Name:
Thrinaxodon
(Trident tooth).
Phonetic: Thrin-ax-o-don.
Named By: Harry Govier Seeley - 1894.
Classification: Chordata, Synapsida,
Therapsida, Cynodontia, Thrinaxodontidae.
Species: T. liorhinus (type).
Diet: Carnivore.
Size: 40-50 centimetres long.
Known locations: South Africa. Antartica.
Time period: Olenekian of the Triassic.
Fossil representation: Several specimens.
As
a cynodont Thrinaxodon was the result of the
on-going evolution of
mammal like reptiles that had can be traced all the way back to the
early synapsids. Thrinaxodon however displays
numerous features that
place it much closer to mammals than its reptile origin would imply.
The
first and most evolutionary significant feature of Thrinaxodon
is the
division of the main body into thoracic and abdominal areas. In
earlier animals the ribs extended down from all of the vertebrae
usually ending where the sacrum (pelvis) begins resulting in a rib
cage that ran the entire length of the main body. In Thrinaxodon
however the ribs only drop down from the frontal dorsal vertebrae,
the dorsal vertebra between these and the sacrum being devoid of
ribs. This meant that the key organs like the lungs and heart were
still protected by the ribs while the intestines were not. It has
also been speculated that a muscular diaphragm would have been present
to separate these two areas.
Another
feature to take note of are small pits in the snout of the skull.
These seem to have been the anchor points for whiskers. The presence
of whiskers denotes the ability to grow hairs, and while fossils have
not yet been found to prove the existence of anything other than these
whiskers, it’s certainly possible that Thrinaxodon
may have had a
coat of hair in life. Hairs would have developed to serve a purpose,
possibly for insulation which may suggest a warm blooded metabolism.
Thrinaxodon
had a secondary palate that allowed the nasal passages to be separate
from the mouth. This means that no matter how long Thrinaxodon
kept
food in its mouth for it could still breathe. Mashing food in the
mouth for longer would reduce the amount of time necessary for
digestion. This also suggests a faster metabolism that needed a
quicker intake of energy and nutrients in order to maintain its speed.
Apart
from suggesting a warm blooded metabolism as found in true mammals,
these adaptations also point to a lifestyle that could see Thrinaxodon
living in burrows to survive. Firstly the division of the body into
abdominal and thoracic area with a rib cage (and least flexible part
of the body) meant that Thrinaxodon would have
been able to turn
around in a much tighter space like a tunnel. This is a feat that
would have been physically impossible for a creature with the older
arrangement of ribs all the way to the sacrum.
You
also need to consider the development of whiskers. Whiskers are
sensory hairs that radiate out so that an animal knows if a gap or hole
is wide enough for them to pass through. If the animal cannot pass
through without the whiskers getting bent back it either avoids going
down or digs the hole wider.
Another
thing to look at is the overall shape and proportions of the body.
The legs are short and do not sprawl out wide to the sides. The tail
is also reduced and the body is proportionately much longer than it is
wide. This shape allows for much more efficient travelling down
narrow tunnels. Living in tunnels would also have been a good
survival strategy as archosaurs were roaming the land always on the
lookout for easy Thrinaxodon sized meals.
Thrinaxodon
itself was probably a generalist of small prey items like lizards and
invertebrates. The arrangement of small sharp teeth would have been
better suited for these kinds of prey, which themselves would have
provided a high protein diet better suited for maintaining a fast
metabolism. If Thrinaxodon was indeed a burrowing
animal it may have
remained in the burrow during the daytime coming out to feed at night
where it could use the cover of darkness to hide its activities. Even
so, Thrinaxodon probably would not stray too far
from the burrow.
Further reading
- Researches on the Structure, Organisation, and Classification of the
Fossil Reptilia. Part IX. Section 1. On the Therosuchia. -
Philosophical Transactions of the Royal Society of London, Series B
185:987-1018. - Harry Govier Seeley - 1894.
- Note on a new skeleton of Thrinaxodon Liorhinus.
- A. S. Brink - 1959.
- Cranial anatomy of the cynodont reptile Thrinaxodon
liorhinus. -
Museum of comparative Zoology, Harvard University, 125: 165-180. - R.
Estes - 1961.
- Growth patterns of Thrinaxodon liorhinus, a
non-mammalian cynodont
from the lower Triassic of South Africa. - Paleontology. 48(2):
385-394. - J. Botha & A. Chinsamy - 2005.
- Ontogeny of the Early Triassic Cynodont Thrinaxodon
liorhinus
(Therapsida): Cranial Morphology. - The Anatomical Record. - Sandra C.
Jasinoski, Fernando Abdala & Vincent Fernandez - 2015.