Author: Kathleen Hunt (firstname.lastname@example.org)
Title: Transitional Fossils FAQ
I've recently been re-reading Colbert's _Evolution of the
Vertebrates_, and was reminded of the old "there aren't any
transitional fossils" complaint that pops up on t.o. every now and
then. That argument has long been obsolete and inaccurate, as a brief
glance at the fossil record shows. I thought it might be of interest
to have a list of some of the transitional vertebrate fossils known,
so that future t.o.discussions of the fossil record can be somewhat
more up-to-date and interesting (I can dream, can't I?).
A couple people have asked me to post this as a f.a.q. file to t.o.
So here goes. First, I'll present a *partial* list of known
transitional fossils, compiled from Colbert's _Evolution of the
Vertebrates_ (ref at end). Also at the end I have a short note about
the significance of "transitional fossils".
The fossils mentioned in this list are from species and/or genuses
thought to represent transitions from one vertebrate group to another.
This list is necessarily highly incomplete, because:
a) I skipped entire sections of Colbert's text (rodents, bovids,
dinosaurs, teleosts, and more).
b) Colbert's text is *not* an encyclopedic list of all known fossils,
but instead has detailed descriptions of *particular* fossils that
Colbert thought were representative of that group at that time, or
that were otherwise of special interest.
c) Colbert's text is from 1980 and thus somewhat outdated. I've added
in some recently discovered bird, whale, horse, and primate fossils.
Please let me know of other recent discoveries.
[We start off with primitive jawless fish.]
Transition from primitive jawless fish to sharks, skates, and rays:
Cladoselachians (e.g., _Cladoselache_).
Hybodonts (e.g. _Hybodus_)
Heterodonts (e.g. _Heterodontus_)
Hexanchids (e.g. _Chlamydoselache_)
Transition from primitive bony fish to holostean fish:
Palaeoniscoids (e.g. _Cheirolepis_); living chondrosteans such as
_Polypterus_ and _Calamoichthys_, and also the living acipenseroid
chondrosteans such as sturgeons and paddlefishes.
Primitive holosteans such as _Semionotus_.
Transition from holostean fish to advanced teleost fish:
Leptolepidomorphs, esp. _Leptolepis_, an excellent holostean-teleost
Elopomorphs, both fossil and living (tarpons, eels)
Clupeomorphs (e.g. _Diplomystus_)
Osteoglossomorphs (e.g. _Portheus_)
Transition from primitive bony fish to amphibians:
Paleoniscoids again (e.g. _Cheirolepis_)
_Osteolepis_ -- one of the earliest crossopterygian lobe-finned fishes,
still sharing some characters with the lungfish (the other group of
lobe-finned fish). Had paired fins with a leg-like arrangement of bones,
and had an early-amphibian-like skull and teeth.
_Eusthenopteron_ (and other rhipidistian crossopterygian fish) --
intermediate between early crossopterygian fish and the earliest
amphibians. Skull very amphibian-like. Strong amphibian-like backbone.
Fins very like early amphibian feet.
Icthyostegids (such as _Icthyostega_ and _Icthyostegopsis_) --
Terrestrial amphibians with many of _Eusthenopteron_'s fish features
(e.g., the fin rays of the tail were retained). Some debate about
whether _Icthyostega_ should be considered a fish or an amphibian;
it is an excellent transitional fossil.
Labyrinthodonts (e.g., _Pholidogaster_, _Pteroplax_) -- still have some
icthyostegid features, but have lost many of the fish features (e.g.,
the fin rays are gone, vertebrae are stronger and interlocking, the
nasal passage for air intake is well defined.)
Transition from amphibians to reptiles:
Seymouriamorph labyrinthodonts (e.g. _Seymouria_) -- classic labyrinthodont
skull and teeth, with reptilian vertebrae, pelvis, humerus, and digits;
Cotylosaurs (e.g. _Hylonomus_, _Limnoscelis_) -- slightly amphibian
skull (e.g. with amphibian-type pineal opening), with rest of skeleton
The cotylosaurs gave rise to many reptile groups of tremendous variety. I
won't go into the transitions from cotylosaurs to the advanced anapsid
reptiles (turtles and possibly mesosaurs), to the euryapsid reptiles
(icthyosaurs, plesiosaurs, and others), or to the lepidosaurs (eosuchians,
lizards, snakes, and the tuatara), or to most of the dinosaurs, since I don't
have infinite time. Instead I'll concentrate on the synapsid reptiles (which
gave rise to mammals) and the archosaur reptiles (which gave rise to birds).
Transition from reptiles to mammals:
Pelycosaur synapsids -- classic reptilian skeleton, intermediate between
the cotylosaurs (the earliest reptiles) and the therapsids (see next)
Therapsids (e.g. _Dimetrodon_) -- the numerous therapsid
fossils show gradual transitions from reptilian features to
mammalian features. For example: the hard palate forms, the teeth
differentiate, the occipital condyle on the base of the skull doubles,
the ribs become restricted to the chest instead of extending down the
whole body, the legs become "pulled in" instead of sprawled out, the ilium
(major bone of the hip) expands forward.
Cynodont theriodonts (e.g. _Cynognathus_) -- very mammal-like reptiles.
Or is that reptile-like mammals? Highly differentiated teeth (a classic
mammalian feature), with accessory cusps on cheek teeth; strongly
differentiated vertebral column (with distinct types of vertebrae for
the neck, chest, abdomen, pelvis, and tail -- very mammalian), mammalian
scapula, mammalian limbs, mammalian digits (e.g. reduction of number of
bones in the first digit). But, still has unmistakably *reptilian*
Tritilodont theriodonts (e.g. _Tritylodon_, _Bienotherium_) -- skull
even more mammalian (e.g. advanced zygomatic arches). Still has
reptilian jaw joint.
Ictidosaur theriodonts (e.g. _Diarthrognathus_) -- has all the mammalian
features of the tritilodonts, and has a *double* jaw joint; both the
reptilian jaw joint and the mammalian jaw joint were present, side-by-side,
in _Diarthrognathus_'s skull. A really stunning transitional fossil.
Morganucodonts (e.g. _Morganucodon_) -- early mammals. Double jaw joint,
but now the mammalian joint is dominant (the reptilian joint bones are
beginning to move inward; in modern mammals these are the bones of
the middle ear).
Eupantotheres (e.g. _Amphitherium_) -- these mammals begin to show the
complex molar cusp patterns characteristic of modern marsupials and
eutherians (placental mammals). Mammalian jaw joint.
Proteutherians (e.g. _Zalambdalestes_) -- small, early insectivores with
molars intermediate between eupantothere molars and modern eutherian
Those wondering how egg-laying reptiles could make the transition to
placental mammals may wish to study the reproductive biology of the
monotremes (egg-laying mammals) and the marsupials. The monotremes
in particular could almost be considered "living transitional fossils".
[see Peter Lamb's suggested marsupial references at end]
Transition from reptiles to birds:
_Lisboasaurus estesi_ and other "troodontid dinosaur-birds" -- a bird-like
reptile with very bird-like teeth (that is, teeth very like those of
early toothed birds [modern birds have no teeth]). May not have been
a direct ancestor; may have been a "cousin" of the birds instead.
_Protoavis_ -- this is a *highly controversial* fossil that may or may not be
an extremely early bird. Not enough of the fossil was recovered to
determine if it is definitely related to the birds, or not. I mention it
in case people have heard about it recently.
_Archeopteryx_ -- reptilian vertebrae, pelvis, tail, skull, teeth, digits,
claws, sternum. Avian furcula (wishbone, for attachment of flight
muscles), forelimbs, and lift-producing flight feathers. _Archeopteryx_
could probably fly from tree to tree, but couldn't take off from
the ground, since it lacked a keeled breastbone (for attachment of large
flight muscles) and had a weak shoulder (relative to modern birds).
"Chinese bird" [I don't know what name was given to this fossil] --
A fossil dating from 10-15 million years after _Archeopteryx_.
Bird-like claws on the toes, flight-specialized shoulders, fair-sized
sternal keel (modern birds usually have large sternal keel); also
has reptilian stomach ribs, reptilian unfused hand bones, & reptilian
pelvis. This bird has a fused tail ("pygostyle"), but I don't know how
long it was, or if it was all fused or just part of it was fused.
"Las Hoyas bird" [I don't know what name was given to this fossil] --
This fossil dates from 20-30 m.y. after _Archeopteryx_. It still
has reptilian pelvis & legs, with bird-like shoulder. Tail is
medium-length with a fused tip (_Archeopteryx_ had long, unfused tail;
modern birds have short, fused tail). Fossil down feather was found with
the Las Hoyas bird.
Toothed Cretaceous birds, e.g. _Hesperornis_ and _Ichthyornis_. Skeleton
further modified for flight (fusion of pelvis bones, fusion of hand
bones, short & fused tail). Still had true socketed teeth, which are
missing in modern birds.
[note: a classic study of chicken embryos showed that chicken bills can
be induced to develop teeth, indicating that chickens (and perhaps other
modern birds) still retain the genes for making teeth.]
Now, on to some of the classes of mammals.
Transitional fossils from early eutherian mammals to primates:
Early primates -- paromomyids, carpolestids, plesiadapids. Lemur-like
clawed primates with generalized nails.
_Notharctus_, an early Eocene lemur
_Parapithecus_, a small Old World monkey (Oligocene)
_Propliopithecus_, a small primate intermediate between _Parapithecus_
and the more recent O.W. monkeys. Has several ape-like characters.
_Aegyptopithecus_, an early ape.
_Limnopithecus_, a later ape showing similarities to the modern gibbons.
_Dryopithecus_, a later ape showing similarities to the non-gibbon apes.
_Ramapithecus_, a dryopithecine-like ape showing similarities to the
hominids but now thought to be an orang ancestor.
_Australopithecus_ spp., early hominids. Bipedal.
_Homo erectus_. Numerous fossils across the Old World.
_Homo sapiens sapiens_. This is us. (NB: "Cro-magnon man" belongs
here too. Cro-magnons were a specific population of modern humans.)
_Homo sapiens neanderthalensis_ (not on the direct line to _H. sapiens
sapiens_, but worth mentioning).
[I haven't described these fossils in detail because they're fairly well
covered in any intro biology text, or in any of several good general-
interest books on human evolution.]
Transitional fossils from early eutherian mammals to rodents:
Paramyids, e.g. _Paramys_ -- early "primitive" rodent
_Paleocastor_ -- transitional from paramyids to beavers
[yick. I was going to summarize rodent fossils but _Paramys_ and its
friends gave rise to 5 enormous and very diverse groups of rodents, with
about ten zillion fossils. Never mind.]
Transitional fossils among the cetaceans (whales & dolphins):
_Pakicetus_ -- the oldest fossil whale known. Only the skull was found.
It is a distinct whale skull, but with nostrils in the position of a
land animal (tip of snout). The ears were *partially* modified for
hearing under water. This fossil was found in association with fossils
of land mammals, suggesting this early whale *maybe* could walk on land.
_Basilosaurus isis_ -- a recently discovered "legged" whale from the
Eocene (after _Pakicetus_). Had hind feet with 3 toes and a tiny remnant
of the 2nd toe (the big toe is totally missing). The legs were small and
must have been useless for locomotion, but were specialized for swinging
forward into a locked straddle position -- probably an aid to copulation
for this long-bodied, serpentine whale.
Archaeocetes (e.g. _Protocetus_, _Eocetus_) -- have lost hind legs entirely,
but retain "primitive whale" skull and teeth, with forward nostrils.
Squalodonts (e.g. _Prosqualodon_) -- whale-like skull with *dorsal*
nostrils (blowhole), still with un-whale-like teeth.
_Kentriodon_, an early toothed whale with whale-like teeth.
_Mesocetus_, an early whalebone whale
[note: very rarely a modern whale is found with tiny hind legs, showing
that some whales still retain the genes for making hind legs.]
Transitional fossils from early eutherian mammals to the carnivores:
Miacids (e.g. _Viverravus_ and _Miacis_) -- small weasel-like animals
with very carnivore-like teeth, esp. the carnassial teeth.
Arctoids (e.g. _Cynodictis_, _Hesperocyon_) -- intermediate between
miacids and dogs. Limbs have elongated, carnassials are more
specialized, braincase is larger.
_Cynodesmus_, _Tomarctus_ -- transitional fossils between arctoids
and the modern dog genus _Canis_.
_Hemicyon_, _Ursavus_ -- heavy doglike fossils between the arctoids
and the bears.
_Indarctos_ -- early bear. Carnassial teeth have no shearing action,
molars are square, short tail, heavy limbs. Transitional to the
modern genus _Ursus_.
_Phlaocyon_ -- a climbing carnivore with non-shearing carnassials,
transitional from the arctoids to the procyonids (raccoons et al.)
Meanwhile back at the ranch,
_Plesictis_, transitional between miacids (see above) and mustelids
(weasels et al.)
_Stenoplesictis_ and _Palaeoprionodon_, early civets related to the
miacids (see above)
_Tunguricits_, transitional between early civets and modern civets
_Ictitherium_, transitional between early civets to hyenas
_Proailurus_, transitional from early civets to early cats
_Dinictis_, transitional from early cats to modern "feline" cats
_Hoplophoneus_, transitional from early cats to "saber-tooth" cats
Transitional fossils from early eutherians to hoofed animals:
Arctocyonid condylarths -- insectivore-like small mammals with classic
mammalian teeth and clawed feet.
Mesonychid condylarths -- similar to the arctocyonids, but with blunt
crushing-type cheek teeth, and flattened nails instead of claws.
Late condylarths, e.g. _Phenocodus_ -- a fair-sized animal with
hoofs on each toe (all toes were present), a continuous series of
crushing-type cheek teeth with herbivore-type cusps, and no collarbone
(like modern hoofed animals).
Transitional fossils from early hoofed animals to perissodactyls:
[Perissodactyls are animals with an *odd* number of toes; most of the
weight is borne by the central 3rd toe. Horses, rhinos, tapirs.]
_Tetraclaeonodon_ -- a Paleocene condylarth showing perissodactyl-like
_Hyracotherium_ -- the famous "dawn horse", an early perissodactyl, with
more elongated digits and interlocking ankle bones, and slightly
different tooth cusps, compared to to _Tetraclaeonodon_. A small, doggish
animal with an arched back, short neck, and short snout; had 4 toes
in front and 3 behind. Omnivore teeth.
[The rest of horse evolution will be covered in an upcoming "horse
fossils" post in a few weeks. To whet your appetite:]
_Orohippus_ -- small, 4/3 toed, developing browser tooth crests
_Epihippus_ -- small, 4/3 toed, good tooth crests, browser
_Epihippus (Duchesnehippus)_ -- a subgenus with _Mesohippus_-like teeth
_Mesohippus_ -- 3 toed on all feet, browser, slightly larger
_Miohippus_ -- 3 toed browser, slightly larger [gave rise to lots of
successful three-toed browsers]
_Parahippus_ -- 3 toed browser/grazer, developing "spring foot"
_'Parahippus' leonensis_ -- a _Merychippus_-like species of _Parahippus_
_'Merychippus' gunteri_ -- a _Parahippus_-like species of _Merychippus_
_'Merychippus' primus_ -- a more typical _Merychippus_, but still very
_Merychippus_ -- 3 toed grazer, spring-footed, size of small pony
(gave rise to tons of successful three-toed grazers)
_Merychippus (Protohippus)_ -- a subgenus of _Merychippus_ developing
_Pliohippus_ & _Dinohippus_ -- *one*-toed grazers, spring-footed
_Equus (Plesippus)_ -- like modern equines but teeth slightly simpler.
_Equus (Hippotigris)_, the modern 1-toed spring-footed grazing zebras.
_Equus (Equus)_, the modern 1-toed spring-footed grazing horses & donkeys.
[note: very rarely a horse is born with small visible side toes, indicating
that some horses retain the genes for side toes.]
Meanwhile back at the ranch,
Hyrachyids -- transitional from perissodactyl-like condylarths to tapirs
Heptodonts, e.g. _Lophiodont_ -- a small horse-like tapir, transitional
to modern tapirs
_Protapirus_ -- a probable descendent of _Lophiodont_, much like modern
tapirs but without the flexible snout.
_Miotapirus_ -- an almost-modern tapir with a flexible snout, transitional
between _Protapirus_ and the modern _Tapirus_.
Hyracodonts -- early "running rhinoceroses", transitional to modern rhinos
_Caenopus_, a large, hornless, generalized rhino transitional between the
hyracodonts and the various later groups of modern & extinct rhinos.
Transitional fossils from early hoofed animals to some of the artiodactyls
Dichobunoids, e.g. _Diacodexis_, transitional between condylarths
and all the artiodactyls (cloven-hoofed animals). Very condylarth-like
but with a notably artiodactyl-like ankle.
_Propalaeochoerus_, an early pig, transitional between _Diacodexis_ and
_Protylopus_, a small, short-necked, four-toed animal, transitional between
dichobunoids and early camels. From here the camel lineage goes through
_Protomeryx_, _Procamelus_, _Pleauchenia_, _Lama_ (which are still alive;
these are the llamas) and finally _Camelus_, the modern camels.
_Archeomeryx_, a rabbit-sized, four-toed animal, transitional between the
dichobunoids and the early deer. From here the deer lineage goes through
_Eumeryx_, _Paleomeryx_ and _Blastomeryx_, _Dicrocerus_ (with antlers) and
then a shmoo of successful groups that survive today as modern deer --
muntjacs, cervines, white-tail relatives, moose, reindeer, etc., etc.
_Palaeotragus_, transitional between early artiodactyls and the okapi &
giraffe. Actually the okapi hasn't changed much since _Palaeotragus_ and
is essentially a living Miocene giraffe. After _Palaeotragus_ came
_Giraffa_, with elongated legs & neck, and _Sivatherium_, large ox-like
giraffes that *almost* survived to the present.
So, there's a *partial* list of transitional fossils.
This really only scratches the surface since I left out all groups
that have no surviving relatives, didn't discuss modern amphibians or
reptiles, left out most of the birds, ignored the diversity in modern
fish, didn't discuss the bovids or elephants or rodents or many other
mammal groups....I hope this gives a taste of the richness of the
fossil record and the abundance of transitional fossils between major
By the way, notice that this list mostly includes transitional fossils
that happened to lead to modern, familiar animals. This may
unintentionally give the impression that fossil lineages proceed in a
"straight line" from one fossil to the next. That's not so; generally
at any one time there are a whole raft of successful species, only a
few of which happened to leave modern descendents. The horse family is
a good example; _Merychippus_ gave rise to something like 19 new
three-toed grazing horse species, which traveled all over the Old and
New Worlds and were very successful at the time. Only one of these
lines happened to lead to _Equus_, though, so that's the only line I
talked about. Evolution is not a ladder, it's a branching bush.
And now, for those of you who are still with me...
I have a few comments about "transitional fossils" in general. When
_The Origin Of Species_ was first published, the fossil record was
poorly known. At that time, the complaint about the lack of
transitional fossils bridging the major vertebrate taxa was perfectly
reasonable. Opponents of Darwin's theory of common descent (the
theory that evolution has occurred; not to be confused with the
separate theory that evolution occurs specifically by natural
selection) were justifiably skeptical of such ideas as birds being
related to reptiles. The discovery of _Archeopteryx_ only two years
after the publication of _The Origin of Species_ was seen a stunning
triumph for Darwin's theory of common descent. _Archeopteryx_ has
been called the single most important natural history specimen ever
found, "comparable to the Rosetta Stone" (Alan Feduccia, in "The Age
Of Birds"). O.C. Marsh's groundbreaking study of the evolution of
horses was another dramatic example of transitional fossils, this time
demonstrating a whole sequence of transitions within a single family.
Within a few decades after the _Origin_, these and other fossils,
along with many other sources of evidence (such as developmental
biology and biogeography) had convinced the majority of educated
people that evolution *had* occured, and that organisms *are* related
to each other by common descent. (Whether evolution occurs by natural
selection, rather than by some other mechanism, is *another question
entirely* and is the topic of current evolutionary research.)
Since then *many* more transitional fossils have been found.
Typically, the only people who still demand to see transitional
fossils are creationists who have been reading 100-year-old
anti-evolution arguments, and who are either unaware of the currently
known fossil record or are unwilling to believe it for some reason.
When presented with a transitional fossil, such creationists often
then want to see the transitions between the transitions - - or, as
Pilbeam complained, "as soon as you find a missing link, you've just
created two more missing links". Alternatively, creationists will
often state that the two groups being bridged by the transitional
fossil are really the same "kind" (a term that has *no* meaning in
modern biology) and that therefore "real evolution" hasn't occurred.
This often leads to a weasely backtracking in which *no* transitional
fossil, however dramatic, no matter what disparate groups it connects,
will ever be accepted by a creationist. Biologists justifiably find
this attitude irritating, and any creationist taking this tack can
expect to have testy biologists demanding that he/she clearly define
"kind" before the discussion goes any further.
Creationists also sometimes say "All right, so you have a transitional
fossil from X to Y -- but you don't from Y to Z!" It is unreasonable
to expect the fossil record to be absolutely complete. It is highly
unlikely for *any* organism to get fossilized, and to demand a perfect
sequence of fossils of all species from all times and all locations,
perfectly preserved in rocks that are not plowed under or eroded away,
and not taken by private collectors and sold for thousands of dollars
at some auction or used as a doorstop or a paperweight, but instead
are exposed just as one of the few working paleontologists in the
world happens to walk by -- well, we're lucky that the known fossil
record is as good as it is. Remember that even if only *ONE*
transitional fossil were known, it would be a tremendous support for
evolutionary theory. (Thus the tremendous impact of _Archeopteryx_ in
1861). We now know of HUNDREDS of transitional fossils. It is
logically absurd to demand that a *particular* gap be filled, and if
it can't be filled to then say that evolution has been falsified --
meanwhile ignoring all the gaps that *have* been filled.
I'll leave it at that. This has been a partial list of transitional
fossils among some of the major taxa of vertebrates. This list has
been brought to you by the numbers 1 and 7 and the letter E.
"Chinese bird fossil: mix of old and new". 1990. Science News 138: 246-247
[this fossil was described at the 1990 annual meeting of the Society of
Vertebrate Paleontology, so there's probably a paper on it in the collected
Colbert, E. 1980. _Evolution of the Vertebrates_, 3rd ed. John Wiley &
Sons, New York.
Gould, S.J. 1983. _Hen's Teeth And Horse's Toes_. W.W. Norton, New York.
[The title essay discusses evidence that some species retain old genes
for traits that they no longer express -- teeth in chickens, side toes in
Feduccia, A. 1980. _The Age Of Birds_. Harvard University Press,
Gingerich, P.D., Smith, B.H., Simons, E.L. 1990. Hind limb of Eocene
_Basilosaurus_: evidence of feet in whales. Science 249:154.
_The Lonely Bird_. 1991. Science News 140:104-105. [an article on the
controversy surrounding _Protoavis_. A monograph on _Protoavis_'s skull was
published in June 1991 in Phil. Trans. Royal Soc. London, if anyone cares;
this was the first publication on _Protoavis_, which was found years ago but
has been jealously guarded by its discoverer for some time.]
Milner, A.R., and S.E. Evans. 1991. The Upper Jurassic diapsid
_Lisboasaurus estesi_ -- a maniraptoran theropod. Paleontology 34:503-513.
[this is the bird-like archosaurian reptile]
Sanz, J.L., Bonaparte, J.F., and A. Lacassa. 1988. Unusual Early Cretaceous
birds from Spain. Nature 331:433-435. [This is about the Las Hoyas bird.
Also see Science News 133:102, "Bird fossil reveals history of flight", for
a brief synopsis.]
Horse references will be in horse post.
Marsupial references (suggested by Peter Lamb):
 Mervyn Griffiths, "The Platypus", Scientific American, May 1988 pp 60-67.
 Mervyn Griffiths, "The Biology of the Monotremes", Academic Press,
New York a.o., 1978
 Terence J.Dawson, "Monotremes and Marsupials: the other Mammals",
Arnold, London, 1983