Probainognathus

''Probainognathus'' meaning “progressive jaw” is an extinct genus of cynodonts that lived around 235 to 221.5 million years ago, during the Late Triassic in what is now South America. ''Probainognathus'' is a member of the family Probainognathidae, and is a close relative of the family Chiniquodontidae.Fossilworks: Gateway to the Paleobiology Database. Probainognathus Romer 1970 (therapsid). PaleoDB taxon number: 39225 , http://www.fossilworks.org/cgi-bin/bridge.pl?a=taxonInfo&taxon_no=80895 The various similarities to Chiniquodontidae led Alfred Romer to initially suggest ''Probainognathus'' be placed within that family, but it was subsequently decided that the differences were enough to warrant its placement within Probainognathidae.Romer, A.S. 1973. The Chanares (Argentina) Triassic reptile fauna. XVIII. Probelesodon minor, a new species of carnivorous cynodont; family Probainognathidae nov. Breviora, 401:1-4

''Probainognathus jenseni'' was a species of small, carnivorous cynodonts which possessed features that provide a connection between cynodonts and mammals. The major feature being jaw joint articulation that not only included the quadrate and articular bones, but also the squamosal and dentary bones."Jaws to Ears in the Ancestors of Mammals." Jaws to Ears in the Ancestors of Mammals. University of California, Berkeley, n.d. Web. 05 Mar. 2017. , http://evolution.berkeley.edu/evolibrary/article/evograms_05 This development in the jaw joint is an important step in the evolution of mammals as this squamosal-dentary articulation is the joint all extant mammals possess. These findings provide evidence that ''Probainognathus'' should be placed on the line ascending towards Mammalia.

Discovery and naming

''Probainognathus'' was discovered in the Chañares Formation in La Rioja Province, Argentina. Harvard collector James A. Jensen headed the group that discovered ''Probainognathus'' jenseni that was described in 1970, and Jensen is also the specific namesake for ''Probainognathus jenseni''. These specimens were collected 3 km north of the terminus of the Rio Chañares at the Campo de Talampaya, and were described by Alfred Sherwood Romer in 1970.

''Probainognathus'' has also been collected from the Ischigualasto Formation in the Ischigualasto-Villa Unión Basin in northwestern Argentina."Digimorph - Probainognathus Sp. (fossil Eucynodont)." Digimorph - Probainognathus Sp. (fossil Eucynodont). University of Texas, Austin, n.d. Web. 05 Mar. 2017. . A juvenile skull unearthed from this formation was sent through a CT scan at the University of Texas at Austin, and was, ultimately, made available to the public. This allows for current three-dimensional analysis of this specimen in a digital medium, and provides morphological context beyond the limits of two-dimensional pictures.
In addition to this, in 1994, the holotype of ''Probainognathus jenseni'', along with several other specimens, were stolen from the National University of La Rioja in Argentina.Hultz, Thomas R. "Missing La Rioja Specimens." Missing La Rioja Specimens. N.p., 1994. Web. 05 Mar. 2017. .

''Probainognathus'' was first described and named by Harvard paleontologist, Alfred Sherwood Romer in 1970. ''Probainognathus jenseni'' is the focus of this description, and was named generically for its advanced jaw articulation, literally meaning “progressive jaw” in Greek, and specifically for James A. Jensen, a Harvard collector on the trips to recover the specimen.

Description

''Probainognathus'' was a small cynodont, the apparent body size of which is inconsistent across sources, ranging from being about the size of a rat, to the size of a small dog."Cynodont - Probainognathus Jenseni Skull - Bone Clones, Inc. - Osteological Reproductions." Bone Clones, Inc. - Osteological Reproductions. Bone Clones, Inc., n.d. Web. 05 Mar. 2017. ."Triassic Cynodonts; Cynognathidae, Probainognathidae and 'Allies', an Internet Directory."Triassic Cynodonts; Cynognathidae, Probainognathidae and 'Allies', an Internet Directory. N.p., n.d. Web. 05 Mar. 2017. .

Teeth and diet

The diet of ''Probainognathus'' is thought to have been carnivorous, which can be extrapolated from its teeth. The teeth include dully-tipped, partially developed canines, postcanines, four upper incisors that are positioned vertically, and three lower incisors that lean slightly forward.Soares, Marina Bento, Fernando Abdala, and Cristina Bertoni-Machado. "A Sectorial Toothed Cynodont (Therapsida) from the Triassic Santa Cruz Do Sul Fauna, Santa Maria Formation, Southern Brazil." Geodiversitas 33.2 (2011): 265-78. Science Press. Web. 5 Mar. 2017. . The cheek teeth are slender from the sides, but are lengthened anteroposteriorly, with several cusps along the row. This pattern is consistent with that exhibited by other meat-eating cynodonts, which indicates ''Probainognathus'' was a carnivore, as well. This tooth pattern is similar to that of chiniquodontids, and is part of the proposed connection between them and ''Probainognathus''.Colbert, E., & Morales, Michael. (1991). Evolution of the vertebrates : A history of the backboned animals through time (4th ed.). New York: Wiley-Liss.

Brain

Based on an endocranial cast of ''Probainognathus'', several details have been extrapolated about its brain. ''Probainognathus''’ cerebral hemispheres were elongated, it had established olfactory bulbs, and a significant cerebellum and flooculi. These were among other, less pronounced, structures such as anterior colliculi. Because of these features, it has been suggested that ''Probainognathus''’ brain had well-developed visual, auditory, motor, and body sensory systems. And, as a whole, the brain of ''Probainognathus'' indicates that it was in the process of developing endothermy.

Skull

The snout of ''Probainognathus'' extends beyond the premaxillary processes, and is very thin. The apparent fragility of this structure has led to it being broken off several specimens, and has led to an increased difficulty in studying it. There are several features that have been identified, however. The suborbital arch and the zygomatic arch are both shaped relatively sharply, and there is an extension of the squamosal posteriorly on the zygomatic arch. ''Probainognathus'' has a long secondary palate as it stretches all the way to the posterior end of the tooth row. In addition, the secondary palate is also formed largely by the palatine, and it is somewhat narrow. This narrowing of the palate allows for the conservation of space to be able to fit the back, lower molars. Moreover, the vomer is posterior to the secondary palate, and significant pterygoid flanges are present, as well.

The skull of ''Probainognathus'' is, on average, around 7 cm in length, and is missing the parietal foramen. The skull is broad in proportion to its length, the face is short and narrow, but its zygomatic region is relatively large, seemingly for jaw muscle attachment. The skull also possesses a double occipital condyle, which is a typical mammalian feature. In comparison to close relatives, such as '' Probelesodon'', the braincase of ''Probainognathus'' is more broad in the parietal region, and the oticoccipital region of the skull is deeper. Likewise, the parietals have more depth, the occipital crest is higher, and the orbits are proportionately larger relative to the rest of its head. The sagittal crest is also lengthened posteriorly, and splits into the occipital crests. This leads to a straighter posterior margin, and a more concave occiput in ''Probainognathus''.

Jaw

The jaw of ''Probainognathus'' is of particular phylogenetic importance. Morphologically, the dentary makes up most of the lower jaw, and it curves and extends down posteriorly to the area of the articular and jaw articulation. Correspondingly, in the upper jaw, the squamosal bone becomes situated next to the quadrate. The posterior end of this enlarged dentary fits into a small nook in the squamosal of the upper jaw, and displays the beginning of the evolution of the squamosal-dentary jaw joint. This squamosal-dentary jaw articulation is the same joint exhibited by mammals, and the inclusion of this joint in ''Probainognathus'' indicates that it is a precursor to mammals.

Along with this osteological feature, the jaw of ''Probainognathus'' is suggested to have had mammal-like soft tissue advancements, as well. The jaw muscles in ''Probainognathus'' were thought to have been positioned farther forward, with the masseter splitting into two separate muscles: the superficial masseter and the deep masseter.Ridley, Mark. "Evolution - Fossils and the History of Life." Evolution - Fossils and the History of Life. Blackwell Publishing, n.d. Web. 05 Mar. 2017. .Bard, J. (2017). Principles of evolution : System, species, and the history of life. London ; New York: Garland Science, Taylor & Francis Group. Despite this new development, the reptilian quadrate-articular jaw joint persists in ''Probainognathus'', and a paired jaw joint is the result. Because of this, the jaw of ''Probainognathus'' remains distinct from that of mammals due mostly to the presence of the articular and the quadrate.
Once the dentary-squamosal articulation becomes more established, the former bones involved in jaw articulation, the articular and quadrate, can become integrated into the inner ear as the malleus and incus, respectively. This has not yet happened in the case of ''Probainognathus'', but the reduced size of the quadrate, as well as its loose association with the squamosal and proximity to the stapes indicates the quadrate to incus process is underway. This combination of evidence further solidifies ''Probainognathus''’ phylogenetic placement on the line to Mammalia, and provides a sound evolutionary connection between reptiles and mammals.

Paleoecology

''Probainognathus'' was collected from the Chañares Formation in La Rioja Province, Argentina. This locale is known for its preservation of Middle Triassic tetrapods, which are largely fossilized in volcanic concretions.Rogers, R., A. B. Arucci, F. Abdala, P. C. Sereno, C. A. Foster, and C. L. May. "Paleoenvironment and Taphonomy of the Chañares Formation Tetrapod Assemblage (Middle Triassic), Northwestern Argentina: Spectacular Preservation in Volcanogenic Concretions." Palaios 16 (2001): 461-81.Fernando Abdala (PhD). Web. 05 Mar. 2017. .
When ''Probainognathus'' inhabited this area during the Middle Triassic, it is thought to have been a lacustrine region within a rift basin that got a large influx of sedimentary debris and volcanic ash.

Various pieces of evidence suggest tetrapod mass mortality was the cause of death in the Chañares Formation. Fossils of both young and adult specimens were found at this locale, and these are not limited to ''Probainognathus'', but rather represent many taxa that fell victim to the event. There is also an unusual concentration of herbivores, carnivores, and omnivores in this formation. Numerous fossils of each type were found in close proximity to one another, despite the fact that this intermingling would normally be uncommon. It has, subsequently, been suggested that this unusual distribution of specimens could be due to the death event leaving these animals stranded with no other place to go, perhaps along a waterfront of some kind. The evidence at hand, especially the nature of volcanic preservation, has led to the postulation that volcanism played a role in the death event in some capacity. Suggested events include a large outpouring of ash, lethal gas surges, or, more likely, volcanism-induced flooding. Volcanic activity may have caused damming or other water diversion, which led to major widespread flooding in the area, and the death of ''Probainognathus'' and other tetrapods. Unfortunately, however, it remains uncertain whether volcanism was the direct cause of this major death event, or if it just aided in preservation after the fact.

References

{{Taxonbar, from=Q3406217

Prehistoric probainognathians
Prehistoric cynodont genera
Carnian genera
Late Triassic synapsids of South America
Fossils of Argentina
Triassic Argentina
Chañares Formation
Ischigualasto Formation
Fossil taxa described in 1970