Poposaurus
Name:
Poposaurus
(Popo lizard - after the Popo Agie Formation).
Phonetic: Pop-o-sore-us.
Named By: M. G. Mehl - 1915.
Synonyms: Lythrosuchus.
Classification: Chordata, Reptilia, Rauisuchia,
Paracrocodylomorpha, Poposauroidea, Poposauridae.
Species: P. gracilis
(type), P. langstoni.
Diet: Herbivore.
Size: Up to 4 meters long.
Known locations: USA, including Arizona -
Chinle Formation (Blue Mesa Member, Mesa Redondo Member, Owl
Rock Member, Petrified Forest Member, Sonsela Member), Texas
- Colorado City Formation, Cooper Canyon Formation, Tecovas
Formation, Utah - Chinle Formation (Monitor Butte Member) and
Wyoming - Popo Agie Formation.
Time period: Carnian to Norian of the Triassic.
Fossil representation: Fossils of various
individuals revealing almost all of the post cranial skeleton.
Partial skull remains are also known.
The
rauisuchians
were the top land predators of the Triassic, and
remained so until the appearance of the large theropod dinosaurs.
These rauisuchians are usually portrayed as long quadrupedal
predators, and for the most part this is true. However there is a
surprisingly little known group of bipedal rauisuchians called the
poposaurids, of which Poposaurus is the type
genus. Further to this
the poposaurids of the Poposauridae are situated within the
Poposauroidea, a group of rauisuchians that is noted for the presence
of several unusual members.
Poposaurus
was first named upon the basis of partial post cranial remains, and
over the years these have been interpreted as belonging to a variety of
different creatures, including various types of dinosaurs from
ornithischians, like iguanodonts and stegosaurs,
to even
sauropodomorphs.
Towards the end of the twentieth century the
classification of Poposaurus steadily became more
accurate,
eventually resting within the Rauisuchia. Eventually in 1995,
the description of new Poposaurus fossils by
Robert Long and Phillip
Murry saw Poposaurus split from the Rauisuchidae
which contains genera
such as Saurosuchus
and Postosuchus,
and established the
Poposauridae.
More
new fossils of Poposaurus were discovered in
2007, and these led to
another genus named Lythrosuchus being identified
as a synonym to
Poposaurus. Then in 2011 another and this time
almost complete
Poposaurus missing only the skull was located.
2013 saw the first
description of Poposaurus cranial remains by
Parker &
Nesbitt, giving a clearer indication as to the form of the
Poposaurus skull.
The
modern twenty-first century reconstruction of Poposaurus
is of a swift
bipedal predator capable of running down fast and agile prey, quite a
bit different from the heavier set quadrupedal members of the related
rauisuchidae that are perceived to be masters of ambush hunting. This
bipedal nature however was actually first speculated in the original
1915 description of Poposaurus fossils by M.
G. Mehl, though
he also considered the form of the hip of the Poposaurus
holotype to be
an indicator that it was a dinosaur.
With
more complete specimens found, and more accurate studies made,
Poposaurus has been confirmed as an obligatory
bipedal animal,
though one that it thought to have developed its stance totally
independently from the emerging dinosaurs of the Triassic. This
stance is thought to be developed from an ancestral ability to ‘high
walk’. Today we can observe crocodiles
'high walking’ by
adjusting their limbs so that they are almost vertical to the ground,
enabling them to lift their belly and often also their tail clear off
the ground. Amongst modern reptiles this ability is unique to the
crocodiles, and though not directly related to rauisuchians like
Poposaurus, they are still distant cousins.
Reconstructions
of the hind legs of Poposaurus show that the most
developed muscles
were those concerning the back and forth movement of the leg, a very
clear indication of bipedal locomotion. The fore legs are
proportionately much shorter than the rear, which also indicates that
there was no weight bearing purpose for them. Bipedal locomotion has
also been proposed for other rauisuchian genera that have more
traditionally been seen as quadrupedal, however at this time it seems
that Poposaurus is an exception and not a rule.
For example,
exceptionally well preserved remains of a hind leg upon a specimen of
Prestosuchus
have confirmed a very different muscle development to
Poposaurus, with a preference for limb rotation,
indicating that
members of the rauisuchidae were most likely quadrupeds, though with
the ability to rear up on hind legs.
Further reading
- The skull of Paleorhinus, a Wyoming
phytosaur. - The Journal
of Geology 15 (2): 121–151. doi:10.1086/621382. - J.
H. Lees - 1907.
- Poposaurus gracilis, a new reptile from the
Triassic of Wyoming.
- The Journal of Geology 23 (6): 516–522.
doi:10.1086/622268. - M. G. Mehl - 1915.
- Late Triassic (Carnian and Norian) tetrapods from the
southwestern United States. - Bulletin of the New Mexico Museum of
Natural History and Science 4: 1–254. - R. A. Long
&
P. A. Murry - 1995.
- A revision of Poposaurus gracilis
(Archosauria: Suchia) based
on two new specimens from the Late Triassic of the southwestern U.S.A.
- Pal�ontologische Zeitschrift 81 (2): 131–145.
doi:10.1007/BF02988388. - J. C. Weinbaum & A.
Hungerb�hler - 2007.
- Anatomical reconstructions of respiratory morphology and hindlimb
musculature in Poposaurus gracilis (Archosauria:
Poposauroidea)
and related dinosauriformes (Ph.D. thesis). University of
Pennsylvania. - E. R. Schachner - 2010.
- The bipedal stem crocodilian Poposaurus gracilis:
inferring
function in fossils and innovation in archosaur locomotion. -
Bulletin of the Peabody Museum of Natural History 52 (1):
107–126. doi:10.3374/014.052.0102. - J. A. Gauthier, S.
J. Nesbitt, E. R. Schachner, G. S. Bever & W.
G. Joyce - 2011.
- Pelvic and hindlimb myology of the basal archosaur Poposaurus
gracilis (archosauria: Poposauroidea). - Journal of
Morphology 272 (12): 1464–1491. doi:10.1002/jmor.10997.
- E. R. Schachner, P. L. Manning & P. Dodson
-
2011.
- Cranial remains of Poposaurus gracilis
(Pseudosuchia:
Poposauroidea) from the Upper Triassic, the distribution of the
taxon, and its implications for poposauroid evolution. -
Geological Society, London, Special Publications. v. 379.
http://dx.doi.org/10.1144/SP379.3. - W. Parker & S.
Nesbitt - 2013.
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