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.‭ ‬-‭ ‬Paläontologische Zeitschrift‭ ‬81‭ (‬2‭)‬:‭ ‬131‭–‬145.‭ ‬doi:10.1007/BF02988388.‭ ‬-‭ ‬J.‭ ‬C.‭ ‬Weinbaum‭ & ‬A.‭ ‬Hungerbühler‭ ‬-‭ ‬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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