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Mesozoic turtle papers: Proterochersis + Bauruemys



Ben Creisler
bcreisler@gmail.com

Two new papers in PeerJ preprints:

Tomasz Szczygielski (2015)
New data on the oldest turtles: revision and reconsideration of
Proterochersidae.
PeerJ PrePrints 3:e1059
doi: http://dx.doi.org/10.7287/peerj.preprints.858v1
https://peerj.com/preprints/858/

Background. The origin of turtles, their earliest evolution and the
homologies of the elements building their shell are still enigmatic
and remain an object of ongoing discussion and research. Although the
oldest fully shelled turtle – Proterochersis robusta from the Lower
Stubensandstein (Norian) of Germany – was described more than a
century ago, until recently it was mostly ignored by researchers. This
is surprising, not only because of its notable stratigraphic position,
but also due to the critical significance of this taxon in two
competing hypotheses of turtle interrelationships. The divergence time
of two main branches of Testudines crown group depends on whether
Proterochersis is a basal pleurodire or a stem turtle. Methods. A
detailed study of the German material of Proterochersis and
Murrhardtia was performed by the author and the available specimens
were compared with still growing collection of proterochersid remains
from the Norian location in Poręba (Poland). Results. Two
controversial taxa from Germany, Proterochersis intermedia and
Murrhardtia staeschei are proved to be the synonyms of P. robusta.
Establishment of another two proterochersid taxa is proposed and new
primitive postcranial characters are recognized, supporting the stem
position of Proterochersidae on the turtle phylogenetic tree.
Discussion. Two general hypotheses concerning the split time of
Cryptodira and Pleurodira are functioning in the literature, and
Proterochersis plays a crucial role in these considerations. According
to traditional view, this taxon is the oldest side-necked turtle,
based on the sutural connection between its pelvis and shell. Numerous
recent analyses tended to allocate the Pleurodira much higher on the
tree, resulting in stem position of Proterochersis, though such
placement usually lacked a strong support. This was caused by an
incompletness of Proterochersis material (only shells) and errors in
interpretation of some characters. New data, gathered mostly from the
Polish – much more complete and well preserved – specimens, supports
the basal position of Proterochersidae and provides new insights in
the ancestral structure of the turtle shell.


==


Thiago F. Mariani & Pedro S. R. Romano (2015)
Skull morphometrics of the Late Cretaceous side-necked turtle
Bauruemys elegans (Pleurodira, Podocnemididae) from Presidente
Prudente Formation, São Paulo, Brazil.
PeerJ PrePrints 3:e1060
doi:  http://dx.doi.org/10.7287/peerj.preprints.859v1
https://peerj.com/preprints/859/

Background. Previous quantitative studies about Bauruemys elegans
(Suárez, 1969) shell variation, as well as the taphonomy
interpretation of its type locality, have suggested that all specimens
collected in this locality may have belonged to the same population.
We rely on this hypothesis in a morphometric study of the skull. Also,
we tried to assess the eating preference habits differentiation that
might be explained as due to ontogenetic changes.Methods. We took 29
linear measurements from 21 skulls of B. elegans. First, a Principal
Components Analysis (PCA) was performed using 27 measurements
(excluding total length and width characters) in order to plot the
overall individual dispersion in PCs to visualize patterns of
scattering based on the form variance. Secondly, PCA was carried out
using ratios of length and width of each original measurement to
assess shape variation among individuals. Finally, original
measurements were log-transformed to describe allometries along the
ontogenetic development.Results. The first three PCs of the first
analysis comprising 70.2% of the variance. PC1 was related to size
variation and all others related to shape variation. MCZ4123 and
MN7071-V have been plotted outside the 95% ellipse in PC1xPC2 axes.
The first three PCs of the second analysis comprising 64% of the
variance. When considering PC1xPC2 and PC2xPC3, all specimens have
been plotted inside the 95% ellipse, which is in contrast to PC1xPC3
in which two individuals (MCT1753- R and MN6750-V) are outliers. In
the third analysis, five measurements were positively allometric, 18
were negatively allometric and four represent truly negatively
allometry. All bones of the posterior and the lateral emarginations,
as well as the squamosal, lengthen due to size increasing, different
from the jugal and the quadratojugal which decrease in
width.Discussion. Some specimens show small differences in form
(MCZ4123 and MN7071-V) and shape (MCT1753-R and MN6750-V). Form
differences were already detected in a shell morphometry study, but
interpreted as due to ontogeny, which might be the case of the present
data. Moreover, all outlier specimens are crushed and/or distorted,
thus the form/shape differences might be partially due to taphonomy.
The allometric lengthen of parietal, quadrate, squamosal, maxilla,
associated with the narrowing of jugal and quadratojugal may be
related to changes in feeding habit between different stages of
development. This change in shape might represent a progressive skull
stretching and enlargement of posterior and lateral emargination
during ontogeny, and consequently, the increment of the
feeding-apparatus musculature. Smaller individuals may have fed of
softer diet whereas bigger ones probably have had a harder diet, as
seen in some living species of Podocnemis. We conclude that the skull
variation is higher than expected and might be related to differences
in feeding habits along the ontogeny of B. elegans.