Rhinesuchidae
Rhinesuchidae is a family of tetrapods that lived primarily in the Permian period. They belonged to the broad group Temnospondyli, a successful and diverse collection of semiaquatic tetrapods which modern amphibians are probably descended from. Rhinesuchids can be differentiated from other temnospondyls by details of their skulls, most notably the interior structure of their otic notches at the back of the skull. They were among the earliest-diverging members of the Stereospondyli, a subgroup of temnospondyls with flat heads and aquatic habits. Although more advanced stereospondyls evolved to reach worldwide distribution in the Triassic period, rhinesuchids primarily lived in the high-latitude environments of Gondwana during the Guadalupian and Lopingian epochs of the Permian. The taxonomy of this family has been convoluted, with more than twenty species having been named in the past; a 2017 review recognized only eight of them to be valid. While several purported members of this group have been reported to have lived in the Triassic period, most are either dubious or do not belong to the group. However, at least one valid genus of rhinesuchid is known from the early Triassic, a small member known as Broomistega. The most recent formal definition of Rhinesuchidae, advocated by Mariscano et al. is that of a stem-based clade containing all taxa more closely related to Rhinesuchus whaitsi than to Lydekkerina huxleyi or Peltobatrachus pustulatus. A similar alternate definition is that Rhinesuchidae is a stem-based clade containing all taxa more closely related to Uranocentrodon senekalensis than to Lydekkerina huxleyi, Trematosaurus brauni, or Mastodonsaurus giganteus.
Description
Rhinesuchids generally had a conventional body type for tetrapods, with four limbs and a moderately long tail. In addition, their bodies were also somewhat elongated and their limbs were small and weak but still rather well-developed. Some were very large, up to 3 meters in length. Like most stereospondyls, their skulls were flattened and triangular, with upward-pointing eyes. Most rhinesuchids had relatively short snouts, although the snout of Australerpeton was very long and thin. The only other giant long-snouted Permian amphibians were members of the family Archegosauridae, such as Prionosuchus and Konzhukovia.Otic notch
Like most ancient amphibians, rhinesuchids had a pair of indentations at the rear edge of the skull known as otic notches. While sometimes considered to have housed hearing organs such as a tympanum, these notches are more likely to have held spiracles, fleshy holes used for breathing. Rhinesuchids can be characterized by a unique system of ridges and grooves within the inner cavity of each otic notch. The walls of the otic notch cavity are mainly made up of the ascending branch of the pterygoid bones. Nevertheless, the inside edge of each cavity is formed by a tabular bone. The tabular bones are a pair of triangular bones along the rear edge of the skull which form pointed structures known as tabular horns. The upper part of the outer wall of the cavity is also formed partly from the squamosal bones, which mostly occupy the flat upper face of the skull. The portion of the squamosal which forms the cavity wall is separated by the portion outside of the cavity by a pronounced boundary known as a falciform crest.The outer wall of the cavity has a long and pronounced groove, known as a stapedial groove, which extends lengthwise along the wall. The lower edge of the groove is formed by a ridge/crest known as an oblique ridge, although it has also been called a crista obliqua, otic flange, or simply an oblique crest. The upper edge of the stapedial groove is formed by another ridge/crest bordering the squamosal bone, which Eltink et al. named the 'dorsal pterygoid crest'. However, Mariscano et al. preferred to use the name "lamella" for this structure so that it would not be confused with a different ridge present in lydekkerinids, which is sometimes termed an 'oblique crest of the pterygoid', but more commonly called a 'tympanic crest'. Confusingly enough, many rhinesuchids are also known to possess a tympanic crest. This ridge was positioned further back than the other ridges and extends down along the rear face of the cheek. The inner edge of the outer wall of the cavity was formed by a ledge which most studies simply label 'membrane'. This convention exists as a result of the old and likely incorrect hypothesis that otic notches housed eardrums. Under this hypothesis, the inner ledge may have attached to a membrane stretching along the inner cavity of the ear.
This combination of otic cavity grooves and ridges is unique to rhinesuchids. The lamella and stapedial groove are unknown in any other groups, although they are present in practically every rhinesuchid. The tympanic crest is present in most rhinesuchids but absent in a few, and it is additionally present in lydekkerinids. The oblique ridge/crest and falciform crest are present in most other stereospondyls, while the 'membrane' ledge is present in practically every stereospondylomorph.
Palate and braincase
Various bones and openings comprised the palate in rhinesuchids, as in other amphibians. At the tip of the palate lied the vomers, while the areas near the edge of the mouth were made of the palatine and ectopterygoid bones. In the middle of the rear part of the mouth was a rectangular bone known as a parasphenoid. Most of the parasphenoid formed the lower face of the flattened braincase, although it also possesses a thin forward-projecting rod known as a cultriform process, which extends down the midline of the skull to meet the vomers. Towards the back of the mouth, there were the multi-pronged pterygoid bones on each side of the skull. Each pterygoid had several branches, including the posterior branch which stretches back and to the side of the skull, the short medial branch which extends inwards and connects to the parasphenoid bone, an ascending branch which projects upwards to form the otic notch, and finally the anterior branch which extends forward along the palatine and ectopterygoid. The pterygoids of most rhinesuchids have very long anterior branches. In most members of this family, the anterior branch reaches as far forward as the vomers, although Australerpeton has relatively short anterior branches. A pair of large openings, known as interpterygoid vacuities, fill the areas between these bones, making the majority of the palate open space.When seen from behind, the upper branches of the braincase extends from side to side, partially concealing the ascending branch of the pterygoids. Each paroccipital process is also perforated by a small hole, known as post-temporal fenestrae. These holes are very thin in rhinesuchids. Above these paroccipital processes lie the otic notches as well as the tabular bones. The paroccipital processes also point backwards to some extent, forming horns which in some rhinesuchids are slightly longer than those of the tabulars. When seen from below, the most prominent portion of the braincase is the parasphenoid bone. The rear corners of the parasphenoid have small 'pockets' bordered by ridges. These ridges may have anchored muscles capable of maneuvering the head on the neck.
Other skull and jaw features
Many bones made up the upper side of the skull, although a particular pair of bones acquired a specific design in rhinesuchids. These bones were the elongated jugal and prefrontal bones, which formed the front edge of the orbits. In most rhinesuchids, the edge between the two bones possessed a 'stepped' shape, with a triangular outer extension of the prefrontal pushing the suture with the jugal towards a more lateral position. However, the suture is more straight in Australerpeton, like in other stereospondyls.The lower jaw has a pair of holes only visible from the inside edge of the jaw. The larger hole at the rear part of the bone complex, known as a posterior Meckelian foramen, was thin and elongated in rhinesuchids. An additional hole on the underside of the jaw joint is only visible from below. This hole, the chorda tympanic foramen, was large in this family. On the upper side of the jaw joint, a thin groove known as an arcadian groove stretches towards the lingual side of the jaw and separates other bony bumps located among the jaw joint. As a whole, the grooves and ridges of the jaw joint were poorly developed in rhinesuchids compared to that of many other stereospondyl groups, instead resembling the simple joint of archegosaurids such as Melosaurus.
Paleobiology
Most rhinesuchids are only known from skull material, although a few members of the group include specimens preserving a significant portion of the rest of the skeleton. A juvenile specimen of Broomistega had ankles and vertebrae which were poorly ossified, indicating that its joints had a large amount of cartilaginous material to supplement the low amount of bone. This trait is often correlated with an aquatic lifestyle. Features of the skull, such as upwards-pointing eyes, also support this hypothesis.File:Odd couple.png|thumb|left|260x260px|Broomistega preserved in a fossilized burrow with Thrinaxodon
Despite this support for an aquatic lifestyle, other pieces of evidence show that rhinesuchids were capable of some terrestrial movement. Although rhinesuchids did not possess any adaptations for digging, the poorly ossified juvenile specimen of Broomistega was found in a flooded burrow which was also inhabited by a Thrinaxodon. Various conditions of the way these animals were preserved indicate that they co-inhabited the burrow peacefully, likely to survive a drought by aestivating. The fact that a Broomistega was able to enter the burrow of a terrestrial animal such as Thrinaxodon indicates that rhinesuchids were not exclusively aquatic.
In addition, it has been noted that larger temnospondyls generally have more well-ossified joints. For example, large specimens of Australerpeton possessed robust hips, several completely bony ankle bones, and ossified pleurocentra. Nevertheless, these skeletons were not as strongly built as those of Eryops, with smaller shoulder girdles and less prominent sites for muscle attachment. Dias & Schultz suggested that the lifestyle of Australerpeton was that of a semiaquatic piscivore, preferring to hunt in shallow bodies of freshwater yet retaining the ability to walk on land during droughts.
A Histological study of several indeterminate rhinesuchid fossils indicate that members of the family grew seasonally, as in modern amphibians. Individuals also had fairly long life span, with one specimen being 30 to 35 years old at the time of its death based on the number of lines of arrested growth present in a hip fragment. Some lines of arrested growth were very narrow, indicating that the individuals could reduce their growth and metabolism during times of hardship. This ability may be the reason why rhinesuchids were rather successful at the end of the Permian, as well as how a few small members of the group survived the Permian-Triassic extinction event.