Transitional Vertebrate Fossils FAQ
Part 1B
[Last Update: March 17, 1997]
Transition from amphibians to amniotes (first reptiles)
The major functional difference between the ancient, large amphibians and the
first little reptiles is the amniotic egg. Additional differences include
stronger legs and girdles, different vertebrae, and stronger jaw muscles. For
more info, see Carroll (1988) and Gauthier et al. (in Benton, 1988)
- Proterogyrinus or another early anthracosaur (late Mississippian)
-- Classic labyrinthodont-amphibian skull and teeth, but with reptilian
vertebrae, pelvis, humerus, and digits. Still has fish skull hinge. Amphibian
ankle. 5-toed hand and a 2-3-4-5-3 (almost reptilian) phalangeal count.
- Limnoscelis, Tseajaia (late Carboniferous) -- Amphibians
apparently derived from the early anthracosaurs, but with additional reptilian
features: structure of braincase, reptilian jaw muscle, expanded neural
arches.
- Solenodonsaurus (mid-Pennsylvanian) -- An incomplete fossil,
apparently between the anthracosaurs and the cotylosaurs. Loss of palatal
fangs, loss of lateral line on head, etc. Still just a single sacral vertebra,
though.
- Hylonomus, Paleothyris (early Pennsylvanian) -- These are
protorothyrids, very early cotylosaurs (primitive reptiles). They were quite
little, lizard-sized animals with amphibian-like skulls (amphibian pineal
opening, dermal bone, etc.), shoulder, pelvis, & limbs, and intermediate
teeth and vertebrae. Rest of skeleton reptilian, with reptilian jaw muscle, no
palatal fangs, and spool-shaped vertebral centra. Probably no eardrum yet.
Many of these new "reptilian" features are also seen in little
amphibians (which also sometimes have direct-developing eggs laid on land), so
perhaps these features just came along with the small body size of the first
reptiles.
The ancestral amphibians had a rather weak skull and paired "aortas"
(systemic arches). The first reptiles immediately split into two major lines
which modified these traits in different ways. One line developed an aorta on
the right side and strengthened the skull by swinging the quadrate bone down and
forward, resulting in an enormous otic notch (and allowed the later development
of good hearing without much further modification). This group further split
into three major groups, easily recognizable by the number of holes or
"fenestrae" in the side of the skull: the anapsids (no fenestrae), which
produced the turtles; the diapsids (two fenestrae), which produced the dinosaurs
and birds; and an offshoot group, the eurapsids (two fenestrae fused into one),
which produced the ichthyosaurs.
The other major line of reptiles developed an aorta on left side only, and
strengthened the skull by moving the quadrate bone up and back, obliterating the
otic notch (making involvement of the jaw essential in the later development of
good hearing). They developed a single fenestra per side. This group was the
synapsid reptiles. They took a radically different path than the other reptiles,
involving homeothermy, a larger brain, better hearing and more efficient teeth.
One group of synapsids called the "therapsids" took these changes particularly
far, and apparently produced the mammals.
Some transitions among reptiles
I will review just a couple of the reptile phylogenies, since there are so
many.... Early reptiles to turtles: (Also see Gaffney & Meylan, in Benton
1988)
- Captorhinus (early-mid Permain) -- Immediate descendent of the
protorothryids.
Here we come to a controversy; there are two related groups of early
anapsids, both descended from the captorhinids, that could have been ancestral
to turtles. Reisz & Laurin (1991, 1993) believe the turtles descended from
procolophonids, late Permian anapsids that had various turtle-like skull
features. Others, particularly Lee (1993) think the turtle ancestors are
pareiasaurs:
- Scutosaurus and other pareiasaurs (mid-Permian) -- Large bulky
herbivorous reptiles with turtle-like skull features. Several genera had bony
plates in the skin, possibly the first signs of a turtle shell.
- Deltavjatia vjatkensis (Permian) -- A recently discovered
pareiasaur with numerous turtle-like skull features (e.g., a very high
palate), limbs, and girdles, and lateral projections flaring out some of the
vertebrae in a very shell-like way. (Lee, 1993)
- Proganochelys (late Triassic) -- a primitive turtle, with a fully
turtle-like skull, beak, and shell, but with some primitive traits such as
rows of little palatal teeth, a still-recognizable clavicle, a simple
captorhinid-type jaw musculature, a primitive captorhinid- type ear, a
non-retractable neck, etc..
- Recently discovered turtles from the early Jurassic, not yet described.
Mid-Jurassic turtles had already divided into the two main groups of modern
turtles, the side-necked turtles and the arch-necked turtles. Obviously these
two groups developed neck retraction separately, and came up with totally
different solutions. In fact the first known arch-necked turtles, from the Late
Jurassic, could not retract their necks, and only later did their descendents
develop the archable neck. Early reptiles to diapsids: (see Evans, in Benton
1988, for more info)
- Hylonomus, Paleothyris (early Penn.) -- The primitive
amniotes described above
- Petrolacosaurus, Araeoscelis (late Pennsylvanian) -- First
known diapsids. Both temporal fenestra now present. No significant change in
jaw muscles. Have Hylonomus-style teeth, with many small marginal teeth &
two slightly larger canines. Still no eardrum.
- Apsisaurus (early Permian) -- A more typical diapsid. Lost canines.
(Laurin, 1991)
GAP: no diapsid fossils from the mid-Permian.
- Claudiosaurus (late Permian) -- An early diapsid with several
neodiapsid traits, but still had primitive cervical vertebrae & unossified
sternum. probably close to the ancestry of all diapsides (the lizards &
snakes & crocs & birds).
- Planocephalosaurus(early Triassic) -- Further along the line that
produced the lizards and snakes. Loss of some skull bones, teeth, toe bones.
- Protorosaurus, Prolacerta (early Triassic) -- Possibly among
the very first archosaurs, the line that produced dinos, crocs, and birds. May
be "cousins" to the archosaurs, though.
- Proterosuchus (early Triassic) -- First known archosaur.
- Hyperodapedon, Trilophosaurus (late Triassic) -- Early
archosaurs.
Some species-to-species transitions:
- De Ricqles (in Chaline, 1983) documents several possible cases of gradual
evolution (also well as some lineages that showed abrupt appearance or stasis)
among the early Permian reptile genera Captorhinus,
Protocaptorhinus, Eocaptorhinus, and Romeria.
- Horner et al. (1992) recently found many excellent transitional dinosaur
fossils from a site in Montana that was a coastal plain in the late
Cretaceous. They include:
- Many transitional ceratopsids between Styracosaurus and Pachyrhinosaurus
- Many transitional lambeosaurids (50! specimens) between Lambeosaurus and
Hypacrosaurus.
- A transitional pachycephalosaurid between Stegoceras and
Pachycephalosaurus
- A transitional tyrannosaurid between Tyrannosaurus and Daspletosaurus.
All of these transitional animals lived during the same brief 500,000
years. Before this site was studied, these dinosaur groups were known from the
much larger Judith River Formation, where the fossils showed 5 million years
of evolutionary stasis, following by the apparently abrupt appearance of the
new forms. It turns out that the sea level rose during that 500,000 years,
temporarily burying the Judith River Formation under water, and forcing the
dinosaur populations into smaller areas such as the site in Montana. While the
populations were isolated in this smaller area, they underwent rapid
evolution. When sea level fell again, the new forms spread out to the
re-exposed Judith River landscape, thus appearing "suddenly" in the Judith
River fossils, with the transitional fossils only existing in the Montana
site. This is an excellent example of punctuated equilibrium (yes, 500,000
years is very brief and counts as a "punctuation"), and is a good example of
why transitional fossils may only exist in a small area, with the new species
appearing "suddenly" in other areas. (Horner et al., 1992) Also note the
discovery of Ianthosaurus, a genus that links the two synapsid families
Ophiacodontidae and Edaphosauridae. (see Carroll, 1988, p. 367)
Transition from synapsid reptiles to mammals
This is the best-documented transition between vertebrate classes. So far
this series is known only as a series of genera or families; the transitions
from species to species are not known. But the family sequence is quite
complete. Each group is clearly related to both the group that came before, and
the group that came after, and yet the sequence is so long that the fossils at
the end are astoundingly different from those at the beginning. As Rowe recently
said about this transition (in Szalay et al., 1993), "When sampling artifact is
removed and all available character data analyzed [with computer phylogeny
programs that do not assume anything about evolution], a highly corroborated,
stable phylogeny remains, which is largely consistent with the temporal
distributions of taxa recorded in the fossil record." Similarly, Gingerich has
stated (1977) "While living mammals are well separated from other groups of
animals today, the fossil record clearly shows their origin from a reptilian
stock and permits one to trace the origin and radiation of mammals in
considerable detail." For more details, see Kermack's superb and readable little
book (1984), Kemp's more detailed but older book (1982), and read Szalay et
al.'s recent collection of review articles (1993, vol. 1).
This list starts with pelycosaurs (early synapsid reptiles) and continues
with therapsids and cynodonts up to the first unarguable "mammal". Most of the
changes in this transition involved elaborate repackaging of an expanded brain
and special sense organs, remodeling of the jaws & teeth for more efficient
eating, and changes in the limbs & vertebrae related to active,
legs-under-the-body locomotion. Here are some differences to keep an eye on:
|
# |
Early Reptiles |
Mammals
|
|
1 |
No fenestrae in skull |
Massive fenestra exposes all of braincase
|
2 |
Braincase attached loosely |
Braincase attached firmly to skull
|
3 |
No secondary palate |
Complete bony secondary palate
|
4 |
Undifferentiated dentition |
Incisors, canines, premolars, molars
|
5 |
Cheek teeth uncrowned points |
Cheek teeth (PM & M) crowned & cusped
|
6 |
Teeth replaced continuously |
Teeth replaced once at most
|
7 |
Teeth with single root |
Molars double-rooted
|
8 |
Jaw joint quadrate-articular |
Jaw joint dentary-squamosal (*)
|
9 |
Lower jaw of several bones |
Lower jaw of dentary bone only
|
10 |
Single ear bone (stapes) |
Three ear bones (stapes, incus, malleus)
|
11 |
Joined external nares |
Separate external nares
|
12 |
Single occipital condyle |
Double occipital condyle
|
13 |
Long cervical ribs |
Cervical ribs tiny, fused to vertebrae
|
14 |
Lumbar region with ribs |
Lumbar region rib-free
|
15 |
No diaphragm |
Diaphragm
|
16 |
Limbs sprawled out from body |
Limbs under body
|
17 |
Scapula simple |
Scapula with big spine for muscles
|
18 |
Pelvic bones unfused |
Pelvis fused
|
19 |
Two sacral (hip) vertebrae |
Three or more sacral vertebrae
|
20 |
Toe bone #'s 2-3-4-5-4 |
Toe bones 2-3-3-3-3
|
21 |
Body temperature variable |
Body temperature constant
|
|
(*) The presence of a dentary-squamosal jaw joint has been arbitrarily
selected as the defining trait of a mammal.
- Paleothyris (early Pennsylvanian) -- An early captorhinomorph
reptile, with no temporal fenestrae at all.
- Protoclepsydrops haplous (early Pennsylvanian) -- The earliest
known synapsid reptile. Little temporal fenestra, with all surrounding bones
intact. Fragmentary. Had amphibian-type vertebrae with tiny neural processes.
(reptiles had only just separated from the amphibians)
- Clepsydrops (early Pennsylvanian) -- The second earliest known
synapsid. These early, very primitive synapsids are a primitive group of
pelycosaurs collectively called "ophiacodonts".
- Archaeothyris (early-mid Pennsylvanian) -- A slightly later
ophiacodont. Small temporal fenestra, now with some reduced bones
(supratemporal). Braincase still just loosely attached to skull. Slight hint
of different tooth types. Still has some extremely primitive,
amphibian/captorhinid features in the jaw, foot, and skull. Limbs, posture,
etc. typically reptilian, though the ilium (major hip bone) was slightly
enlarged.
- Varanops (early Permian) -- Temporal fenestra further enlarged.
Braincase floor shows first mammalian tendencies & first signs of stronger
attachment to rest of skull (occiput more strongly attached). Lower jaw shows
first changes in jaw musculature (slight coronoid eminence). Body narrower,
deeper: vertebral column more strongly constructed. Ilium further enlarged,
lower-limb musculature starts to change (prominent fourth trochanter on
femur). This animal was more mobile and active. Too late to be a true
ancestor, and must be a "cousin".
- Haptodus (late Pennsylvanian) -- One of the first known
sphenacodonts, showing the initiation of sphenacodont features while retaining
many primitive features of the ophiacodonts. Occiput still more strongly
attached to the braincase. Teeth become size-differentiated, with biggest
teeth in canine region and fewer teeth overall. Stronger jaw muscles.
Vertebrae parts & joints more mammalian. Neural spines on vertebrae
longer. Hip strengthened by fusing to three sacral vertebrae instead of just
two. Limbs very well developed.
- Dimetrodon, Sphenacodon or a similar sphenacodont (late
Pennsylvanian to early Permian, 270 Ma) -- More advanced pelycosaurs, clearly
closely related to the first therapsids (next). Dimetrodon is almost
definitely a "cousin" and not a direct ancestor, but as it is known from very
complete fossils, it's a good model for sphenacodont anatomy. Medium-sized
fenestra. Teeth further differentiated, with small incisors, two huge deep-
rooted upper canines on each side, followed by smaller cheek teeth, all
replaced continuously. Fully reptilian jaw hinge. Lower jaw bone made of
multiple bones & with first signs of a bony prong later involved in the
eardrum, but there was no eardrum yet, so these reptiles could only hear
ground-borne vibrations (they did have a reptilian middle ear). Vertebrae had
still longer neural spines (spectacularly so in Dimetrodon, which had a
sail), and longer transverse spines for stronger locomotion muscles.
- Biarmosuchia (late Permian) -- A therocephalian -- one of the
earliest, most primitive therapsids. Several primitive, sphenacodontid
features retained: jaw muscles inside the skull, platelike occiput, palatal
teeth. New features: Temporal fenestra further enlarged, occupying virtually
all of the cheek, with the supratemporal bone completely gone. Occipital plate
slanted slightly backwards rather than forwards as in pelycosaurs, and
attached still more strongly to the braincase. Upper jaw bone (maxillary)
expanded to separate lacrymal from nasal bones, intermediate between early
reptiles and later mammals. Still no secondary palate, but the vomer
bones of the palate developed a backward extension below the palatine bones.
This is the first step toward a secondary palate, and with exactly the same
pattern seen in cynodonts. Canine teeth larger, dominating the dentition.
Variable tooth replacement: some therocephalians (e.g Scylacosaurus)
had just one canine, like mammals, and stopped replacing the canine after
reaching adult size. Jaw hinge more mammalian in position and shape, jaw
musculature stronger (especially the mammalian jaw muscle). The amphibian-like
hinged upper jaw finally became immovable. Vertebrae still
sphenacodontid-like. Radical alteration in the method of locomotion, with a
much more mobile forelimb, more upright hindlimb, & more mammalian femur
& pelvis. Primitive sphenacodontid humerus. The toes were approaching
equal length, as in mammals, with #toe bones varying from reptilian to
mammalian. The neck & tail vertebrae became distinctly different from
trunk vertebrae. Probably had an eardrum in the lower jaw, by the jaw hinge.
- Procynosuchus (latest Permian) -- The first known cynodont -- a
famous group of very mammal-like therapsid reptiles, sometimes considered to
be the first mammals. Probably arose from the therocephalians, judging from
the distinctive secondary palate and numerous other skull characters. Enormous
temporal fossae for very strong jaw muscles, formed by just one of the
reptilian jaw muscles, which has now become the mammalian masseter. The large
fossae is now bounded only by the thin zygomatic arch (cheekbone to you &
me). Secondary palate now composed mainly of palatine bones (mammalian),
rather than vomers and maxilla as in older forms; it's still only a partial
bony palate (completed in life with soft tissue). Lower incisor teeth was
reduced to four (per side), instead of the previous six (early mammals had
three). Dentary now is 3/4 of lower jaw; the other bones are now a small
complex near the jaw hinge. Jaw hinge still reptilian. Vertebral column starts
to look mammalian: first two vertebrae modified for head movements, and lumbar
vertebrae start to lose ribs, the first sign of functional division into
thoracic and lumbar regions. Scapula beginning to change shape. Further
enlargement of the ilium and reduction of the pubis in the hip. A diaphragm
may have been present.
- Dvinia [also "Permocynodon"] (latest Permian) -- Another early
cynodont. First signs of teeth that are more than simple stabbing points --
cheek teeth develop a tiny cusp. The temporal fenestra increased still
further. Various changes in the floor of the braincase; enlarged brain. The
dentary bone was now the major bone of the lower jaw. The other jaw bones that
had been present in early reptiles were reduced to a complex of smaller bones
near the jaw hinge. Single occipital condyle splitting into two surfaces. The
postcranial skeleton of Dvinia is virtually unknown and it is not therefore
certain whether the typical features found at the next level had already
evolved by this one. Metabolic rate was probably increased, at least
approaching homeothermy.
- Thrinaxodon (early Triassic) -- A more advanced "galesaurid"
cynodont. Further development of several of the cynodont features seen
already. Temporal fenestra still larger, larger jaw muscle attachments. Bony
secondary palate almost complete. Functional division of teeth: incisors (four
uppers and three lowers), canines, and then 7-9 cheek teeth with cusps for
chewing. The cheek teeth were all alike, though (no premolars & molars),
did not occlude together, were all single- rooted, and were replaced
throughout life in alternate waves. Dentary still larger, with the little
quadrate and articular bones were loosely attached. The stapes now touched the
inner side of the quadrate. First sign of the mammalian jaw hinge, a
ligamentous connection between the lower jaw and the squamosal bone of the
skull. The occipital condyle is now two slightly separated surfaces, though
not separated as far as the mammalian double condyles. Vertebral connections
more mammalian, and lumbar ribs reduced. Scapula shows development of a new
mammalian shoulder muscle. Ilium increased again, and all four legs fully
upright, not sprawling. Tail short, as is necessary for agile quadrupedal
locomotion. The whole locomotion was more agile. Number of toe bones is
2.3.4.4.3, intermediate between reptile number (2.3.4.5.4) and mammalian
(2.3.3.3.3), and the "extra" toe bones were tiny. Nearly complete skeletons of
these animals have been found curled up - a possible reaction to conserve
heat, indicating possible endothermy? Adults and juveniles have been found
together, possibly a sign of parental care. The specialization of the lumbar
area (e.g. reduction of ribs) is indicative of the presence of a diaphragm,
needed for higher O2 intake and homeothermy. NOTE on hearing: The eardrum had
developed in the only place available for it -- the lower jaw, right
near the jaw hinge, supported by a wide prong (reflected lamina) of the
angular bone. These animals could now hear airborne sound, transmitted through
the eardrum to two small lower jaw bones, the articular and the quadrate,
which contacted the stapes in the skull, which contacted the cochlea. Rather a
roundabout system and sensitive to low-frequency sound only, but better than
no eardrum at all! Cynodonts developed quite loose quadrates and articulars
that could vibrate freely for sound transmittal while still functioning as a
jaw joint, strengthened by the mammalian jaw joint right next to it. All early
mammals from the Lower Jurassic have this low-frequency ear and a double jaw
joint. By the middle Jurassic, mammals lost the reptilian joint (though it
still occurs briefly in embryos) and the two bones moved into the nearby
middle ear, became smaller, and became much more sensitive to high-frequency
sounds.
- Cynognathus (early Triassic, 240 Ma; suspected to have existed even
earlier) -- We're now at advanced cynodont level. Temporal fenestra larger.
Teeth differentiating further; cheek teeth with cusps met in true occlusion
for slicing up food, rate of replacement reduced, with mammalian-style tooth
roots (though single roots). Dentary still larger, forming 90% of the
muscle-bearing part of the lower jaw. TWO JAW JOINTS in place, mammalian and
reptilian: A new bony jaw joint existed between the squamosal (skull) and the
surangular bone (lower jaw), while the other jaw joint bones were reduced to a
compound rod lying in a trough in the dentary, close to the middle ear. Ribs
more mammalian. Scapula halfway to the mammalian condition. Limbs were held
under body. There is possible evidence for fur in fossil pawprints.
- Diademodon (early Triassic, 240 Ma; same strata as
Cynognathus) -- Temporal fenestra larger still, for still stronger jaw
muscles. True bony secondary palate formed exactly as in mammals, but didn't
extend quite as far back. Turbinate bones possibly present in the nose
(warm-blooded?). Dental changes continue: rate of tooth replacement had
decreased, cheek teeth have better cusps & consistent wear facets (better
occlusion). Lower jaw almost entirely dentary, with tiny articular at the
hinge. Still a double jaw joint. Ribs shorten suddenly in lumbar region,
probably improving diaphragm function & locomotion. Mammalian toe bones
(2.3.3.3.3), with closely related species still showing variable numbers.
- Probelesodon (mid-Triassic; South America) -- Fenestra very large,
still separate from eyesocket (with postorbital bar). Secondary palate longer,
but still not complete. Teeth double-rooted, as in mammals. Nares separated.
Second jaw joint stronger. Lumbar ribs totally lost; thoracic ribs more
mammalian, vertebral connections very mammalian. Hip & femur more
mammalian.
- Probainognathus (mid-Triassic, 239-235 Ma, Argentina) -- Larger
brain with various skull changes: pineal foramen ("third eye") closes, fusion
of some skull plates. Cheekbone slender, low down on the side of the eye
socket. Postorbital bar still there. Additional cusps on cheek teeth. Still
two jaw joints. Still had cervical ribs & lumbar ribs, but they were very
short. Reptilian "costal plates" on thoracic ribs mostly lost. Mammalian #toe
bones.
- Exaeretodon (mid-late Triassic, 239Ma, South America) -- (Formerly
lumped with the herbivorous gomphodont cynodonts.) Mammalian jaw prong forms,
related to eardrum support. Three incisors only (mammalian). Costal plates
completely lost. More mammalian hip related to having limbs under the body.
Possibly the first steps toward coupling of locomotion & breathing. This
is probably a "cousin" fossil not directly ancestral, as it has several new
but non-mammalian teeth traits.
GAP of about 30 my in the late Triassic, from about 239-208 Ma. Only one
early mammal fossil is known from this time. The next time fossils are found in
any abundance, tritylodontids and trithelodontids had already appeared, leading
to some very heated controversy about their relative placement in the chain to
mammals. Recent discoveries seem to show trithelodontids to be more mammal-
like, with tritylodontids possibly being an offshoot group (see Hopson 1991,
Rowe 1988, Wible 1991, and Shubin et al. 1991). Bear in mind that both these
groups were almost fully mammalian in every feature, lacking only the final
changes in the jaw joint and middle ear.
- Oligokyphus, Kayentatherium (early Jurassic, 208 Ma) --
These are tritylodontids, an advanced cynodont group. Face more mammalian,
with changes around eyesocket and cheekbone. Full bony secondary palate.
Alternate tooth replacement with double-rooted cheek teeth, but without
mammalian-style tooth occlusion (which some earlier cynodonts already had).
Skeleton strikingly like egg- laying mammals (monotremes). Double jaw joint.
More flexible neck, with mammalian atlas & axis and double occipital
condyle. Tail vertebrae simpler, like mammals. Scapula is now substantially
mammalian, and the forelimb is carried directly under the body. Various
changes in the pelvis bones and hind limb muscles; this animal's limb
musculature and locomotion were virtually fully mammalian. Probably cousin
fossils (?), with Oligokyphus being more primitive than
Kayentatherium. Thought to have diverged from the trithelodontids
during that gap in the late Triassic. There is disagreement about whether the
tritylodontids were ancestral to mammals (presumably during the late Triassic
gap) or whether they are a specialized offshoot group not directly ancestral
to mammals.
- Pachygenelus, Diarthrognathus (earliest Jurassic, 209 Ma) --
These are trithelodontids, a slightly different advanced cynodont group. New
discoveries (Shubin et al., 1991) show that these animals are very close to
the ancestry of mammals. Inflation of nasal cavity, establishment of
Eustachian tubes between ear and pharynx, loss of postorbital bar. Alternate
replacement of mostly single- rooted teeth. This group also began to develop
double tooth roots -- in Pachygenelus the single root of the cheek
teeth begins to split in two at the base. Pachygenelus also has
mammalian tooth enamel, and mammalian tooth occlusion. Double jaw joint, with
the second joint now a dentary-squamosal (instead of surangular), fully
mammalian. Incipient dentary condyle. Reptilian jaw joint still present but
functioning almost entirely in hearing; postdentary bones further reduced to
tiny rod of bones in jaw near middle ear; probably could hear high frequencies
now. More mammalian neck vertebrae for a flexible neck. Hip more mammalian,
with a very mammalian iliac blade & femur. Highly mobile, mammalian-style
shoulder. Probably had coupled locomotion & breathing. These are probably
"cousin" fossils, not directly ancestral (the true ancestor is thought to have
occurred during that late Triassic gap). Pachygenelus is pretty close,
though.
- Adelobasileus cromptoni (late Triassic; 225 Ma, west Texas) -- A
recently discovered fossil proto-mammal from right in the middle of that late
Triassic gap! Currently the oldest known "mammal." Only the skull was found.
"Some cranial features of Adelobasileus, such as the incipient
promontorium housing the cochlea, represent an intermediate stage of the
character transformation from non-mammalian cynodonts to Liassic mammals"
(Lucas & Luo, 1993). This fossil was found from a band of strata in the
western U.S. that had not previously been studied for early mammals. Also note
that this fossil dates from slightly before the known tritylodonts and
trithelodonts, though it has long been suspected that tritilodonts and
trithelodonts were already around by then. Adelobasileus is thought to
have split off from either a trityl. or a trithel., and is either identical to
or closely related to the common ancestor of all mammals.
- Sinoconodon (early Jurassic, 208 Ma) -- The next known very ancient
proto-mammal. Eyesocket fully mammalian now (closed medial wall). Hindbrain
expanded. Permanent cheekteeth, like mammals, but the other teeth were still
replaced several times. Mammalian jaw joint stronger, with large dentary
condyle fitting into a distinct fossa on the squamosal. This final refinement
of the joint automatically makes this animal a true "mammal". Reptilian jaw
joint still present, though tiny.
- Kuehneotherium (early Jurassic, about 205 Ma) -- A slightly later
proto-mammal, sometimes considered the first known pantothere (primitive
placental-type mammal). Teeth and skull like a placental mammal. The three
major cusps on the upper & lower molars were rotated to form interlocking
shearing triangles as in the more advanced placental mammals & marsupials.
Still has a double jaw joint, though.
- Eozostrodon, Morganucodon, Haldanodon (early
Jurassic, ~205 Ma) -- A group of early proto-mammals called "morganucodonts".
The restructuring of the secondary palate and the floor of the braincase had
continued, and was now very mammalian. Truly mammalian teeth: the cheek teeth
were finally differentiated into simple premolars and more complex molars, and
teeth were replaced only once. Triangular- cusped molars. Reversal of the
previous trend toward reduced incisors, with lower incisors increasing to
four. Tiny remnant of the reptilian jaw joint. Once thought to be ancestral to
monotremes only, but now thought to be ancestral to all three groups of modern
mammals -- monotremes, marsupials, and placentals.
- Peramus (late Jurassic, about 155 Ma) -- A "eupantothere" (more
advanced placental-type mammal). The closest known relative of the placentals
& marsupials. Triconodont molar has with more defined cusps. This fossil
is known only from teeth, but judging from closely related eupantotheres (e.g.
Amphitherium) it had finally lost the reptilian jaw joint, attaing a
fully mammalian three-boned middle ear with excellent high-frequency hearing.
Has only 8 cheek teeth, less than other eupantotheres and close to the 7 of
the first placental mammals. Also has a large talonid on its "tribosphenic"
molars, almost as large as that of the first placentals -- the first
development of grinding capability.
- Endotherium (very latest Jurassic, 147 Ma) -- An advanced
eupantothere. Fully tribosphenic molars with a well- developed talonid. Known
only from one specimen. From Asia; recent fossil finds in Asia suggest that
the tribosphenic molar evolved there.
- Kielantherium and Aegialodon (early Cretaceous) -- More
advanced eupantotheres known only from teeth. Kielantherium is from
Asia and is known from slightly older strata than the European
Aegialodon. Both have the talonid on the lower molars. The wear on it
indicates that a major new cusp, the protocone, had evolved on the upper
molars. By the Middle Cretaceous, animals with the new tribosphenic molar had
spread into North America too (North America was still connected to Europe.)
- Steropodon galmani (early Cretaceous) -- The first known definite
monotreme, discovered in 1985.
- Vincelestes neuquenianus (early Cretaceous, 135 Ma) -- A
probably-placental mammal with some marsupial traits, known from some nice
skulls. Placental-type braincase and coiled cochlea. Its intracranial arteries
& veins ran in a composite monotreme/placental pattern derived from
homologous extracranial vessels in the cynodonts. (Rougier et al., 1992)
- Pariadens kirklandi (late Cretaceous, about 95 Ma) -- The first
definite marsupial. Known only from teeth.
- Kennalestes and Asioryctes (late Cretaceous, Mongolia) --
Small, slender animals; eyesocket open behind; simple ring to support eardrum;
primitive placental-type brain with large olfactory bulbs; basic primitive
tribosphenic tooth pattern. Canine now double rooted. Still just a trace of a
non-dentary bone, the coronoid, on the otherwise all-dentary jaw. "Could have
given rise to nearly all subsequent placentals." says Carroll (1988).
- Cimolestes, Procerberus, Gypsonictops (very late
Cretaceous) -- Primitive North American placentals with same basic tooth
pattern.
So, by the late Cretaceous the three groups of modern mammals were in place:
monotremes, marsupials, and placentals. Placentals appear to have arisen in East
Asia and spread to the Americas by the end of the Cretaceous. In the latest
Cretaceous, placentals and marsupials had started to diversify a bit, and after
the dinosaurs died out, in the Paleocene, this diversification accelerated. For
instance, in the mid- Paleocene the placental fossils include a very primitive
primate-like animal (Purgatorius - known only from a tooth, though, and
may actually be an early ungulate), a herbivore-like jaw with molars that have
flatter tops for better grinding (Protungulatum, probably an early
ungulate), and an insectivore (Paranyctoides).
The decision as to which was the first mammal is somewhat subjective. We are
placing an inflexible classification system on a gradational series. What
happened was that an intermediate group evolved from the 'true' reptiles, which
gradually acquired mammalian characters until a point was reached where we have
artificially drawn a line between reptiles and mammals. For instance,
Pachygenulus and Kayentatherium are both far more mammal-like than
reptile-like, but they are both called "reptiles".
Transition from diapsid reptiles to birds
In the mid-1800's, this was one of the most significant gaps in vertebrate
fossil evolution. No transitional fossils at all were known, and the two groups
seemed impossibly different. Then the exciting discovery of Archeopteryx
in 1861 showed clearly that the two groups were in fact related. Since then,
some other reptile-bird links have been found. On the whole, though, this is
still a gappy transition, consisting of a very large-scale series of "cousin"
fossils. I have not included Mononychus (as it appears to be a digger,
not a flier, well off the line to modern birds). See Feduccia (1980) and Rayner
(1989) for more discussion of the evolution of flight, and Chris Nedin's
excellent Archeopteryx FAQ for more info on that critter.
- Coelophysis (late Triassic) -- One of the first theropod dinosaurs.
Theropods in general show clear general skeletal affinities with birds (long
limbs, hollow bones, foot with 3 toes in front and 1 reversed toe behind, long
ilium). Jurassic theropods like Compsognathus are particularly similar
to birds.
- Deinonychus, Oviraptor, and other advanced theropods (late
Jurassic, Cretaceous) -- Predatory bipedal advanced theropods, larger, with
more bird-like skeletal features: semilunate carpal, bony sternum, long arms,
reversed pubis. Clearly runners, though, not fliers. These advanced theropods
even had clavicles, sometimes fused as in birds. Says Clark (1992): "The
detailed similarity between birds and theropod dinosaurs such as
Deinonychus is so striking and so pervasive throughout the skeleton
that a considerable amount of special pleading is needed to come to any
conclusion other than that the sister-group of birds among fossils is one of
several theropod dinosaurs." The particular fossils listed here are are
not directly ancestral, though, as they occur after
Archeopteryx.
- Lisboasaurus estesi & other "troodontid dinosaur-birds"
(mid-Jurassic) -- A bird-like theropod reptile with very bird-like teeth (that
is, teeth very like those of early toothed birds, since modern birds have no
teeth). These really could be ancestral.
GAP: The exact reptilian ancestor of Archeopteryx, and the first
development of feathers, are unknown. Early bird evolution seems to have
involved little forest climbers and then little forest fliers, both of which are
guaranteed to leave very bad fossil records (little animal + acidic forest soil
= no remains). Archeopteryx itself is really about the best we could ask
for: several specimens has superb feather impressions, it is clearly
related to both reptiles and birds, and it clearly shows that the
transition is feasible.
- One possible ancestor of Archeopteryx is Protoavis
(Triassic, ~225 Ma) -- A highly controversial fossil that may or may not be an
extremely early bird. Unfortunately, not enough of the fossil was recovered to
determine if it is definitely related to the birds.
- Archeopteryx lithographica (Late Jurassic, 150 Ma) -- The several
known specimes of this deservedly famous fossil show a mosaic of reptilian and
avian features, with the reptilian features predominating. The skull and
skeleton are basically reptilian (skull, teeth, vertebrae, sternum, ribs,
pelvis, tail, digits, claws, generally unfused bones). Bird traits are limited
to an avian furcula (wishbone, for attachment of flight muscles; recall that
at least some dinosaurs had this too), modified forelimbs, and -- the real
kicker -- unmistakable lift-producing flight feathers. Archeopteryx
could probably flap from tree to tree, but couldn't take off from the ground,
since it lacked a keeled breastbone for large flight muscles, and had a weak
shoulder compared to modern birds. May not have been the direct ancestor of
modern birds. (Wellnhofer, 1993)
- Sinornis santensis ("Chinese bird", early Cretaceous, 138 Ma) -- A
recently found little primitive bird. Bird traits: short trunk, claws on the
toes, flight-specialized shoulders, stronger flight- feather bones, tightly
folding wrist, short hand. (These traits make it a much better flier than
Archeopteryx.) Reptilian traits: teeth, stomach ribs, unfused hand
bones, reptilian-shaped unfused pelvis. (These remaining reptilian traits
wouldn't have interfered with flight.) Intermediate traits: metatarsals
partially fused, medium-sized sternal keel, medium-length tail (8 vertebrae)
with fused pygostyle at the tip. (Sereno & Rao, 1992).
- "Las Hoyas bird" or "Spanish bird" [not yet named; early Cretaceous, 131
Ma) -- Another recently found "little forest flier". It still has reptilian
pelvis & legs, with bird-like shoulder. Tail is medium-length with a fused
tip. A fossil down feather was found with the Las Hoyas bird, indicating
homeothermy. (Sanz et al., 1992)
- Ambiortus dementjevi (early Cretaceous, 125 Ma) -- The third known
"little forest flier", found in 1985. Very fragmentary fossil.
- Hesperornis, Ichthyornis, and other Cretaceous diving birds
-- This line of birds became specialized for diving, like modern cormorants.
As they lived along saltwater coasts, there are many fossils known. Skeleton
further modified for flight (fusion of pelvis bones, fusion of hand bones,
short & fused tail). Still had true socketed teeth, a reptilian trait.
[Note: a classic study of chicken embryos showed that chicken bills can be
induced to develop teeth, indicating that chickens (and perhaps other modern
birds) still retain the genes for making teeth. Also note that molecular data
shows that crocodiles are birds' closest living relatives.]
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