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

[4]
Part 1B [5]
Contents [6]
Part 2B

PART 2

Overview of the Cenozoic

The Cenozoic fossil record is much better than the older Mesozoic
record, and _much_ better than the very much older Paleozoic record.
The most extensive Cenozoic gaps are early on, in the Paleocene and in
the Oligocene. From the Miocene on it gets better and better, though
it's still never perfect. Not surprisingly, the very recent
Pleistocene has the best record of all, with the most precisely known
lineages and most of the known species-to-species transitions. For
instance, of the 111 modern mammal species that appeared in Europe
during the Pleistocene, at least 25 can be linked to earlier European
ancestors by species-to-species transitional morphologies (see Kurten,
1968, and Barnosky, 1987, for discussion).

Timescale

Pleistocene 2.5-0.01 Ma Excellent mammal record
Pliocene 5.3-2.5 Ma Very good mammal record
Miocene 24-5.3 Ma Pretty good mammal record
Oligocene 34-24 Ma Spotty mammal record. Many gaps in various lineages
Eocene 54-34 Ma Surprisingly good mammal record, due to uplift and
exposure of fossil-bearing strata in the Rockies
Paleocene 67-54 Ma Fair record early on, but late Paleocene is lousy

For the rest of this FAQ, I'll walk through the known fossil records
for the major orders of modern placental mammals. For each order, I'll
describe the known lineages leading from early unspecialized
placentals to the modern animals, point out some of the remaining
gaps, and list several of the known species-to-species transitions. I
left out some of the obscure orders (e.g. hyraxes, anteaters), groups
that went completely extinct, and some of the families of particularly
diverse orders. 

Primates

I'll outline here the lineage that led to humans. Notice that there
were many other large, successful branches (particularly the lemurs,
New World monkeys, and Old World monkeys) that I will only mention in
passing. Also see Jim Foley's fossil hominid FAQ for detailed
information on hominid fossils.

GAP: "The modern assemblage can be traced with little question to the
base of the Eocene" says Carroll (1988). But before that, the origins
of the very earliest primates are fuzzy. There is a group of Paleocene
primitive primate-like animals called "plesiadapids" that may be
ancestral to primates, or may be "cousins" to primates. (see Beard, in
Szalay et al., 1993.)
* _Palaechthon_, _Purgatorius_ (middle Paleocene) -- Very primitive
plesiadapids. To modern eyes they looks nothing like primates,
being simply pointy-faced, small early mammals with mostly
primitive teeth, and claws instead of nails. But they show the
first signs of primate-like teeth; lost an incisor and a premolar,
and had relatively blunt-cusped, squarish molars.
* _Cantius_ (early Eocene) -- One of the first true primates (or
"primates of modern aspect"), more advanced than the plesiadapids
(more teeth lost, bar behind the eye, grasping hand & foot) and
beginning to show some lemur-like arboreal adaptations.
* _Pelycodus_ & related species (early Eocene) -- Primitive
lemur-like primates.

The tarsiers, lemurs, and New World monkeys split off in the Eocene.
The Old World lineage continued as follows:
* _Amphipithecus_, _Pondaungia_ (late Eocene, Burma) -- Very early
Old World primates known only from fragments. Larger brain,
shorter nose, more forward-facing eyes (halfway between
plesiadapid eyes and modern ape eyes).

GAP: Here's that Oligocene gap mentioned above in the timescale. Very
few primate fossils are known between the late Eocene and early
Oligocene, when there was a sharp change in global climate. Several
other mammal groups have a similar gap.
* _Parapithecus_ (early Oligocene) -- The O.W. monkeys split from
the apes split around now. _Parapithecus_ was probably at the
start of the O.W. monkey line. From here the O.W. monkeys go
through _Oreopithecus_ (early Miocene, Kenya) to modern monkey
groups of the Miocene & Pliocene.
* _Propliopithecus_, _Aegyptopithecus_ (early Oligocene, Egypt) --
From the same time as _Parapithecus_, but probably at the
beginning of the ape lineage. First ape characters (deep jaw, 2
premolars, 5- cusped teeth, etc.).
* _Aegyptopithecus_ (early-mid Oligocene, Egypt) -- Slightly later
anthropoid (ape/hominid) with more ape features. It was a
fruit-eating runner/climber, larger, with a rounder brain and
shorter face.
* _Proconsul africanus_ (early Miocene, Kenya.) -- A sexually
dimorphic, fruit-eating, arboreal quadruped probably ancestral to
all the later apes and humans. Had a mosaic of ape-like and
primitive features; Ape-like elbow, shoulder and feet; monkey-
like wrist; gibbon-like lumbar vertebrae.
* _Limnopithecus_ (early Miocene, Africa) -- A later ape probably
ancestral to gibbons.
* _Dryopithecus_ (mid-Miocene) -- A later ape probably ancestral to
the great apes & humans. At this point Africa & Asia connected via
Arabia, and the non-gibbon apes divided into two lines:
1. _Sivapithecus_ (including "Gigantopithecus" & "Ramapithecus",
mid- Miocene) -- Moved to Asia & gave rise to the orangutan.
2. _Kenyapithecus_ (mid-Miocene, about 16 Ma) -- Stayed in
Africa & gave rise to the African great apes & humans.

GAP: There are no known fossil hominids _or_ apes from Africa between
14 and 4 Ma. Frustratingly, molecular data shows that this is when the
African great apes (chimps, gorillas) diverged from hominids, probably
5-7 Ma. The gap may be another case of poor fossilization of forest
animals. At the end of the gap we start finding some very ape-like
bipedal hominids:
* _Australopithecus ramidus_ (mid-Pliocene, 4.4 Ma) -- A recently
discovered very early hominid (or early chimp?), from just after
the split with the apes. Not well known. Possibly bipedal (only
the skull was found). Teeth both apelike and humanlike; one baby
tooth is very chimp-like. (White et al., 1994; Wood 1994)
* _Australopithecus afarensis_ (late Pliocene, 3.9 Ma) -- Some
excellent fossils ("Lucy", etc.) make clear that this was fully
bipedal and definitely a hominid. But it was an extremely ape-like
hominid; only four feet tall, still had an ape-sized brain of just
375-500 cc (finally answering the question of which came first,
large brain or bipedality) and ape-like teeth. This lineage
gradually split into a husky large-toothed lineage and a more
slender, smaller- toothed lineage. The husky lineage (A. robustus,
A. boisei) eventually went extinct.
* _Australopithecus africanus_ (later Pliocene, 3.0 Ma) -- The more
slender lineage. Up to five feet tall, with slightly larger brain
(430-550 cc) and smaller incisors. Teeth gradually became more and
more like _Homo_ teeth. These hominds are almost perfect ape-
human intermediates, and it's now pretty clear that the slender
australopithecines led to the first _Homo_ species.
* _Homo habilis_ (latest Pliocene/earliest Pleistocene, 2.5 Ma) --
Straddles the boundary between australopithecines and humans, such
that it's sometimes lumped with the australopithecines. About five
feet tall, face still primitive but projects less, molars smaller.
Brain 500-800 cc, overlapping australopithecines at the low end
and and early Homo erectus at the high end. Capable of rudimentary
speech? First clumsy stone tools.
* _Homo erectus_ (incl. "Java Man", "Peking Man", "Heidelberg Man";
Pleist., 1.8 Ma) -- Looking much more human now with a brain of
775-1225 cc, but still has thick brow ridges & no chin. Spread out
of Africa & across Europe and Asia. Good tools, first fire.
* Archaic _Homo sapiens_ (Pleistocene, 500,000 yrs ago) -- These
first primitive humans were perfectly intermediate between H.
erectus and modern humans, with a brain of 1200 cc and less robust
skeleton & teeth. Over the next 300,000 years, brain gradually
increased, molars got still smaller, skeleton less muscular.
Clearly arose from _H erectus_, but there are continuing arguments
about where this happened.
* One famous offshoot group, the Neandertals, developed in Europe
125,000 years ago. They are considered to be the same species as
us, but a different subspecies, _H. sapiens neandertalensis_. They
were more muscular, with a slightly _larger_ brain of 1450 cc, a
distinctive brow ridge, and differently shaped throat (possibly
limiting their language?). They are known to have buried their
dead.
* _H. sapiens sapiens_ (incl. "Cro-magnons"; late Pleist., 40,000
yrs ago) -- All modern humans. Average brain size 1350 cc. In
Europe, gradually supplanted the Neanderthals.

Known species-species transitions in primates:

Phillip Gingerich has done a lot of work on early primate transitions.
Here are some of his major findings in plesiadapids, early lemurs, and
early monkeys:
* Plesiadapids: Gingerich (summarized in 1976, 1977) found smooth
transitions in plesiadapid primates linking four genera together:
_Pronothodectes_, _Nannodectes_, two lineages of _Plesiadapis_,
and _Platychoerops_. In summary: _Pronothodectes matthewi_ changed
to become _Pro. jepi_, which split into _Nannodectes intermedius_
and _Plesiadapis praecursor_. _N. intermedius_ was the first
member of a gradually changing lineage that passed through three
different species stages (N. gazini, N. simpsoni, and N. gidleyi).
_Ples. praecursor_ was the first member of a separate, larger
lineage that slowly grew larger (passing through three more
species stages), with every studied character showing continuous
gradual change. Gingerich (1976) noted "Loss of a tooth, a
discrete jump from one state to another, in several instances
proceeded continuously by continuous changes in the frequencies of
dimorphism -- the percentage of specimens retaining the tooth
gradually being reduced until it was lost entirely from the
population." The Plesiadapis lineage then split into two more
lineages, each with several species. One of these lineages shows a
gradual transition from _Plesiadapis_ to _Platychoerops_,"where
the incisors were considerably reorganized morphologically and
functionally in the space of only 2-3 million years."
* Early lemur-like primates: Gingerich (summarized in 1977) traced
two distinct species of lemur-like primates, _Pelycodus frugivorus_
and _P. jarrovii_, back in time, and found that they converged on
the earlier _Pelycodus abditus_ "in size, mesostyle development,
and every other character available for study, and there can be
little doubt that each was derived from that species." Further
work (Gingerich, 1980) in the same rich Wyoming fossil sites found
species-to-species transitions for _every step_ in the following
lineage: _Pelycodus ralstoni_ (54 Ma) to _P. mckennai_ to _P.
trigonodus_ to _P. abditus_, which then forked into three
branches. One became a new genus, _Copelemur feretutus_, and
further changed into _C. consortutus_. The second branch became
_P. frugivorus_. The third led to _P. jarrovi_, which changed into
another new genus, _Notharctus robinsoni_, which itself split into
at least two branches, _N. tenebrosus_, and _N. pugnax_ (which
then changed to _N. robustior_, 48 Ma), and possibly a third,
_Smilodectes mcgrewi_ (which then changed to _S. gracilis_). Note
that this sequence covers _at least_ three and possibly four
genera, with a timespan of 6 million years.
* Early monkey-like primates: Gingerich (1982, also discussed in
Gingerich, 1983) also describes gradual species-species
transitions in a lineage of early Eocene primate: _Cantius
ralstoni_ to _C. mckennai_ to _C. trigonodus_.

And here are some transitions found by other researchers:
* Rose & Bown (1984) analyzed over 600 specimens of primates
collected from a 700-meter-thick sequence representing
approximately 4 million years of the Eocene. They found smooth
transitions between _Teilhardina americana_ and _Tetonoides
tenuiculus_, and also beween _Tetonius homunculus_ and
_Pseudotetonius ambiguus._ "In both lines transitions occurred not
only continuously (rather than by abrupt appearance of new
morphologies followed by stasis), but also in mosaic fashion, with
greater variation in certain characters preceding a shift to
another character state." The _T. homunculus - P. ambiguus_
transition shows a dramatic change in dentition (loss of P2,
dramatic shrinkage of P3 with loss of roots, shrinkage of C and
I2, much enlarged I1) that occurs gradually and smoothly during
the 4 million years. The authors conclude "...our data suggest
that phyletic gradualism is not only more common than some would
admit but also capable of producing significant adaptive
modifications."
* Delson (discussed in Gingerich, 1985) has studied transitions in
primates from the Miocene to the present. For instance, in a 1983
paper (see Chaline, 1983), he discussed a possible smooth
transition from _Theropithecus darti_ to _T. oswaldi_, and
discusses transitions in hominids, concluding that _Homo sapiens_
clearly shows gradual changes over the last 800,000 years.
* Kurten (1968) reports a smooth transition linking _Macaca
florentina_ to _M. sylvana_

Bats

GAP: One of the least understood groups of modern mammals -- there are
no known bat fossils from the entire Paleocene. The first known fossil
bat, _Icaronycteris_, is from the (later) Eocene, and it was already a
fully flying animal very similar to modern bats. It did still have a
few "primitive" features, though (unfused & unkeeled sternum, several
teeth that modern bats have lost, etc.)
* Fruit bats and horseshoe bats first appear in the Oligocene.
Modern little vespertiliontids (like the little brown bat) first
appear in the Miocene.

Carnivores

* Creodonts -- early placental mammals with minor but interestingly
carnivore-like changes in the molars and premolars. Had a
carnivore- like shearing zone in the teeth, though the zone moved
throughout life instead of staying in particular teeth. Also had a
carnivore- like bony sheet in the brain dividing cerebrum &
cerebellum, details of ankle. Closely related to & possibly
ancestral to carnivores. The origin of the creodonts is unclear.
They probably were derived from condylarths.
* _Cimolestes_ (late Cretaceous) -- This creodont (?) lost the last
molar & then later enlarged the last upper premolar and first
lower molar. (In modern carnivores, these two teeth are very
enlarged to be the wickedly shearing carnassial teeth, the
hallmark of carnivores.) Still unfused feet & unossified bulla.
This genus is probably ancestral to two later lines of Eocene
carnivores called "miacoids". Miacoids were relatively
unspecialized meat-eaters that seem to have split into a
"viverravid" line (with cat/civet/hyena traits) and a "miacid"
line (with dog/bear/weasel traits). These two lines may possibly
have arisen from these slightly different species of _Cimolestes_:
* _Cimolestes incisus_ & _Cimolestes cerberoides_ (Cretaceous) --
These are two species that lost their third molar, and may have
given rise to the viverravid line of miacoids (see Hunt & Tedford,
in Szalay et al., 1993).
* _Cimolestes sp._ (Paleocene) -- A later, as yet unnamed species
that has very miacid-like teeth.
* _Simpsonictis tenuis_ (mid-Paleocene) -- A very early viverravid.
The upper carnassial was large; the lower carnassial was of
variable size in different individuals.
* _Paroodectes_, _Vulpavus_ (early Eocene) -- Early miacids.
Enlarged carnassials now specialized for shearing. Still had
unfused foot bones, short limbs, plantigrade feet, unossified
bulla.

GAP: few miacoid skulls are known from the rest of the Eocene -- a
real pity because for early carnivore relationships, skulls
(particularly the skull floor and ear capsule) are more useful than
teeth. There are some later skulls from the early Oligocene, which are
already distinguishable as canids, viverrids, mustelids, & felids (a
dog-like face, a cat-like face, and so on). Luckily some new
well-preserved miacoid fossils have just been found in the last few
years (mentioned in Szalay et al., 1993). They are still being studied
and will probably clarify exactly which miacoids gave rise to which
carnivores. Meanwhile, analysis of teeth has revealed at least one
ancestor:
* _Viverravus sicarius_ (mid-Eocene) -- Hunt & Tedford (in Szalay et
al., 1993) think this viverravid may be the ancestral aeluroid. It
has teeth & skeletal traits similar to the first known Oligocene
aeluroids (undifferentiated cat/civet/hyenas).

From the Oligocene onward, the main carnivore lineages continued to
diverge. First, the dog/bear/weasel line.

Dogs:
* _Cynodictis_ (late Eocene) -- First known arctoid
(undifferentiated dog/bear).
* _Hesperocyon_ (early Oligocene) -- A later arctoid. Compared to
miacids like _Paroodectes_, limbs have elongated, carnassials are
more specialized, braincase is larger. From here, the main line of
canid evolution can be traced in North America, with bears
branching out into a Holarctic distribution.
* _Cynodesmus_ (Miocene) -- First true dog. The dog lineage
continued through _Tomarctus_ (Pliocene) to the modern dogs,
wolves, & foxes, _Canis_ (Pleistocene).

Bears:
* _Cynodictis_ (see above)
* _Hesperocyon_ (see above)
* _Ursavus elmensis_ (mid-Oligocene) -- A small, heavy doglike
animal, intermediate between arctoids and bears. Still had slicing
carnassials & all its premolars, but molars were becoming squarer.
Later specimens of _Ursavus_ became larger, with squarer, more
bear-like, molars.
* _Protursus simpsoni_ (Pliocene; also "Indarctos") --
Sheepdog-sized. Carnassial teeth have no shearing action, molars
are square, shorter tail, heavy limbs. Transitional to the modern
genus _Ursus_.
* _Ursus minimus_ (Pliocene) -- First little bear, with very
bearlike molars, but still had the first premolars and slender
canines. Shows gradual tooth changes and increase in body size as
the ice age approached. Gave rise to the modern black bears (_U.
americanus_ & _U. thibetanus_), which haven't changed much since
the Pliocene, and also smoothly evolved to the next species, _U.
etruscus_:
* _Ursus etruscus_ (late Pliocene) -- A larger bear, similar to our
brown bear but with more primitive dentition. Molars big & square.
First premolars small, and got smaller over time. Canines stouter.
In Europe, gradually evolved into:
* _Ursus savini_ (late Pleistocene, 1 Ma) -- Very similar to the
brown bear. Some individuals didn't have the first premolars at
all, while others had little vestigial premolars. Tendency toward
domed forehead. Slowly split into a European population and an
Asian population.
* _U. spelaeus_ (late Pleistocene) -- The recently extinct giant
cave bear, with a highly domed forehead. Clearly derived from the
European population of U. savini, in a smooth transition. The
species boundary is arbitrarily set at about 300,000 years ago.
* _U. arctos_ (late Pleistocene) -- The brown ("grizzly") bear,
clearly derived from the Asian population of _U. savini_ about
800,000 years ago.. Spread into the Europe, & to the New World.
* _U. maritimus_ (late Pleistocene) -- The polar bear. Very similar
to a local population of brown bear, _U. arctos beringianus_ that
lived in Kamchatka about 500,000 years ago (Kurten 1964).

The transitions between each of these bear species are very well
documented. For most of the transitions there are superb series of
transitional specimens leading right across the species "boundaries".
See Kurten (1976) for basic info on bear evolution.

Raccoons (procyonids):
* _Phlaocyon_ (Miocene) -- A climbing carnivore with non-shearing
carnassials and handlike forepaws, transitional from the arctoids
to the procyonids (raccoons et al.). Typical raccoons first
appeared in the Pliocene.

Weasels (mustelids):
* _Plesictis_ (early Oligocene) -- Transitional between miacids (see
above) and mustelids (weasels etc.)
* _Potamotherium_ (late Oligocene) -- Another early mustelid, but
has some rather puzzling traits that may mean it is not a direct
ancestor of later mustelids. Mustelids were diversifying with
"bewildering variety" by the early Miocene.

Pinniped relationships have been the subject of extensive discussion
and analysis. They now appear to be a monophyletic group, probably
derived from early bears (or possibly early weasels?).

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

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

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

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

Species-species transitions among carnivores:
* Ginsburg (in Chaline, 1983) describes gradual change in the early
cats, from _Haplogale media_ to _Proailurus lemansis_, to (in
Europe) _Pseudaelurus transitorius_ to _Ps. lorteti_ to _Ps.
rmoieviensis_ to _Ps. quadridentatus_. These European lineages
gave rise to the modern _Lynx_, _Panthera_, etc. Different
lineages of _Pseudaelurus_ evolved in North American, Africa, and
Asia.
* Hecht (in Chaline, 1983) describes polar bear evolution; the first
"polar bear" subspecies, _Ursus maritimus tyrannus_, was a
essentially a brown bear subspecies, with brown bear dimensions
and brown bear teeth. Over the next 20,000 years, body size
reduced and the skull elongated. As late as 10,000 years ago,
polar bears still had a high frequency of brown-bear-type molars.
Only recently have they developed polar-bear-type teeth.
* Kurten (1976) describes bear transitions: "From the early Ursus
minimus of 5 million years ago to the late Pleistocene cave bear,
there is a perfectly complete evolutionary sequence without any
real gaps. The transition is slow and gradual throughout, and it
is quite difficult to say where one species ends and the next
begins. Where should we draw the boundary between U. minimus and
U. etruscus, or between U. savini and U. spelaeus? The history of
the cave bear becomes a demonstration of evolution, not as a
hypothesis or theory but as a simple fact of record." He adds, "In
this respect the cave bear's history is far from unique."
* Kurten (1968) also described the following known species-species
transitions:
+ _Felis issiodorensis_ to _Felis pardina_ (leopards)
+ _Gulo schlosseri_ to _Gulo gulo_ (wolverines)
+ _Cuon majori_ to _Cuon alpinus_ (dholes, a type of
short-faced wolf)
* Lundelius et al. (1987) describe a study by Schultz in 1978 that
showed an increase in canine length leading from the dirk-tooth
cat _Megantereon hesperus_ to _Megantereon/Smilodon gracilis_,
then to _Smilodon fatalis_ (a saber-toothed cat), and then to
_Smilodon californicus_. Note the genus transition and the
accompanying striking change in morphology.
* Werdelin & Solounias (1991) wrote an extensive monograph on
hyenids. They discuss over one hundred (!) named species, with
extensive discussion of the eighteen best-known species, and
cladistic analysis of _hundreds_ of specimens from the _SIXTY-ONE_
"reasonably well known" hyaenid fossil species. They concluded:

"We view the evolution of hyaenids as overwhelmingly gradual. The
species, when studied with regard to their total variability, often
grade insensibly into each other, as do the genera. Large specimens
of Hyaenotherium wongii are, for example, difficult to distinguish
from small specimens of Hyaenictitherium hyaenoides, a distinct
genus. Viewed over the entire family, the evolution of hyaenids
from small, fox-like forms to large, scavenging, "typical" hyenas
can be followed step by step, and the assembly of features defining
the most derived forms has taken place piecemeal since the Miocene.
Nowhere is there any indication of major breaks identifying
macroevolutionary steps."

Rodents

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

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

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

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

Cavies:

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

[7]
Part 1B [8]
Contents [9]
Part 2B

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