New Papers: Pachypleurosaurid growth and procolophonid anatomy

[Ben Creisler <bcreisler@gmail.com>]

From: Ben Creisler
bcreisler@gmail.com

A couple of new papers about non-archosauromorph Triassic amniotes:


Jasmina Hugi & Torsten M. Scheyer (2012)
Ossification sequences and associated ontogenetic changes in the bone
histology of pachypleurosaurids from Monte San Giorgio
(Switzerland/Italy).
Journal of Vertebrate Paleontology 32(2): 315-327
DOI:10.1080/02724634.2012.646376
http://www.tandfonline.com/doi/abs/10.1080/02724634.2012.646376


Evolutionary changes in lifestyle (e.g., terrestrial vs. aquatic
habits) influence tetrapod limb morphology. Similar evolutionary
trends in osteogenetic sequences (order of bone ossification) and bone
microstructure are often shared in extant tetrapods with similar habit
preferences: such data are sometimes available from extinct taxa. The
pachypleurosaurids from Monte San Giorgio (Switzerland, Italy) are
Triassic marine reptiles with high quality, complete ontogenetic
series. We studied osteogenesis and bone histology in the four species
from this locality, comparing these with data from Recent terrestrial
lizards and secondarily aquatic reptiles in order to determine if the
osteogenetic sequences of pachypleurosaurids were similar to either
the hypothesized plesiomorphic condition for terrestrial eureptilians
or those of Recent aquatic reptiles. Pachypleurosaurian limb
osteogenesis occurs in two steps: (1) developmental sequences of
ossification during embryology and/or in early neonates, (2a)
additional primary periosteal compaction processes, and (2b)
additional primary and secondary endosteal compaction processes during
neonate ontogeny. Taphonomic patterns reveal information on the order
of the initiation and termination of these steps, which are (A) onset
of ossification, (B) onset of additional compaction processes (early
phase), and (C) termination of additional compaction processes (final
phase). An event pairing analysis found that ossification of forelimb
elements in pachypleurosaurids precedes that of the hind limb elements
in all osteogenetic stages except for initiation of ossification (A).
The order of their early phase compaction processes is similar to the
hypothesized plesiomorphic eureptilian condition, whereas their final
phase compaction processes varies among pachypleurosaurids, with S.
mirigiolensis showing minor heterochronic shifts and N. edwardsii
showing many heterochronic shifts relative to the hypothesized
ancestral condition. Pachypleurosaurids from Monte San Giorgio
increase the number of heterochronic shifts with decreasing
stratigraphic age, showing a transition from more ‘terrestrial’ to
more ‘aquatic’ osteogenetic sequences in comparison to data on
ossification sequences of Recent aquatic reptiles.

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Mark J. MacDougall, Sean P. Modesto, and Jennifer Botha-Brink (2012)
The postcranial skeleton of the Early Triassic parareptile
Sauropareion anoplus, with a discussion of possible life history.
Acta Palaeontologica Polonica (in press)
doi:10.4202/app.2011.0099
http://app.pan.pl/article/item/app20110099.html

The skeletal anatomy of the Early Triassic (Induan) procolophonid
reptile Sauropareion anoplus is described on the basis of three
partial skeletons from Vangfontein, Middelburg District, South Africa.
Together these three specimens preserve the large majority of the
pectoral and pelvic girdles, articulated forelimbs and hindlimbs, and
all but the caudal portion of the vertebral column, elements hitherto
undescribed. Our phylogenetic analysis of the Procolophonoidea is
consonant with previous work, positing S. anoplus as the sister taxon
to a clade composed of all other procolophonids exclusive of Coletta
seca. Previous studies have suggested that procolophonids were
burrowers, and this seems to have been the case for S. anoplus, based
on comparisons with characteristic skeletal anatomy of living digging
animals, such as the presence of a spade-shaped skull, robust
phalanges, and large unguals.