Secodontosaurus
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Name:
Secodontosaurus
(cutting-tooth lizard).
Phonetic: Sek-o-dont-o-sore-us.
Named By: Alfred S. Romer - 1936 - more
detail in main text.
Synonyms: Dimetrodon longiramus.
Classification: Chordata, Synapsida,
Pelycosauria, Sphenacodontidae.
Species: S. longiramus, S.
obtusidens, S. willistoni
Diet: Carnivore.
Size: Between 2 and 2.7 meters long.
Known locations: USA, Texas - Arroyo
Formation, Belle Plains Formation.
Time period: Sakmarian to Kungurian of the Permian.
Fossil representation: Several individuals
represented by skulls and partial post cranial remains.
Secodontosaurus
is a fantastic genus of pelycosaur
as not only do individuals have
large sails on their backs, but they have skulls that are quite
different to the other sphenacodontid pelycosaurs that they are related
to.
The
naming of the Secodonotsaurus genus was actually a
long time coming.
The first Secodontosaurus fossils were actually
named as a species of
Theropleura, T. obusidens,
by Edward Drinker Cope. Later
though the type species of Theropleura was found to
be a synonym to
Ophiacodon,
though what would be named as Secodontosaurus
fossils
could not be a part of the Ophiacodon genus. Then
in 1907 another
American palaeontologist named Ermine Case described a new fossil
specimen as a new species of Dimetrodon,
D. longiramus. Mainly
based upon the description of a lower jaw, Dimetrodon
longiramus
would eventually become Secodontosaurus longiramus
later in the
twentieth century.
The
first actual mention of Secodontosaurus as a genus
name came in 1925
when Samuel Williston listed the name as a member of the
Ophiacodontidae, with reference to fossils initially mentioned in an
earlier paper that he authored in 1916. The mention though was not
a formal description of a genus however, and it was not until 1936
when Alfred Romer, a palaeontologist best remembered for his work
with Permian era fauna, made the genus official with a proper
description. In honour of Williston and his prior work with the
genus, Romer also established the species S. willistoni.
The
most famous sail-backed pelycosaur is without a doubt Dimetrodon,
and
it is this genus that Secodontosaurus has a
particularly close
association. The skull of Secodontosaurus is very
different to the
skull of Dimetrodon, but so far there is only one
known difference in
the post cranial skeleton between these two pelycosaurs. This
difference is the axis vertebra, the second cervical (neck)
vertebra that is the main pivot for the skull to move on the neck.
In Dimetrodon, the neural spine of this vertebra
is quite high,
but the same vertebra in Secodontosaurus is
proportionately lower.
Aside from being a key physical difference, this may indicate that
the head of Secodontosaurus had a greater range of
backwards motion.
Unfortunately,
Secodontosaurus is still only known from very
incomplete post cranial
remains which makes further differences hard to establish. The known
elements though are seen as being very similar to Dimetrodon,
so much
so that it has been speculated more than once that fossils labelled as
belonging to Dimetrodon and currently sitting in
storage across
different museums may actually represent further Secodontosaurus.
Until more definitive Secodontosaurus post cranial
fossils can be
confirmed, reconstructions of Secodontosaurus
usually rely upon
comparative Dimetrodon remains to fill in the gaps.
The
sail on the back of Secodontosaurus is also
considered to have been
similar in form to the one on Dimetrodon. Also
like with Dimetrodon
and other sail backed pelycosaurs for there are many, the sail of
Secodontosaurus seems to have been for a primarily
thermoregulatory
purpose, warming and cooling the blood as needed in order to maintain
optimum body temperature and by extension function. The sail may have
also been a display device and although potentially similar in form to
the sail of Dimetrodon, it may have been coloured
or patterned in a
different way as to be unique. This would then allow Secodontosaurus
to recognise others of their own species, and not confuse them with
he other kinds of sail-backed pelycosaurs that were active in the same
area, which not only included Dimetrodon, but
other genera such as
Edaphosaurus
and Ctenospondylus
to name just two that we know about.
The
key question concerning Secodontosaurus is why did
it have different
jaws? The simple answer is that Secodontosaurus
probably had a
different kind of prey specialisation. Others such as the
aforementioned Dimetrodon and Ctenospondylus
were probably predators of
other medium to large vertebrates, probably even including other
pelycosaurs, and their taller more robust skulls would have been
better able to withstand the stresses imposed upon them from larger
struggling prey.
The
jaws of Secodontosaurus are comparatively narrow
and were better able
to reach into places like gaps between rocks and perhaps even burrows,
suggesting that Secodontosaurus may have been more
specialised in
hunting smaller vertebrate animals that were too small for other larger
pelycosaurs to bother with. It would be also interesting to see if
Secodontosaurus was a hunter of fish, since the
long and thin jaws
would have been quite suited to snatching fish out of shallow water in
a similar manner to a crocodiles
or possibly even the much later
spinosaurid
dinosaurs. A reduction in the size of the neural spine of
the axis vertebra may have increased the range of up and down pitching
of the head allowing Secodontosaurus to more easily
lift a fish up out
of the water, though such an adaptation would have also been of a
benefit if Secodontosaurus spent their time pulling
animals up and out
of burrows and other tight spaces.
The
smaller neural spine of the axis vertebra of Secodontosaurus
could also
be an argument for Secodontosaurus not specialising
in the hunting of
larger animals, since the taller neural spine of the axis vertebra of
Dimetrodon would have been a larger attachment
point of muscle to
hold the head steady when dealing with large prey that was putting up a
fight. Therefore, Secodontosaurus may not have
had such an enlarged
neural spine, because it didn’t require one. A lower skull form
would also mean less room for the jaw closing muscles which means that
the bite of Secodontosaurus would not have been as
strong as other
sphenacodontid pelycosaurs that had the more characteristic deep
skulls. This further infers that Secodontosaurus
was better suited to
dealing with smaller animals than larger ones which may have been heavy
and strong enough to break free from the mouth grip of a
Secodontosaurus.
Ctenospondylus, Dimetrodon, Edaphosaurus, Ianthasaurus, Secodontosaurus.
Further reading
- Second contribution to the History of the Vertebrata of the Permian
Formation of Texas - Proceedings of the American Philosophical
Society 19: 38–58. - Edward Drinker Cope - 1880.
- Revision of the Pelycosauria of North America. - Carnegie
Institution of Washington 55:3-176 - E. C. Case - 1907.
- The osteology of some American Permian vertebrates, II -
Contribution from Walker Museum 1: 165–192. - Samuel W.
Williston - 1916.
- Studies on American Permo-Carboniferous tetrapods - Problems of
Paleontology, USSR 1: 85–93. - A. S. Romer - 1936.
- Review of the Pelycosauria - Geological Society of America
Special Paper 28: 1–538. - A. S. Romer & L.I
Price - 1940.
- The cranial anatomy and relationships of Secodontosaurus,
an
unusual mammal-like reptile (Pelycosauria: Sphenacodontidae) from
the early Permian of Texas - Zoological Journal of the Linnean
Society 104: 127–184. - R. R. Reisz, D. S. Berman
& D. Scott - 1992.
- Interrelationships of basal synapsids: cranial and postcranial
morphological partitions suggest different topologies - R. J.
Benson - 2012.
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