Transitional Vertebrate Fossils FAQ
[Last Update: March 17, 1997]
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).
||Excellent mammal record
||Very good mammal record
||Pretty good mammal record
||Spotty mammal record. Many gaps in various
||Surprisingly good mammal record, due to
uplift and exposure of fossil-bearing strata in the Rockies
||Fair record early on, but late Paleocene is
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
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
- 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
- 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:
- Sivapithecus (including "Gigantopithecus" &
"Ramapithecus", mid- Miocene) -- Moved to Asia & gave rise to
- 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
- 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
- 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
- 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
- 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
- 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
- Kurten (1968) reports a smooth transition linking Macaca
florentina to M. sylvana
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,
- Fruit bats and horseshoe bats first appear in the Oligocene.
Modern little vespertiliontids (like the little brown bat) first
appear in the Miocene.
- 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 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
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.
- Cynodictis (late Eocene) -- First known arctoid
- 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).
- 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
- 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
- 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
- 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
- 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
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.
- 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.
- 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
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,
- Odobenidae: the walrus family. Started with Neotherium
14 my, then Imagotaria, which is probably ancestral to
- 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
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
- More advanced modern civets appeared in the Miocene.
- Haplogale (late Oligocene, 30 Ma) -- A slightly cat-like
- "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 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
- 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 &
- 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
- Plioviverrops orbignyi (& 3 closely related
- Tungurictis spocki, a mid-Miocene fox-sized hyenid.
Truly hyena-like ear capsule.
- Ictitherium viverrinum (& 6 closely related
- 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
- 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
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
- 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
- Felis issiodorensis to Felis pardina
- Gulo schlosseri to Gulo gulo (wolverines)
- Cuon majori to Cuon alpinus (dholes, a type of
- 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.
"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."
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.
- 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".
- 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.
- Paramys (see above)
- Eomyids -- later Eocene rodents with a few tooth and eyesocket
features that show they had branched off from the squirrel
- 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
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
- 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.
- Jaeger (in Chaline, 1983) describes gradual shifts in tooth
size and shape two genera of early mice, related to the development
- 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,
- 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
- 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.
Home Page | Browse | Search | Feedback | Links
The FAQ | Must-Read Files | Index | Creationism | Evolution | Age of the Earth | Flood Geology | Catastrophism | Debates