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Transitional Vertebrate Fossils FAQ
Part 2B
Copyright © 1994-1997 by [3]Kathleen Hunt
[Last Update: March 17, 1997]

[4]
Part 2A [5]
Contents [6]
Part 2C

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.

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. 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_) 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_ found.

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 1994)
* _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
mesonychids.
* _Dorudon intermedius_ -- a late Eocene whale probably ancestral to
modern whales.

In the Oligocene, whales split into two lineages:
1. 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.
2. 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."
(MacFadden, 1988)

So here's the summary of the horse sequence. For more info, see the
[7]Horse Evolution FAQ.
* _Loxolophus_ (early Paleocene) -- A primitive condylarth with
rather low-crowned molars, probably ancestral to the phenacodontid
condylarths.
* _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_-like teeth.
* _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 teeth.
* _Merychippus primus_ (mid-Miocene, ~17 Ma) -- Slightly more
advanced.
* _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 clear, right?

SMALL GAP: It is not known which Merychippus species (stylodontus?
carrizoensis?) gave rise to the first Dinohippus species (Evander, in
Prothero & S 1988).
* _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 smaller fossae.
* _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.]

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:
* _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 teeth.
* _Heptodon_ (late early Eocene) -- A small early tapiroid showing
one more tooth cusp change. Split into two lineages:
1. _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.
2. _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.

Species-species transitions:
* 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_ species.

Elephants

* _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 &
anthracobunids.
* _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 transverse ridges.
* 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 trunk.
* _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:
* _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
throughout life.
* _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 Mammuthus:
1. _Loxodonta adaurora_ (5 Ma). Gave rise to the modern African
elephant _Loxodonta africana_ about 3.5 Ma.
2. _Elephas ekorensis_ (5 Ma), an early Asian elephant with
rather primitive molars, clearly derived directly from _P.
gomphotheroides_. Led directly to:
o _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_.
o _Elephas maximus_, the modern Asian elephant, clearly
derived from
o _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).
3. _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_.

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...."

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. primigenius_.
* 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
too.

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 Hungary.
* _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:
1. 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.
2. 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 manatees.

[8]
Part 2A [9]
Contents [10]
Part 2C

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8. http://www.talkorigins.org/faqs/faq-transitional/part2a.html
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11. http://www.talkorigins.org/scripts/imagemap/ltrailer.map
12. LYNXIMGMAP:file://localhost/www/sat/files/part2b.htm#ltrailermap
13. http://www.talkorigins.org/
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24. http://www.talkorigins.org/origins/faqs-flood.html
25. http://www.talkorigins.org/origins/faqs-catastrophism.html
26. http://www.talkorigins.org/origins/faqs-debates.html