The postcranial skeleton of the Early Triassic non-mammalian cynodont Galesaurus planiceps: implications for biology and lifestyle
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Date
2009-12
Authors
Butler, Elize
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State
Abstract
Newly discovered skeletons of the Early Triassic basal cynodont,
Galesaurus
planiceps, has enabled a detailed morphological redescription of the postcrania
of this genus. The examination of
Galesaurus
reveals two distinct morphs,
namely a gracile and a robust morph.
The primary differences between each
morph lie in the pectoral and pelvic girdles with further subtle differences in the
fore- and hind limbs. The morphological differences between the two morphs
may be attributed to ontogeny, sexual dimorphism or the presence of two
subspecies.
The morphology and high cortical thickness in the limb bones of
Galesaurus
indicates that it was a more robust animal compared to its closely related sister
taxon
Thrinaxodon liorhinus.
Galesaurus
was thus, capable of being an active
burrower and may have used burrows to escape the harsh environmental
conditions of the Early Triassic.
The bone microstructure of
Galesaurus
reveals uninterrupted fibro-lamellar bone,
indicating fast continuous initial grow
th, with a change to lamellar-zonal bone,
indicating a decrease in growth rate. T
he presence of annuli and LAGs in the
peripheral lamellar-zonal bone indicates
interrupted slow growth and suggests
that
Galesaurus
may have been susceptible to environmental fluctuations. The
growth patterns of
Galesaurus
and
Thrinaxodon
are similar, but can be
distinguished from one another by the presence of lamellar-zonal bone in the
former and parallel-fibred bone in the la
tter genus. Annuli and LAGs are absent
in
Thrinaxodon
, implying that
Thrinaxodon
was less susceptible to environmental
fluctuations than
Galesaurus,
as growth did not decrease or cease periodically.
Abstract
iv
Galesaurus,
with a short biostratigraphic range from the Palingkloof Member,
Balfour Formation and lowermost Katberg Formation of the
Lystrosaurus
Assemblage Zone, was previously known only
from cranial and poorly preserved,
isolated postcranial fragments. In contrast, extensive research has been
conducted on the more abundant better-known
Thrinaxodon,
which has a
biostratigraphic range that extends the entire
Lystrosaurus
Assemblage Zone. It
was previously assumed that the postcr
anial skeletons of basal cynodonts were
indistinguishable. However, this study has revealed morphological differences
between
Galesaurus
and
Thrinaxodon
, allowing the taxa to be distinguished in
the absence of cranial material. Examining postcranial material previously
identified as
Thrinaxodon
and ensuring that collection material has been correctly
identified can now test the short stratigraphic range of
Galesaurus
.
Description
Keywords
Dissertation (M.Sc. (Zoology & Entomology))--University of the Free State, 2009, Paleontology, Triassic, Reptiles, Fossil, Therapsida, Fossils -- Morphology, Galesaurus planiceps, Basal cynodont, Early Triassic, Postcranial skeleton, Bone histology