Transitional Vertebrate Fossils FAQ
[Last Update: March 17, 1997]
- Barunlestes (see above) The possible Asian
- 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 (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
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
- 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.
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 bones present and unfused,
pointy-faced, narrow small brain, eyesocket not 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.
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) from Clark's Fork
Basin in Wyoming. Note, however, that several other species from
the same time showed stasis (particularly Ectocion, which
was previously reported to show change, but in fact stayed much the
same), and that not all species transitions are documented. So
transitions are not always found. But sometimes they are
Cetaceans (whales, dolphins)
Just several years ago, there was still a large gap in the fossil
record of the cetaceans. It was thought that they arose from
land-dwelling mesonychids that gradually lost their hind legs and
became aquatic. Evolutionary theory predicted that they must have
gone through a stage where they had were partially aquatic but
still had hind legs, but there were no known intermediate fossils.
A flurry of recent discoveries from India & Pakistan (the
shores of the ancient Tethys Sea) has pretty much filled this gap.
There are still no known species-species transitions, and the
"chain of genera" is not complete, but we now have a partial
lineage, and sure enough, the new whale fossils have legs, exactly
as predicted. (for discussions see Berta, 1994; Gingerich et al.
1990; Thewissen et al. 1994; Discover magazine, Jan. 1995; Gould
- Eoconodon or similar triisodontine arctocyonids (early
Paleocene) Unspecialized condylarths quite similar to the early
oxyclaenid condylarths, but with strong canine teeth (showing first
meat-eating tendencies), blunt crushing cheek teeth, and flattened
claws instead of nails.
- Microclaenodon (mid-Paleocene) -- A transitional genus
intermediate between Eoconodon and the mesonychids, with
molar teeth reorganizing in numerous ways to look like premolars.
Adapted more toward carnivory.
- Dissacus (mid-Paleocene) -- A mesonychid (rather
unspecialized Paleocene meat-eating animal) with molars more like
premolars & several other tooth changes. Still had 5 toes in
the foot and a primitive plantigrade posture.
- Hapalodectes or a very similar mesonychid (early Eocene,
around 55 Ma) -- A small mesonychid with very narrow shearing
molars, a distinctively shaped zygomatic arch, and peculiar
vascularized areas between the molars. Probably a running animal
that could swim by paddling its feet. Hapalodectes itself
may be just too late to be the whale ancestor, but probably was a
close relative of the whale ancestor. Says Carroll (1988): "The
skulls of Eocene whales bear unmistakable resemblances to those of
primitive terrestrial mammals of the early Cenozoic. Early [whale]
genera retain a primitive tooth count with distinct incisors,
canines, premolars,, and multirooted molar teeth. Although the
snout is elongate, the skull shape resembles that of the
mesonychids, especially Hapalodectes...."
- Pakicetus (early-mid Eocene, 52 Ma) -- The oldest fossil
whale known. Same skull features as Hapalodectes, still with
a very terrestrial ear (tympanic membrane, no protection from
pressure changes, no good underwater sound localization), and
therefore clearly not a deep diver. Molars still have very
mesonychid-like cusps, but other teeth are like those of later
whales. Nostrils still at front of head (no blowhole). Whale- like
skull crests and elongate jaws. Limbs unknown. Only about 2.5 m
long. This skull was found with terrestrial fossils and may have
been amphibious, like a hippo.
- Ambulocetus natans (early-mid Eocene, 50 Ma) -- A
recently discovered early whale, with enough of the limbs and
vertebrae preserved to see how the early whales moved on land and
in the water. This whale had four legs! Front legs were stubby.
Back legs were short but well-developed, with enormous broad feet
that stuck out behind like tail flukes. Had no true tail flukes,
just a long simple tail. Size of a sea lion. Still had a long snout
with no blowhole. Probably walked on land like a sea lion, and swam
with a seal/otter method of steering with the front feet and
propelling with the hind feet. So, just as predicted, these early
whales were much like modern sea lions -- they could swim, but they
could also still walk on land. (Thewissen et al., 1994)
- Rodhocetus (mid-Eocene, 46 Ma) -- Another very recent
(1993) fossil whale discovery. Had hind legs a third smaller than
those of A. natans. Could probably still "waddle" a bit on
land, but by now it had a powerful tail (indicated by massive tail
vertebrae) and could probably stay out at sea for long periods of
time. Nostrils had moved back a bit from the tip of the snout.
- Basilosaurus isis, Protocetes, Indocetus
ramani and similar small-legged whales of the mid-late Eocene
(45-42 Ma) -- After Rodhocetus came several whales that
still had hind legs, but couldn't walk on them any more. For
example, B. isis (42 Ma) 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. B. isis may have been a "cousin" to modern whales,
not directly ancestral. Another recent discovery is
Protocetes, a slightly more advanced whale from the late
Eocene. It was about 3m long (dolphin sized), and still had
primitive dentition, nostrils at end of snout, and a large pelvis
attached to the spine; limbs unknown. Finally Indocetus is
known from only fragmentary remains, but these include a tibia.
These late Eocene legged whales still had mesonychid-like teeth,
and in fact, some of the whale fossils were first mis-identified as
mesonychids when only the teeth were found. ( See Gingerich et al.
(1990) for more info on B. isis.)
- Prozeuglodon (late Eocene, 40 Ma) Another recently
discovered whale, found in 1989. Had almost lost the hind
legs, but not quite: still carried a pair of vestigial 6- inch hind
legs on its 15-foot body.
- Eocetus, & similar "archeocete whales" of the late
Eocene These more advanced whales have lost their hind legs
entirely, but retain a"primitive whale" skull and teeth, with
unfused nostrils. They grew to larger body size (up to 25m by the
end of the Eocene), an had an elongate, streamlined body, flippers,
and a cartilaginous tail fluke. The ear was modified for hearing
underwater. Note that this stage of aquatic adaptation was attained
about 15 million years after the first terrestrial
- Dorudon intermedius -- a late Eocene whale probably
ancestral to modern whales.
In the Oligocene, whales split into two lineages:
- Toothed whales:
- Agorophius (late Oligocene) -- Skull partly telescoped,
but cheek teeth still rooted. Intermediate in many ways between
archaeocetes and later toothed whales.
- Prosqualodon (late Oligocene) -- Skull fully telescoped
with nostrils on top (blowhole). Cheek teeth increased in number
but still have old cusps. Probably ancestral to most later toothed
whales (possibly excepting the sperm whales?)
- Kentriodon (mid-Miocene) -- Skull telescoped, still
symmetrical. Radiated in the late Miocene into the modern dolphins
and small toothed whales with asymmetrical skulls.
- Baleen (toothless) whales:
- Aetiocetus (late Oligocene) -- The most primitive known
mysticete whale and probably the stem group of all later baleen
whales. Had developed mysticete-style loose jaw hinge and air
sinus, but still had all its teeth. Later,
- Mesocetus (mid-Miocene) lost its teeth.
- Modern baleen whales first appeared in the late Miocene.
Perissodactyls (horses, tapirs, rhinos)
Here we come to the most famous general lineage of all, the horse
sequence. It was the first such lineage to be discovered, in the
late 1800's, and thus became the most famous. There is an odd rumor
circulating in creationist circles that the horse sequence is
somehow suspect or outdated. Not so; it's a very good sequence that
has grown only more detailed and complete over the years, changing
mainly by the addition of large side-branches. As these various
paleontologists have said recently: "The extensive fossil record of
the family Equidae provides an excellent example of long-term,
large-scale evolutionary change." (Colbert, 1988) "The fossil
record [of horses] provides a lucid story of descent with change
for nearly 50 million years, and we know much about the ancestors
of modern horses."(Evander, in Prothero & Schoch 1989, p. 125)
"All the morphological changes in the history of the Equidae can be
accounted for by the neo-Darwinian theory of microevolution:
genetic variation, natural selection, genetic drift, and
speciation." (Futuyma, 1986, p.409) "...fossil horses do indeed
provide compelling evidence in support of evolutionary theory."
So here's the summary of the horse sequence. For more info, see
the Horse Evolution FAQ.
- Loxolophus (early Paleocene) -- A primitive condylarth
with rather low-crowned molars, probably ancestral to the
- Tetraclaenodon (mid-Paleocene) -- A more advanced
Paleocene condylarth from the phenacodontid family, and almost
certainly ancestral to all the perissodactyls (a different order).
Long but unspecialized limbs; 5 toes on each foot (#1 and #5
smaller). Slightly more efficient wrist.
GAP: There are almost no known perissodactyl fossils from the late
Paleocene. This is actually a small gap; it's only noticeable
because the perissodactyl record is otherwise very complete. Recent
discoveries have made clear that the first perissodactyls arose in
Asia (a poorly studied continent), so hopefully the ongoing new
fossil hunts in Asia will fill this small but frustrating gap. The
first clue has already come in:
- Radinskya yupingae (late Paleocene, China) -- A recently
discovered perissodactyl-like condylarth. (McKenna et al., in
Prothero & Schoch, 1989.)
- Hyracotherium (early Eocene, about 55 Ma; previously
"Eohippus") -- The famous "dawn horse", a small, doggish
perissodactyl, with an arched back, short neck, omnivore teeth, and
short snout. 4 toes in front and 3 behind. Compared to
Tetraclaenodon, has longer toes, interlocking ankle bones,
and slightly different tooth cusps. Probably evolved from
Tetra. in about 4-5 my, perhaps via an Asian species like
Radinskya. Note that Hyrac. differed from other early
perissodactyls (such as tapir/rhino ancestors) only by small
changes in tooth cusps and in body size.
- Hyracotherium vassacciense (early Eocene) -- The
particular species that probably gave rise to the equids.
- Orohippus (mid-Eocene, ~50 Ma) -- Small, 4/3 toed,
developing browser tooth crests.
- Epihippus (late Eocene, ~45 Ma) -- Small, 4/3 toed, good
tooth crests, browser.
- Epihippus (Duchesnehippus) -- A later subgenus with
- Mesohippus celer (latest Eocene, 40 Ma) -- Three-toed on
all feet, browser, slightly larger
- Mesohippus westoni (early Oligocene) -- A slightly
later, more advanced species.
- Miohippus assiniboiensis (mid-Oligocene) -- This species
split off from early Mesohippus via cladogenetic evolution,
after which Miohippus and Mesohippus overlapped for
the next 4 my. Distinctly larger, slightly longer skull, facial
fossa deeper and more expanded, subtly different ankle joint,
variable extra crest on upper cheek teeth. In the early Miocene (24
My) Miohippus began to speciate rapidly. Grasses had just evolved,
& teeth began to change accordingly. Legs, etc., started to
change for fast running.
- Kalobatippus (late Oligocene) -- Three-toed browser
w/foot intermediate between Mio. & Para.
- Parahippus (early Miocene, 23 Ma) -- Three-toed
browser/grazer, developing "spring foot". Permanent establishment
of the extra crest that was so variable in Miohippus. Stronger
tooth crests & slightly taller tooth crowns.
- 'Parahippus' leonensis (mid-Miocene, ~20 Ma) --
Three-toed browser/grazer with the emphasis on grazer. Developing
spring-foot & high-crowned teeth.
- 'Merychippus' gunteri (mid-Miocene, ~18 Ma) --
Three-toed grazer, fully spring-footed with high-crowned
- Merychippus primus (mid-Miocene, ~17 Ma) -- Slightly
- Merychippus spp. of mid-late Miocene (16-15 Ma) --
3-toed grazers, spring-footed, size of small pony. Diversified into
all available grazer niches, giving rise to at least 19 successful
three-toed grazers. Side toes of varying sizes, very small in some
lines. Horsey hoof develops, leg bones fuse. Fully high-crowned
teeth with thick cement & same crests as Parahippus. The line
that eventually produced Equus developed as follows: M.
primus, M. sejunctus, M. isonesus (these last two still had a mix
of primitive, hipparion, and equine features), M. intermontanus, M.
stylodontus, M. carrizoensis. These last two looked quite horsey,
with quite small side toes, and gave rise to a set of larger
three-toed and one-toed horses known as the "true equines". Crystal
SMALL GAP: It is not known which Merychippus species (stylodontus?
carrizoensis?) gave rise to the first Dinohippus species (Evander,
in Prothero & S 1988).
Compare Equus to Hyracotherium and see how much it
has changed. If you think of animals as being divided into "kinds",
do you think Equus and Hyracotherium can be considered the same
"kind"? Tapirs and rhinos:
- Dinohippus (late Miocene, 12 Ma) -- One-toed
grazer, spring-footed. Very equine feet, teeth, and skull, with
straighter teeth & smaller fossae. First was D. spectans,
followed by D. interpolatus and D. leidyanus. A slightly later
species was D. mexicanus, with even straighter teeth and even
- Equus (Plesippus), also called the "E.
simplicidens" group (Pliocene, ~4 My) -- Three closely related
species of one-toed spring-footed high-crowned grazers. No fossae
and very straight teeth. Pony size, fully "horsey" body -- rigid
spine, long neck, long legs, fused leg bones with no rotation, long
nose, flexible muzzle, deep jaw. The brain was a bit larger than in
early Dinohippus. Still had some primitive traits such as simple
teeth & slight facial fossae, which later Equus species lost.
These "simple Equus" species quickly diversified into at least 12
new species in 4 different groups. During the first major
glaciations of the late Pliocene (2.6 Ma), certain Equus
species crossed to the Old World. Worldwide, Equus took over the
niche of "large coarse-grazing plains runner".
- Equus (Hippotigris) (Pleistocene) -- Subgenus of modern
1-toed spring-footed grazing zebras.
- Equus (Equus) (Pleistocene) -- Subgenus of modern 1-toed
spring-footed grazing horses & donkeys. [note: very rarely a
horse is born with small side toes, indicating that some horses
retain the genes for side toes.]
- Loxolophus, see above
- Tetraclaenodon, see above
- Homagalax (early Eocene) -- Very like its sister genus
Hyracotherium, but had cross-lophs on teeth. Note that these
early perissodactyls differed only in slight details of the
- Heptodon (late early Eocene) -- A small early tapiroid
showing one more tooth cusp change. Split into two lineages:
- Helaletes (mid-Eocene) which had a short proboscis, then
Prototapir (late Oligocene), much like modern tapirs but
without such a flexible snout, then Miotapirus (early
Miocene), an almost- modern tapir with a flexible snout, then
Tapirus (Pliocene) the modern tapir.
- Hyrachyus (late Eocene), a tapiroid with increased
shearing function in its teeth. Led to the late Eocene
hyracodontids such as Hyracodon (rhino-tapiroids, or
"running rhinos") that show increasing development of high-crowned
teeth and larger body size. They led to Caenopus (early
Oligocene), a large, hornless, generalized rhino which led to the
modern horned rhinos of the Miocene & Pliocene. Our living
genera first appear in the Pliocene, about 4 Ma.
- Horses: Gingerich (1980) documented speciation from
Hyracotherium grangeri to H. aemulor. Prothero &
Schoch (1989) mention some intermediate fossils that link late
Orohippus to Mesohippus celer. MacFadden (1985) has
documented numerous smooth transitions among the three-toed horses,
particularly among Merychippus and the various hipparions.
Hulbert (in Prothero & Schoch, 1989) showed that
Dinohippus smoothly grades into Equus through
successive Pliocene strata. Simpson (1961) describes gradual loss
of the side toes in Pliohippus through 3 successive strata
of the early Pliocene.
- Rhinos: Wood (1954) said of the rhino fossils "whenever we do
have positive paleontological evidence, the picture is of the most
extreme gradualism" (quoted in Gingerich, 1977), and Kurten (1968)
describes a smooth transition between Dicerorhinus
- Minchenella or a similar condylarth (late Paleocene) --
Known only from lower jaws. Has a distinctive broadened shelf on
the third molar. The most plausible ancestor of the embrithopods
- Phenacolophus (late Paleocene or early Eocene) -- An
early embrithopod (very early, slightly elephant-like condylarths),
thought to be the stem-group of all elephants.
- Pilgrimella (early Eocene) -- An anthracobunid (early
proto-elephant condylarth), with massive molar cusps aligned in two
- Unnamed species of proto-elephant (early Eocene) -- Discovered
recently in Algeria. Had slightly enlarged upper incisors (the
beginnings of tusks), and various tooth reductions. Still had
"normal" molars instead of the strange multi-layered molars of
modern elephants. Had the high forehead and pneumatized skull bones
of later elephants, and was clearly a heavy-boned, slow animal.
Only one meter tall.
- Moeritherium, Numidotherium, Barytherium
(early-mid Eocene) -- A group of three similar very early
elephants. It is unclear which of the three came first. Pig-sized
with stout legs, broad spreading feet and flat hooves. Elephantish
face with the eye set far forward & a very deep jaw. Second
incisors enlarged into short tusks, in upper and lower jaws;
little first incisors still present; loss of some teeth. No
- Paleomastodon, Phiomia (early Oligocene) -- The
first "mastodonts", a medium-sized animals with a trunk, long lower
jaws, and short upper and lower tusks. Lost first incisors and
canines. Molars still have heavy rounded cusps, with enamel bands
becoming irregular. Phiomia was up to eight feet tall.
GAP: Here's that Oligocene gap again. No elephant fossils at all
for several million years.
- Gomphotherium (early Miocene) -- Basically a large
edition of Phiomia, with tooth enamel bands becoming very
irregular. Two long rows cusps on teeth became cross- crests when
worn down. Gave rise to several families of elephant- relatives
that spread all over the world. From here on the elephant lineages
are known to the species level.
- The mastodon lineage split off here, becoming more adapted to a
forest browser niche, and going through Miomastodon
(Miocene) and Pliomastodon (Pliocene), to Mastodon
(or "Mammut", Pleistocene).
Meanwhile, the elephant lineage became still larger, adapting to a
savannah/steppe grazer niche:
The Pleistocene record for elephants is very good. In general,
after the earliest forms of the three modern genera appeared, they
show very smooth, continuous evolution with almost half of the
speciation events preserved in fossils. For instance, Carroll
(1988) says: "Within the genus Elephas, species demonstrate
continuous change over a period of 4.5 million years. ...the
elephants provide excellent evidence of significant morphological
change within species, through species within genera, and through
genera within a family...."
- Stegotetrabelodon (late Miocene) -- One of the first of
the "true" elephants, but still had two long rows of cross-crests,
functional premolars, and lower tusks. Other early Miocene genera
show compression of the molar cusps into plates (a modern feature
), with exactly as many plates as there were cusps. Molars start
erupting from front to back, actually moving forward in the jaw
- Primelephas (latest Miocene) -- Short lower jaw makes it
look like an elephant now. Reduction & loss of premolars. Very
numerous plates on the molars, now; we're now at the modern
elephants' bizarre system of one enormous multi-layered molar being
functional at a time, moving forward in the jaw.
- Primelephas gomphotheroides (mid-Pliocene) -- A later
species that split into three lineages, Loxodonta, Elephas, and
- Loxodonta adaurora (5 Ma). Gave rise to the modern
African elephant Loxodonta africana about 3.5 Ma.
- Elephas ekorensis (5 Ma), an early Asian elephant with
rather primitive molars, clearly derived directly from P.
gomphotheroides. Led directly to:
- Elephas recki, which sent off one side branch, E.
hydrusicus, at 3.8 Ma, and then continued changing on its own
until it became E. iolensis.
- Elephas maximus, the modern Asian elephant, clearly
- E. hysudricus. Strikingly similar to young E.
hysudricus animals. Possibly a case of neoteny (in which "new"
traits are simply juvenile features retained into adulthood).
- Mammuthus meridionalis, clearly derived from P.
gomphotheroides. Spread around the northern hemisphere. In
Europe, led to M. armeniacus/trogontherii, and then to M.
primigenius. In North America, led to M. imperator and
then M. columbi.
Species-species transitions among the elephants:
- Maglio (1973) studied Pleistocene elephants closely. Overall,
Maglio showed that at least 7 of the 17 Quaternary elephant species
arose through smooth anagenesis transitions from their ancestors.
For example, he said that Elephas recki "can be traced
through a progressive series of stages...These stages pass almost
imperceptibly into each other....In the late Pleistocene a more
progressive elephant appears which I retain as a distinct species,
E. iolensis, only as a matter of convenience. Although as a
group, material referred to E. iolensis is distinct from
that of E. recki, some intermediate specimens are known, and
E. iolensis seems to represent a very progressive, terminal
stage in the E. recki specific lineage."
- Maglio also documented very smooth transitions between three
Eurasian mammoth species: Mammuthus meridionalis -->
M. armeniacus (or M. trogontherii) --> M.
- Lister (1993) reanalyzed mammoth teeth and confirmed Maglio's
scheme of gradual evolution in European mammoths, and found
evidence for gradual transitions in the North American mammoths
Sirenians (dugongs & manatees)
GAP: The ancestors of sirenians are not known. No sirenian-like
fossils are known from before the Eocene.
- Early Eocene -- fragmentary sirenian fossils known from
- Prorastomus (mid-Eocene) -- A very primitive sirenian
with an extremely primitive dental formula (including the ancient
fifth premolar that all other mammals lost in the Cretaceous! Could
this mean sirenians split off from all other mammals very early
on?) The skull is somewhat condylarth-like. Had distinctive
sirenian ribs. Not enough of the rest of the skeleton was found to
know how aquatic it was.
- Protosiren (late Eocene) -- A sirenian with an
essentially modern skeleton, though it still had the very primitive
dental formula. Probably split into the two surviving lineages:
- Dugongs: Eotheroides (late Eocene), with a slightly
curved snout and small tusks, still with the primitive dental
formula. Perhaps gave rise to Halitherium (Oligocene) a
dugong-ish sirenian with a more curved snout and longer tusks, and
then to living dugongs, very curved snout & big tusks.
- Manatees: Sirenotherium (early Miocene);
Potamosiren (late Miocene), a manatee-like sirenian with
loss of some cheek teeth; then Ribodon (early Pliocene), a
manatee with continuous tooth replacement, and then the living
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