Dallasaurus
Name:
Dallasaurus
(Dallas lizard).
Phonetic: Dal-las-ore-rus.
Named By: G. L. Bell Jr & M. J.
Polcyn - 2005.
Classification: Chordata, Reptilia, Squamata,
Mosasauridae, Mosasaurinae.
Species: D. turneri (type).
Diet: Carnivore.
Size: Up to 1 meter long.
Known locations: USA, Texas, Dallas County -
Kamp Ranch Limestone.
Time period: Turonian of the Cretaceous.
Fossil representation: 2 partial specimens,
together represented by a partial skull and disarticulated post
cranial remains.
Dallasaurus
was a fascinating discovery, especially for lovers of the marine
reptiles that swam the Mesozoic oceans, more particularly the
mosasaurs
of the Cretaceous. The skeletal features of Dallasaurus
have helped to further cement the theory that the mosasaurs evolved
from terrestrial (land living) reptiles like monitor lizards during
the early/mid Cretaceous. A counter theory to this is that the
mosasaurs evolved from primitive snakes, but this idea has gradually
waned in acceptance with the advent of new fossil discoveries for both
early mosasaurs and Cretaceous snakes. Also of interest is that while
other well-known groups of marine reptiles such as the ichthyosaurs,
plesiosaurs
and pliosaurs
are thought to have also evolved from land
reptiles of the diapsid line, the mosasaurs appeared and adapted
completely independently of them.
Dallasaurus
is seen as a transitory link because while its body and tail show
adaptation for swimming, the legs still have a greater development
for walking on land. This would suggest that Dallasaurus
lived in
coastal waters, perhaps estuarine waters (where rivers meet
oceans) where they could take advantage of an abundant supply of
aquatic prey but still had to return to the land, either to rest or
possibly to hunt in different waters such as coastal lagoons. Roaming
behaviour like this would lead to an oceanic expansion so that
Dallasaurus could have access to an even greater
number of prey. In
later mosasaurs the legs would develop fully into flippers to support a
completely aquatic life.
It
is still too early to say if Dallasaurus laid eggs
or gave birth to
live young as the fossils which indicate either method are still
lacking. However later mosasaurs were too large to conceivably return
to the land, and the reptiles as a group seem remarkably quick to
adapt to live birth particularly in the other lines of marine
reptiles, with evidence suggesting this being found in ichthyosaurs,
but perhaps a more relevant example being that of Keichousaurus.
Beyond this live birth can be seen in some lizards that are alive
today, as well as other reptile groups like the sea snakes.
Reptiles
like Dallasaurus and the another early mosasaur
named Russelosaurus
would go onto to have an impressive lineage of descendants, from the
giant Tylosaurus
that was over fifteen meters long, to the advanced
Platecarpus
which had a tail fluke similar to an ichthyosaur, to
specialist feeders like Globidens
which had crushing teeth to eat
armoured prey.
Further reading
- Dallasaurus turneri, a new primitive mosasauroid
from the Middle
Turonian of Texas and comments on the phylogeny of the Mosasauridae
(Squamata). - Netherlands Journal of Geoscience (Geologie en Mijnbouw)
84 (3) pp. 177-194. - G. L. Bell Jr. & M. J. Polcyn - 2005.
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