Junggarsuchus () is an extinct genus of sphenosuchian crocodylomorph from the Middle or Late Jurassic period of China. The type and only species is J. sloani. The generic name of Junggarsuchus comes from the Junggar Basin (the anglicization of Dzungar), where the fossil was found, and the Greek word "souchos" meaning crocodile. The specific name, "sloani" is in honor of C. Sloan, who is credited with finding the holotype.
Discovery
Junggarsuchus was found in the upper part of the Lower Member of the Shishugou Formation in Xinjiang, China at the Wucaiwan locality. The type and only specimen was described in 2004 by James Clark, Xu Xing, Catherine Forester, and Yuan Wang in Nature, but it did not receive a full osteological description until 2022 when Alexander Ruebenstahl, Michael Klein, and Yi Hongyu published a monograph along with two of the original describers James Clark and Xu Xing.
It is a relatively small animal, with the skull of the holotype only measuring in length. The right forelimb was about long from the shoulder to the metacarpals. This would make the animal in life only about the size of a domestic cat. However, its overall length is unknown because the holotype only preserves a single vertebra from the tail.
The holotype, given the designation IVPP 14010, consists of an almost complete skull with an intact brain-case and lower jaws, most of the left forelimb, the proximal ends of the ulna and radius of the right forelimb along with the right humerus, fifteen cervical and dorsal vertebrae along with most of the ribcage, and an associated caudal vertebra. Most of the skeleton was fully articulated, with some elements being found in association and disarticulated. More of the specimen had yet to be prepared at the time of the monograph's publication, and some of the bones remained obscured by the matrix at the time of its re-description.
The skull of the holotype was transported from the Institute of Vertebrate Paleontology and Paleoanthropology, where it was initially reposited, to George Washington University, where it was studied and comprehensively re-described by Ruebenstahl and colleagues using modern CT imaging technology. The preparation of the skeleton was so exquisite that elements of the skeleton which had been damaged during fossilization were able to be glued together by the preparators with very little unconformity.
Description
In their initial description of the skull, Clark and colleagues noted that there was very little cranial kinesis and there were attachment sites for very powerful jaw muscles, which are derived traits found in modern crocodilians. With regard to the post-cranial skeleton, they noted that the spine of the holotype probably had very low vertical mobility across its length and it was mostly adapted for lateral motion, consistent with modern crocodilians. However, a very significant difference between Junggarsuchus and its modern relatives was the complete lack of osteoderms in the specimen, despite the lack of spinal mobility, when these traits were presumed to have co-evolved. They suggest that the lack of preserved osteoderms may be a product of the taphonomic conditions of the specimen, or that it may have been a juvenile when it died.
As in other sphenosuchians, the limbs of Junggarsuchus were adapted to terrestrial locomotion (movement on land), rather than the semi-aquatic locomotion seen in living crocodilians. These adaptations include: a vertically orientated upper arm bone, ball-and-socket shoulder joint, and a functionally tridactyl (three-fingered) hand due to a reduction of digit 5 and the absence of digit 1. However, the fact that these terrestrial adaptations appear to be common to Junggarsuchus and to more basal animals like Saltoposuchus, means that this probably represents the ancestral condition in crocodylomorphs, rather than a derived trait of "sphenosuchians", and the semi-aquatic adaptations of the crown group only appeared much later in their evolution.
Skull
The specializations observed in the skull of Junggarsuchus are primarily related to the reinforcement of the skull's structure and the development of stronger jaw musculature. Diapsid skulls are ancestrally relatively lightweight, and so to deliver high bite forces without damaging the skull requires significant specializations. This can be seen in the dinosaur Tyrannosaurus, which has a completely fused nasal bone (the bone on the dorsal midline of the snout), whereas most other theropods have sutures in the topology of the bone in that location. Several of the skull's fenestrae have been reduced in size compared to other early crocodylomorphs, most notably the antorbital fenestra and the supratemporal fenestra. In addition, several of the bones of the palate have become enlarged when compared with more basal crocodylomorphs. The quadrate bone is also much closer to the laterosphenoid bone and there is no suture between the parietal bones. Both of these reduce the flexibility of the skull, which has been suggested to enable greater bite forces to be introduced without damaging the skull. However, this trend of an increasing robustness of the skull is not uniform, and two notable adaptations which buck this trend are the appearance of an additional fenestra in the quadrate and a lack of significant contact between the nasal and lacrimal bones. While most of the skull characteristics demonstrate the intermediate placement of Junggarsuchus on the family tree of crocodylomorphs, there are several entirely novel adaptations that the skull shows. The lateral surface of the angular bone has extensive attachment sites for jaw musculature, which is seen in some crown crocodilians, but not in other intermediate forms. This, like several other adaptations, most likely aided in delivering higher bite forces to potential prey items.
Another notable trait of the skull of Junggarsuchus is the appearance of pneumatic spaces in the quadrate, the parabasisphenoid, and possibly the pterygoid. While this seemingly contradicts the trend of skull solidification in Junggarsuchus and related taxa (basal solidocranians), Ruebenstahl and colleagues suggest that it is possible that these pneumatic spaces enabled the tissues of the skull to withstand and absorb shocks that would otherwise damage a structure made of solid bone. However, the authors note that this is only inferred, and the evolution of cranial pneumaticity in crocodylomorphs may instead reflect the invasion of these tissues by cranial sinuses. Poor preservation of the pterygoid of the holotype also makes it difficult to infer the exact degree of pneumaticity or to speculate with regard to any of its possible functions.
Notably, the inner ear anatomy of Junggarsuchus shows significant terrestrial adaptations. The semicircular canal is tall and narrow, unlike in aquatic crocodilians, which is believed to have aided the animal in the orientation of the head and the gaze. This would have been necessary to hunt terrestrial prey and to maintain balance while moving on land.
Despite the exceptional preservation of the skull, there are a few key areas where the arrangement of the skull bones is uncertain due to deformation or damage that the skull suffered during the fossilization process. These areas are the contact between the angular and surangular bones and the contact between the squamosal and postorbital bones. Based on CT data, Ruebenstahl and colleagues suggest that it is likely that the surangular is short in the jaw of Junggarsuchus and does not have significant contact with the angular. However, it is possible that the surangular has significant contact across a significant length of both the articular and angular bones, which would make it similar to the condition of its close relative Dibothrosuchus.
The other uncertainty is in regard to the contacts between the quadrate and the postorbital bones in the rear of the skull. Breaks in the bone as it was being preserved make it difficult to distinguish which fractures are reflective of taphonomic damage and which ones are reflective of actual sutures on the bone. The authors hypothesize that the postorbital only contributes to the dorsal ridge of the supratemporal fenestra, although they consider the possibility that the postorbital extends down the posterior ridge of the fenestra, running parallel to the squamosal rather than meeting it straightforwardly.
Post-cranial skeleton
Junggarsuchus displays numerous adaptations for cursoriality. These include the reduction of the number of digits in contact with the ground from five, which is the ancestral condition in pseudosuchians, to only three in the forelimbs of Junggarsuchus. The reduction of the outer digits such that they are not in contact with the ground has been previously recognized to be a trend conserved among cursorial tetrapods, and digits one and five are very reduced in Junggarsuchus. Other limb specializations include an enlarged surface on the anterior edge of the scapula, which has been interpreted as a site for expanded muscle attachments to aid in limb retraction. The joints of the shoulders and limb bones also appear to be generally oriented in such a way that the limbs would be held directly underneath the body and would have the ability to flex underneath the body of the animal. This distal bones of the arm (the ulna and radius) are also much longer relative to the humerus than in other archosaurs (though they are still not as long as the humerus). This has been interpreted as an additional indication of cursoriality in other crocodylomorphs. The hind limbs are not preserved in the holotype, so it is not certain if the femur, tibia, and fibula also exhibit this relationship.
The notable lack of osteoderms which, in the original description, was suggested to be purely taphonomic, is suggested by later authors to be reflective of a genuine lack of these features as part of a general trend towards more terrestrial mobility and flexibility which is not seen in the modern relatives of Junggarsuchus. Characteristics of the spine including the zygapophyses are suggestive of a much more flexible overall range of motion that most modern crocodiles lack completely. This level of terrestrial flexibility is common in early-diverging crocodylomorphs as well as archosaurs in general. Most of the close relatives of Junggarsuchus suich as Dibothrosuchus, Terrestrisuchus, and Dromicosuchus are similarly gracile and adapted for fast terrestrial movement. This suggests that the aquatic adaptations seen in modern crocodiles, as well as in many of their extinct relatives, like the thalattosuchians, are not the ancestral condition of this group.
Nothing is known about the hind limbs, hips, or tail of Junggarsuchus because the holotype, which is the only known specimen, does not preserve these elements. However, there may be elements of the holotype which have yet to be prepared which could reveal information about these areas of the skeleton once the rock has been removed and any new bones have been described.
Classification The original description of Junggarsuchus by James A. Clark and colleagues found it to be the sister taxon to the clade Crocodyliformes and also found that the traditional grouping, Sphenosuchia, was not a monophyletic group, but rather formed a large polytomy with a grade basal to crocodyliformes. Synapomorphies of the Junggarsuchus-crocodyliformes clade in their analysis included: exoccipitals that meet on the midline above the foramen magnum, a large extension on the bottom sides of the exoccipitals which contacts the quadrate, jugal bones which are strongly arched, a narrowing of the rear portion of the parietal bone, and a fenestra in the quadrate bone. Their analysis is shown below.