[dinosaur] Plesiadapis (Paleocene euarchontan) skeleton (free pdf)

Ben Creisler
[email protected]

A new non-dino monograph on an early Cenozoic mammal that may of interest
to some. The pdf is free.

Free pdf:


Douglas M. Boyer & Philip D. Gingerich (2019)
Skeleton of Late Paleocene Plesiadapis Cookei (Mammalia, Euarchonta): Life
History, Locomotion, and Phylogenetic Relationships.
University of Michigan Papers on Paleontology 38: 268 pp.
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Plesiadapis cookei is a large-bodied plesiadapiform euarchontan (and
potential stem primate) known from many localities of middle Clarkforkian
North American Land Mammal age, late Paleocene epoch, in the Clarks Fork
Basin of northwestern Wyoming. Most specimens are gnathic and dental
remains, but one specimen, University of Michigan Museum of Paleontology
(UM) 87990, is a relatively complete skeleton preserving the skull, much of
the axial skeleton, forelimbs, and hind limbs. Description, measurement,
and illustration of the UM 87990 skeleton of P. cookei provides new
information enabling analysis of proportions within the skeleton and
comparisons across a range of primates and related mammals.

The skull of UM 87990 includes much of the upper and lower dentition,
rostrum, braincase, and basicranium. The dental formula is 2.0.2.3 /
1.0.2.3. Composition of the auditory bulla as petrosal, ectotympanic, or
entotympanic, is uncertain due breakage and some suture-like structures
rimming the promontorium. P. cookei had a very small posterior carotid
foramen and probably lacked a functioning internal carotid artery.

The axial skeleton of UM 87990 is represented by five cervical, 12
thoracic, and six lumbar vertebrae. The sacrum is complete with three
vertebrae. There are 17 caudal vertebrae preserved, with at least three
anterior positions missing. Thus there were at least 20 caudal vertebrae.
Caudal vertebral proportions indicate the tail of P. cookei was relatively
long. We expect the full plesiadapid vertebral formula to have been
7.13.6.3.24, matching the formula in Tupaia and the median formula in
primates. Seven sternebrae are preserved, possibly representing the
complete series. Ribs are slender and lack any notable anteroposterior
expansion.

The pectoral girdle and forelimb of UM 87990 include clavicles, much of one
scapula, humeri, ulnae, radii, and a number of carpal bones. Two sets of
metacarpals, one set slightly longer than the other, were found in
association with UM 87990. One set must have come from a partial skeleton
of Uintacyon (UM 88187) that was preserved next to UM 87990. Manual
proximal phalanges have long, low flexor sheath ridges. Intermediate
phalanges have narrow, deep shafts, and distal articular surfaces
suggesting flexed distal interphalangeal joints. Distal phalanges are all
relatively long and distinctly falciform.

The pelvic girdle and hind limb of UM 87990 include well-preserved
innominates, femora, tibiae, incomplete fibulae, and most tarsal bones
(navicular and entocuneiform excepted). Metatarsals are identified based on
their size and form in comparison to metatarsals described for other
plesiadapids. Pedal phalanges are similar to manual phalanges but can be
distinguished on the basis of size and a more neutrally flexed distal
interphalangeal joint.

A principal components analysis of body proportions places P. cookei and
other plesiadapiforms close to Tupaia, sciurids, and callitrichid primates.
P. cookei is not particularly close to the lorisine slow climber
Nycticebus, nor is it close to the colugo Cynocephalus. The postcranial
skeleton of P. cookei suggests that it was a forest-dwelling arboreal
climber primarily adapted, and possibly constrained, to large diameter
vertical and horizontal supports. Headfirst descent of large supports was
likely accomplished using claw-clinging with a reversed (supinated) foot.
P. cookei would have been more cautious and less scansorial in its
movements than smaller-bodied plesiadapids.

The body weight of P. cookei is predicted to be 1,799 to 2,052 g, based on
long bone lengths and diameters of UM 87990. Brain weight is predicted to
be close to 5 g based on a partial endocast and three-dimensional model.
Comparison of observed with expected brain size for a mammal the size of P.
cookei yields an encephalization quotient in the range of 0.257 to 0.281
and an encephalization residual in the range of −1.958 to −1.831. Both
indicate that the brain of P. cookei was just over one-quarter of the size
expected for an average living mammal of its body weight. A smaller than
expected brain is a common feature of many Paleogene mammals.

We interpret UM 87990 to be a male individual of P. cookei, because the
innominate has a relatively wide superior pubic ramus like that of male
individuals in a range of rodents and primate species. UM 87990 has a fully
erupted adult dentition, little tooth wear, and long-bone epiphyseal fusion
limited to the elbow. We have no independent means of determining sexual
maturity, and thus can only interpret UM 87990 as an advanced and nearly
full-grown subadult, or as a full-grown young adult.

Cladistic analysis of craniodental characters and a geographic character
indicates that Plesiadapidae is monophyletic, with the carpolestid
Elphidotarsius forming the sister taxon of the group. Pronothodectes is
paraphyletic but its species are the most basal within Plesiadapidae.
Chiromyoides, and Platychoerops are monophyletic. Plesiadapis itself is
polyphyletic, with P. cookei recovered as the sister taxon of
Platychoerops. Our reconstructed phylogeny for the family is largely
congruent with that of Gingerich (1976) based on stratophenetic
methodology, but less resolved. A stratocladistic analysis run by adding a
stratigraphic character to the craniodental and geographic character set is
more resolved than our strict consensus using morphology and geography
alone, and it is also more divergent from Gingerich’s (1976) hypothesis in
certain ways. Some notable results include recovery of Nannodectes
intermedius as the common ancestor of all non-Pronothodectes plesiadapids
and recovery of Platychoerops antiquus as ancestral to all later
Platychoerops species.

On a broader scale, cladistic analysis of higher-level taxa, with scorings
for Plesiadapidae revised based on optimization of the ancestral node at
the species level, indicates that plesiadapids and carpolestids exhibit a
greater number of identical character states than previously thought,
though some previously proposed synapomorphies were refuted. Even so,
analysis of combined data from dentition, cranium, and postcrania still
robustly support a link between plesiadapids, saxonellids, and carpolestids
(Plesiadapoidea) and does not contradict previous hypotheses suggesting a
special relationship of plesiadapoids to euprimates (Euprimateformes).


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