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Eunotosaurus skull + amphisbaenian lizard origin + sawfish "teeth"

Ben Creisler

A number of recent papers that may be of interest:

G. S. Bever, Tyler R. Lyson, Daniel J. Field & Bhart-Anjan S. Bhullar (2015)
Evolutionary origin of the turtle skull.
Nature (advance online publication)

Transitional fossils informing the origin of turtles are among the
most sought-after discoveries in palaeontology. Despite strong genomic
evidence indicating that turtles evolved from within the diapsid
radiation (which includes all other living reptiles), evidence of the
inferred transformation between an ancestral turtle with an open,
diapsid skull to the closed, anapsid condition of modern turtles
remains elusive. Here we use high-resolution computed tomography and a
novel character/taxon matrix to study the skull of Eunotosaurus
africanus, a 260-million-year-old fossil reptile from the Karoo Basin
of South Africa, whose distinctive postcranial skeleton shares many
unique features with the shelled body plan of turtles. Scepticism
regarding the status of Eunotosaurus as the earliest stem turtle
arises from the possibility that these shell-related features are the
products of evolutionary convergence. Our phylogenetic analyses
indicate strong cranial support for Eunotosaurus as a critical
transitional form in turtle evolution, thus fortifying a
40-million-year extension to the turtle stem and moving the ecological
context of its origin back onto land. Furthermore, we find unexpected
evidence that Eunotosaurus is a diapsid reptile in the process of
becoming secondarily anapsid. This is important because categorizing
the skull based on the number of openings in the complex of dermal
bone covering the adductor chamber has long held sway in amniote
systematics, and still represents a common organizational scheme for
teaching the evolutionary history of the group. These discoveries
allow us to articulate a detailed and testable hypothesis of fenestral
closure along the turtle stem. Our results suggest that Eunotosaurus
represents a crucially important link in a chain that will eventually
lead to consilience in reptile systematics, paving the way for
synthetic studies of amniote evolution and development.






Mateusz Tałanda (2015)
Cretaceous roots of the amphisbaenian lizards.
ZOOLOGICA SCRIPTA (advance online publication)
DOI: 10.1111/zsc.12138

Amphisbaenians are highly specialized limbless burrowing lizards of
controversial relationships. Among fossil lizards, the Eocene (47 Ma)
Cryptolacerta is allegedly closest to the amphisbaenian ancestor, but
this is put in doubt in this study. Similarities between Cryptolacerta
and amphisbaenians, such as limb reduction and expansion of the skull
roof, may be a result of parallel evolution. Instead, the Late
Cretaceous lizard Slavoia with well-developed limbs and several
plesiomorphic skull characters is proposed to be the oldest known stem
amphisbaenian. This is supported by two different phylogenetic
analyses and observations on numerous specimens together representing
almost the whole skeleton. Among the unique features, that Slavoia
shares with amphisbaenians, the most significant are vomers strongly
underlapping palatines and pterygoid quadrate ramus tightly wrapping
around posteromedial surface of quadrate. The anatomy of Slavoia
suggests that the reinforcement of the snout in amphisbaenian
evolution preceded the elongation of the postorbital part of the
skull, and that of the body, as well as modification of the limbs.
Reduction of its hindlimbs was more advanced than that of the
forelimbs. The ancient geological age of the central Asiatic Slavoia
suggests that diversification of the main North American amphisbaenian
groups may have resulted from a faunal dispersals from Asia after the
Late Cretaceous.


Free pdf:

Monique Welten, Moya Meredith Smith, Charlie Underwood & Zerina Johanson (2015)
Evolutionary origins and development of saw-teeth on the sawfish and
sawshark rostrum (Elasmobranchii; Chondrichthyes).
Royal Society Open Science 2015 2 150189
DOI: 10.1098/rsos.150189

A well-known characteristic of chondrichthyans (e.g. sharks, rays) is
their covering of external skin denticles (placoid scales), but less
well understood is the wide morphological diversity that these skin
denticles can show. Some of the more unusual of these are the
tooth-like structures associated with the elongate cartilaginous
rostrum ‘saw’ in three chondrichthyan groups: Pristiophoridae
(sawsharks; Selachii), Pristidae (sawfish; Batoidea) and the fossil
Sclerorhynchoidea (Batoidea). Comparative topographic and
developmental studies of the ‘saw-teeth’ were undertaken in adults and
embryos of these groups, by means of three-dimensional-rendered
volumes from X-ray computed tomography. This provided data on
development and relative arrangement in embryos, with regenerative
replacement in adults. Saw-teeth are morphologically similar on the
rostra of the Pristiophoridae and the Sclerorhynchoidea, with the same
replacement modes, despite the lack of a close phylogenetic
relationship. In both, tooth-like structures develop under the skin of
the embryos, aligned with the rostrum surface, before rotating into
lateral position and then attaching through a pedicel to the rostrum
cartilage. As well, saw-teeth are replaced and added to as space
becomes available. By contrast, saw-teeth in Pristidae insert into
sockets in the rostrum cartilage, growing continuously and are not
replaced. Despite superficial similarity to oral tooth developmental
organization, saw-tooth spatial initiation arrangement is associated
with rostrum growth. Replacement is space-dependent and more
comparable to that of dermal skin denticles. We suggest these
saw-teeth represent modified dermal denticles and lack the
‘many-for-one’ replacement characteristic of elasmobranch oral