitive
whales, and placoderm fishes (relatively slow-swimming and
quite large) should end up in the same layers. Ichthyosaurs
and porpoises (smaller, faster swimmers with almost identical
body shapes and similar diets) should also occur in the same
layers.
+ there should be no sorting of large rooted structures such as
coral reefs and trees. There should likewise not be
differential sorting of microscopic structures of the same
size and shape, such as pollen grains.
+ sorting, if it occurs at all, should be quite imperfect. With
only 40 days for sorting, there should be occasional examples
of individual fossils that ended up in the "wrong" layer --
the occasional mammal and human fossil in Paleozoic rocks,
for instance, and the occasional trilobite and plesiosaur in
Cenozoic rocks.
+ sorting should _not_ correlate with date of the surrounding
rocks. If all fossils were created by Noah's flood, there is
no conceivable reason that, for instance, lower layers of
fossils should _always_ end up sandwiched between lava rocks
with old radiometric dates.
Finally, some creationists believe that fossils were created by
miraculous processes not operating today. (Many of these
creationists combine this idea with the Flood Theory, as follows:
fossils were created during the Flood, but were "sorted" by a
miraculous process not observable or understandable today.)
Obviously, such a theory makes no testable predictions...except
perhaps for the prediction that geological formations should not
bear any obvious resemblance to processes occurring today.
* 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 of evolutionary theory:_ 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.

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 talk.origins'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). _The information in this FAQ assumes
that these dating methods are accurate_. If you have questions about
the many dating methods used by paleontologists, see the [6]other FAQs
on those topics and get yourself a good textbook of sedimentary
geology. Paleontologists are generally sharp cookies, and are quite
persnickety about using good dating techniques.

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)

Timescale

_CENOZOIC_
(See part 2) 65-0 Ma Mammals & birds & teleost fish dominant
_MESOZOIC_
Cretaceous 144-65 Ma Dinosaurs dominant. Small mammals, birds.
Jurassic 213-144 Ma Dinosaurs dominant. First mammals, then first
birds.
Triassic 248-213 Ma Mammalian reptiles dominant. First dinosaurs.
_PALEOZOIC_
Permian 286-248 Ma Amphibians dominant. First mammal-like reptiles.
Pennsylvanian 320-286 Ma Amphibians dominant. First reptiles.
Mississippian 360-320 Ma Big terrestrial amphibians, 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.) 

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. 

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!). 

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. 

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.

[7]
Contents [8]
Part 1B

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