New synapsid papers

[Ben Creisler <bcreisler@gmail.com>]

From: Ben Creisler
bcreisler@gmail.com

New papers about synapsids in Fieldiana Life and Earth Sciences:


Elizabeth A. Rega, Ken Noriega, Stuart S. Sumida, Adam Huttenlocker,
Andrew Lee, and Brett Kennedy (2012)
Healed Fractures in the Neural Spines of an Associated Skeleton of
Dimetrodon: Implications for Dorsal Sail Morphology and Function.
Fieldiana Life and Earth Sciences Number 5 :104-111
doi: http://dx.doi.org/10.3158/2158-5520-5.1.104
http://www.bioone.org/doi/abs/10.3158/2158-5520-5.1.104

Hyperelongate neural spines forming a prominent dorsal “sail” are
known in eight genera distributed between two families of
pelycosaurian-grade synapsids. Although the function(s) of the sail
remain disputed, most researchers assume that resilient soft tissue
stretched between the elongate neural spines, extending to the distal
tips. Hypotheses to explain the purpose of the sail have included
thermoregulation (Romer & Price, 1940; Bramwell & Fellgett, 1973;
Haack, 1986; Tracy et al., 1986; Bennett, 1996; Florides et al., 1999)
and sexual selection (Tomkins et al., 2010). In this paper, we analyze
the natural pathologies found in the neural spines of a very large
pelycosaur, Dimetrodon giganhomogenes, as a natural experiment: What
would ensue in the event of sail breakage and what does that tell us
about sail structure, development, maintenance, and the orientation of
the sail?

A series of seven associated neural spines from fmnh UC 1134
demonstrate subtle though distinctly abnormal rugosities, a sign most
often indicative of a well-healed hard callus of bone fracture.
Microstructural examination revealed surprising facts: not only did
the abnormal bone areas prove NOT to be fracture hard callus, but the
abnormal tissue reflected underlying material failure resulting from
slippage between adjacent lamellae of bone. Moreover, the
characteristic cranial and caudal orientation of the deep longitudinal
grooves contributing to the classic dimetrodont figure-8 spine cross
section was rapidly reestablished in vivo by a combination of
osteoclastic resorption and additional lamellar deposition of bone to
regain the “correct” pre-injury orientation, underscoring the
architectural importance of the dumbbell shape in resisting lateral
bending. This bone disruption and repair occurred at least five
seasons before death, which explains the well-healed external
appearance of the lesions. The absence of vascular communicating
canals casts doubt on the widely held hypothesis that these grooves
contained blood vessels that supplied a thermoregulatory sail.
Furthermore, the distal morphology of spines in more complete
specimens, including the type fmnh UC 112 and omnh 01727, suggests
that the dorsal margin of the sail was located well proximal to the
tips of the elongate neural spines. The cross-sectional architecture
of the spines suggests a further hypothesis: that the proximal portion
of the sail may have also functioned as an energy storage device,
facilitating fast locomotion in this top predator.


===


Christian F. Kammerer, John J. Flynn, Lovasoa Ranivoharimanana, and
André R. Wyss (2012)
Ontogeny in the Malagasy Traversodontid Dadadon isaloi and a
Reconsideration of its Phylogenetic Relationships.
Fieldiana Life and Earth Sciences Number 5 :112-125
doi: http://dx.doi.org/10.3158/2158-5520-5.1.112
http://www.bioone.org/doi/abs/10.3158/2158-5520-5.1.112


New craniodental material of the traversodontid Dadadon isaloi from
Middle/Upper Triassic basal “Isalo II” beds of southwestern Madagascar
is described. These specimens reveal several new autapomorphies of
Dadadon, including paired foramina on the frontal near the anterior
border of the postorbital and lower incisors with denticulated distal
margins. The new material covers a broad size range, providing the
first information on ontogeny in Dadadon. Larger (presumably older)
specimens of Dadadon isaloi have more postcanine teeth, relatively
longer, narrower snouts, and a higher degree of cranial ornamentation
than smaller specimens. Postcanine replacement in Dadadon was similar
to that of other traversodontids: new teeth erupted at the posterior
end of the postcanine tooth row and moved forward. Using information
from the new specimens, the position of Dadadon was tested in a new
phylogenetic analysis of traversodontids. In the new analysis, Dadadon
is strongly supported as a member of a clade also including the South
American taxa Massetognathus and Santacruzodon, here named
Massetognathinae subfam. nov. This clade is diagnosed by the presence
of denticulated lower incisors, relatively small canines, three cusps
in the labial margin of the upper postcanines, and low, flat skulls.
Massetognathinae is the sister-group of Gomphodontosuchinae, which
includes Gomphodontosuchus, Menadon, Protuberum, Exaeretodon, and
Scalenodontoides. The Laurasian traversodontids (Arctotraversodon,
Boreogomphodon, and Nanogomphodon) form a clade that is the
sister-taxon of Massetognathinae + Gomphodontosuchinae. Denticulated
incisors evolved multiple times in traversodontid evolution (in
massetognathines and Arctotraversodon), and thus this group represents
another possibility (besides various archosauromorphs) to be
considered when attempting to identify isolated Triassic teeth with
denticulated carinae lacking cingula.

==


James A. Hopson (2012)
The Role of Foraging Mode in the Origin of Therapsids: Implications
for the Origin of Mammalian Endothermy.
Fieldiana Life and Earth Sciences Number 5 :126-148
doi: http://dx.doi.org/10.3158/2158-5520-5.1.126
http://www.bioone.org/doi/abs/10.3158/2158-5520-5.1.126

The question of the adaptive basis for the origin of mammalian
endothermy remains unresolved despite a great deal of research effort.
Controversy continues over which physiological adaptations were of
greatest importance in starting ectothermic nonmammalian synapsids of
the Late Paleozoic on the path that culminated in modern endothermic
mammals. Models of the selective basis for the origin of endothermy
fall into two main categories: “thermoregulation first” and “aerobic
capacity first.” Studies of lizards show a dichotomy between a
low-energy “sit-and-wait” (SW) foraging mode in Iguania and a more
energy-intensive “widely foraging” (WF) mode in Autarchoglossa. It is
proposed that in the transition from basal synapsids (“pelycosaurs”)
to therapsids, a shift from the primitive SW mode to the WF mode put
the ancestors of mammals on the path to increased aerobic capacity and
the ability to sustain high levels of foraging activity. Selection for
increased energy expenditure disproportionately increased the amount
of food energy consumed, thus improving foraging efficiency. A shift
from reliance on anaerobic muscle metabolism for short but rapid
dashes to capture prey to a reliance on aerobic metabolism for active
searching for prey necessitated improvements of the cardiovascular
system and lungs for increased aerobic capacity and greater stamina.
Over time, therapsids became locked into high food requirements, which
selected for improvements in aerobic metabolism, locomotor and
food-processing ability, and neurosensory/behavioral specializations.
Evidence of a link between maximum activity metabolism and resting
(basal) metabolism in anurans and rodents suggests that further
increases in aerobic activity metabolism required an increased basal
metabolic rate, which led to high body temperatures and, ultimately,
homeothermy. Therapsids show adaptations for increased activity,
greater food-getting and food-processing ability, and higher metabolic
rates than basal synapsids (“pelycosaurs”). It is argued that the
“foraging mode” model is preferable to the “parental care” model of
Farmer and the “correlated progression” model of Kemp for
understanding the origin of mammalian endothermy.