Temporal range: Triassic, 235–221.5 Ma
Probainognathus meaning “progressive jaw” is a genus of extinct cynodont therapsid that lived around 235 to 221.5 million years ago, during the Middle Triassic in what is now South America. Probainognathus is a member of the family Probainognathidae, and is a close relative of the family Chiniquodontidae. 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. 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. 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.
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. Jenson, a Harvard collector on the trips to recover the specimen.
Description and paleobiology
Teeth and diet
The diet of Probainognathus is thought to have been carnivorous, which can be extrapolated by 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. 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.
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 visual, auditory, motor, and body sensory systems all present. And, as a whole, the brain of Probainognathus indicates that it was in the process of developing endothermy.
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.
The jaw of Probainognathus is of particular phylogenetic importance. Morphologically, the dentary makes up most 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. 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 prosses is underway. This combination of evidence further solidifies Probainognathus’ phylogenic placement on the line to Mammalia, and provides a sound evolutionary connection between reptiles and mammals.
Geological and paleoenvironmental information
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. 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.
Discovery and historical information
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 northwestern Argentina. 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.
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