| Trilophosuchus | |
|---|---|
 | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Archosauria |
| Order: | Crocodilia |
| Clade: | †Mekosuchinae |
| Genus: | †Trilophosuchus Willis, 1993 |
| Type species | |
| †Trilophosuchus rackhami Willis, 1993 | |
Trilophosuchus ("triple crest crocodile") is anextinctgenus ofmekosuchinecrocodilian from Australia. Its fossils have been found at the Ringtail Site in theRiversleigh World Heritage Area (north-westernQueensland) and date to theMiocene epoch. Additional remains have also been found at the older Hiatus Site and extend its range into theOligocene. Like the closely relatedMekosuchus, it is thought to have had a short and blunt snout and large eyes that generally resembles today'sdwarf crocodiles. It also shares similarities with several much older crocodylomorph groups and is commonly thought to have been more terrestrial than any crocodilian living today. Only a single species has been described, thetype speciesT. rackhami.
Trilophosuchus is best known throughholotype specimenQM F16856, which represents a partial skull missing the tip of the snout. Several other isolated bones of the skull were found as well and have been assigned toTrilophosuchus. Most material of this genus has been collected from the middleMiocene Ringtail Site of the inRiversleigh, one of Australia's most famous fossil localities. One referred specimen, a parietal, has been collected the older Hiatus Site, putting the oldest appearance of the genus within theOligocene. This Oligocene material differs subtly and may represent a second species or could simply be the result of age or intraspecific variation.[1] The holotype skull was uncovered in 1985 during an excavation by theUniversity of New South Wales and is currently considered to be among the best preserved fossil crocodilians from Australia, in spite of the missing snout. The genusTrilophosuchus was erected in 1993 with the description of the remains byPaul Willis of the University of New South Wales in theJournal of Vertebrate Paleontology.[2][3] In 2022 and 2023, two papers were published detailing theendocast and the skull anatomy ofTrilophosuchus, greatly expanding upon the research previously published by Willis. The first of these two studies made use of high resolutionμCT scan to map the shape of the crocodilian's brain. This madeTrilophosuchus the second mekosuchine with available information on the neuroanatomy afterPaludirex.[1][4]
The nameTrilophosuchus derives fromAncient Greek and translates to "triple-crested crocodile", referring to the three parallel ridges that run along the animal's skull table. Thespecies name meanwhile honors Alan Rackham Senior, a supporter of the excavations in the Riversleigh WHA.[2]
Trilophosuchus had a short, deep skull and large eyes that face more to the sides than they would in modern semi-aquatic crocodiles. The snout, though incomplete, is thought to have been very robust, showcasing a trapezoid cross-section just before the eyes.[1] Due to its robust and boxy skull,Trilophosuchus has been compared to a variety of other crocodylomorphs by Willis. Among these are bothOsteolaemus (dwarf crocodiles) andPaleosuchus (dwarf caimans), but Willis also points at a variety of Mesozoic forms, in particularnotosuchians,Theriosuchus andProtosuchus.[2] Comparison withMekosuchus is also common, not only due to their similar morphology but the fact that both are known to be closely related.[3] This is evident in part due to the relation between themaxillary bone and the orbital margins. The eye sockets ofMekosuchus are in part formed by the maxilla, which is unique among all crocodilians. While this is not quite the case inTrilophosuchus, the only thing preventing the maxilla from contacting the orbital margin is a small contact between thelacrimal bone andjugal bone.[1][5]
The palatal fenestrae, two openings in the underside of the skull, dominate thepalate and reach up to the level of the sixth maxillary teeth. They are notably wider in this region and taper further towards the back of the skull, where they are constricted by theectopterygoids. Thepalatine bone has an anterior process, or projection, that reaches to the level of the fourth maxillary tooth. Behind the palatine and the palatal fenestra, there are largepterygoids and ectopterygoids that project posteriorly and extend below the skull. The ectopterygoid is unique in that its plate bears a large and shallow cancavity. An unusual feature of the pterygoids meanwhile is that they are partially fused along the midline in the region preceding thechoanae. Thejugal bones, which line the side of the skull below the eyes, project laterally to form prominent ridges. The supratemporal fenestrae, two openings on theskull table, long and narrow and have been compared to the shape of an almond. The entire skull table ofTrilophosuchus is somewhat moved forward, which reveals the underside of thebasioccipital. This effectively creatures what Willis refers to as a "bend" in the occipital face. The upper portion is vertical, while the lower portion is inclined at a 30° angle forward.[2][1]
The most distinct feature ofTrilophosuchus, and the one that gives this taxon its name, is the presence of three prominent crests that extend alongside the length of the crocodilian's skull table. The central crest originates on the broadfrontal bone between the eyes and continues onto theparietal bone before ending approximately at the same level as the supratemporal fenestrae. Two additional crests run parallel to this medial ridge, originating at the orbital margin and stretching from there until the end of the skull table, occupying the space between the midline and the fenestrae. Willis also notes the presence of small knobs on the outer side of the skull table, separated from the three ridges by the supratemporal fenestrae. These may represent an additional pair of parallel crests, tho not as distinct as the central three.[2][3][1][5]
As no specimen ofTrilophosuchus preserves the front of the skull, the precise number of teeth is currently unknown. Assuming that the largest preservedtooth socket is that of the fifth tooth in themaxilla, as in other crocodilians, it may have had between 12 and 13[1] maxillary teeth on either side. The individual tooth sockets, and by extension the teeth, are spaced regularly throughout most of the toothrow. However, between the sixth and seventh alveoli this even spacing is interrupted by adentary tooth of the lower jaw that interlocks with the upper dentition and creates a prominent notch in the maxilla.[2] Based on the known remains,Trilophosuchus shows a high degree of festooning, meaning the maxillae are distinctly wave-like in their form, visible in both side and top view. The degree of festooning is consistent with adult crocodilians, but may also be exaggerated by the shortened rostrum. The first wave of the maxilla peaks with what is thought to be the fifth maxillary tooth before ascending to the level of the dentary notch. A second wave is present behind this one, peaking around the 11th or 12th tooth. No known specimen ofTrilophosuchus preserves the teeth themselves.[1]
Like in other crocodylomorphs, the cranialendocast does not accurately reflect the shape of thebrain itself and is instead also influenced by the extensivedural envelope. The endocast regardless gives a general idea of the brain shape, which is congruent with the linear and elongated endocasts of other crocodylomorphs. One of the most unique features ofTrilophosuchus is the presence of an acute dural peak, an inflation above the hindbrain. This subtriangular peak, in combination with a deeply concave midbrain, gives the endocast ofTrilophosuchus a highly distinct form. Such dural inflations are generally not well developed in other crocodilians, with the onlyeusuchians sharing a clearly distinct dural peak beingAgaresuchus andArenysuchus, though in both of which the peaks are blunt, not acute. Instead, the endocast ofTrilophosuchus compares much more favorably to the notosuchiansAraripesuchus andSebecus as well as thedyrosauridRhabdognathus. Among these threeAraripesuchus in particular is highlighted as the most similar in the overall shape of the endocast.[4]
Theinner ear features a distinctly sub-triangular vestibular apparatus (where thesemicircular canals contact thecommon crus), as opposed to being more rounded like in other modern crocodilians. Although this does somewhat resemble animals likeJunggarsuchus andProtosuchus, the individual elements are still robust as in other eusuchians, rather than slender. A standout feature of the inner ear concerns the ratio between the height of the common crus and various other elements, in particular the height and length of theendosseous labyrinth and the height of the vestibular apparatus. In all three instances, these ratios are amongst the highest of the analyzed crocodylomorphs, often producing values similar to various basal crocodyliforms, crocodylomorphs,sebecosuchians andthalattosuchians.[4]
In addition to elements of the brain itself, the endocast also reveals several details on the pneumaticity ofTrilophosuchus' skull. While crocodilian skulls are generally pneumatized through the presence of air-filled pockets within the skull,Trilophosuchus stands out as having one of the most heavily pneumatized skulls. Among extant forms, it strongly resembles and even exceeds dwarf crocodiles (Osteolaemus) and dwarf caimans (Paleosuchus), and more broadlyalligatoroids as a whole, whereasgavialoids and othercrocodyloids (sansOsteolaemus) are less extensively pneumatized. While this makesTrilophosuchus one of the most highly pneumatized eusuchians, this system of air pockets is not nearly as complex as in many notosuchians.[4]
Several anatomical features ofTrilophosuchus have led to discussion regarding the age at which the holotype specimen died. Both theforamen for thetrigeminal nerve and theforamen magnum (through which thespinal cord attaches to the brain) are large and the supratemporal fenestrae are elongated and compressed. The inclination of theoccipital face also differs from most adults crocodilians. The skull table, though usually flat in adults, slopes inTrilophosuchus from the midline. While all of these features and the overall small size ofTrilophosuchus could be taken as evidence that it was a juvenile, both Willis and Ristevski have argued to the contrary. Notably, both the fusion of the skull bones and the sculpting on their surface indicate that it was in fact an adult at the time of its death. They further point out that although the compression of the supratemporal fenestrae is a juvenile trait, their orientation does not match this interpretation, while the inclination of the occipital face could also be explained as being the ancestral condition. This would suggest that rather than being immature,Trilophosuchus was simply a dwarf species, with the juvenile characteristics being the result ofpaedomorphy.[2][3][1][5] The shape of the endocast also supports this conclusion, showing morphologically mature characters such as the relatively featureless midbrain.[4]
The skull ofTrilophosuchus only measures 79 mm (3.1 in) from the back of the head to the front most preserved part of the rostrum, making it very small even when accounting for the missing material. Although no remains of the body are known,Trilophosuchus is estimated to have been around 70–90 cm (28–35 in) long and weighed 1–2 kg (2.2–4.4 lb). These values are calculated through aregression analysis using the width across the quadrates, based on data collected from modern crocodilians. This would placeTrilophosuchus in the size range of many crocodilians colloquially referred to as dwarf species, including dwarf caimans, dwarf crocodiles, theChinese alligator andMekosuchus, all of which rarely exceed 2 m (6 ft 7 in) in length.[1]
Despite its superficial resemblance to early crocodyliforms such asprotosuchians and notosuchians,Trilophosuchus is an advanced crocodylian. While research on Australian crocodilians was still in its early days during the time ofTrilophosuchus description, it was already recognized that it formed amonophyletic clade with the generaQuinkana,Pallimnarchus,Baru andAustralosuchus. In this firstphylogenetic tree,Trilophosuchus was recovered as the basalmost member of the then still unnamed clade of endemic crocodiles.[2] Additional studies and new discoveries in subsequent years led to a better understanding of the group, eventually dubbed Mekosuchinae following the inclusion ofMekosuchus from New Caledonia and Australia. The addition ofMekosuchus had a major impact on howTrilophosuchus was placed among members of Mekosuchinae, with Willis (1997) suggesting that the two were derived members of the family andsister taxa, withQuinkana as their next closest relative.[3] Meadet al. (2002) also recovered such a clade, but placedTrilophosuchus as the basalmost genus in it, outside of the group formed byQuinkana,Mekosuchus andVolia (which had not been named at that point in time).[6] This marks a trend in coming phylogenies, which recover broadly the same results, but differ in the precise topology of these derived forms. One outlier to this is the phylogenetic tree recovered by Steinet al. (2018), which recovered it as a more basal form related toUltrastenos. However, this result has been deemed irregular and was likely caused by the inclusion of problematic taxa likeHarpacochampsa.[7]
Shown below are three different phylogenetic trees, the first two showing early trees that established the close relationship betweenTrilophosuchus andMekosuchus while the third shows it as the sister taxon toUltrastenos.
Willis (1997) | Meadet al. (2002)
| Steinet al. (2018)
|
Putting aside the results of Steinet al. and minor fluctuations between its closest relatives,Trilophosuchus maintained its relative stability throughout the 21st century, as evidenced by the work of Lee and Yates (2018). In their research,morphological, molecular (DNA sequencing), andstratigraphic (fossil age) data once again showed support forTrilophosuchus being part of an altirostral clade includingVolia,Mekosuchus andQuinkana, only differing slightly in the internal relationships of this grouping.[8] A more recent phylogenetic analysis on mekosuchines was published by Ristevski and colleagues in 2023. While the relation betweenTrilophosuchus,Mekosuchus andVolia once again shifted slightly, the only major departure from prior studies was thatQuinkana was no longer found to be a close relative of them, instead grouping withPaludirex andBaru.[5] The re-evaluation ofUltrastenos and"Baru" huberi eventually came to lend credence to the tree previously published by Steinet al. in 2016, as the results indicate that"Baru" huberi andUltrastenos were the same taxon and the sister to the clade that includesTrilophosuchus,Mekosuchus andVolia while maintaining an overall topology similar to that of Ristevskiet al. 2023.[9]
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In many aspects the jaw musculature ofTrilophosuchus compares well with that of modern crocodilians, with one of the more notable departures being caused by the shape of the supratemporal fenestrae. Due to them being much narrower than in extant forms,Trilophosuchus would rely much more heavily on the musculature of the pterygoid. More substantial are the changes to the neck musculature. The inclination of the back of the skull causes a notable shift in orientation for the musculature that flexes the head ventrally (down) and further indicates that the spinal column would likewise be positioned further down relative to the head. One of the muscles responsible for moving the head side to side in modern crocodiles (the M. obliquus capitis magnus) appears to be relatively poorly developed inTrilophosuchus, with lateral movement instead being more prominently controlled by the longissimus capitis, pars transversalis capitis muscle. The M. spinalis capitis and M. rectus capitis posterior, two muscles responsible for lifting and rotating the head, are positioned higher above the occipital condyle, effectively allowingTrilophosuchus to lift the head with more power.[2]
All of these combined gives several clues as to howTrilophosuchus differed in its posture and lifestyle from modern crocodiles. The inclination of the posterior skull and the neck musculature indicates that this genus held its head higher above the body than most modern taxa, with Willis comparing it to baskingcaimans andagamid lizards. Modern crocodiles have strong obliquus capitis magnus muscle, which allow them to overcome the resistance of the surrounding water when moving their head side to side during hunting. However, as this muscle is only poorly developed inTrilophosuchus, the animal likely didn't have to deal with the same resistances as today's crocs and subsequently needed less power to begin shaking its head. Despite this, the well developed longissimus capitis, pars transversalis capitis muscle indicate that side to side movement was nonetheless important, as this muscle functions to maintain rapid lateral motion while feeding. Similar conclusions were previously reached by Busbey in 1977 in regards toBoverisuchus, a terrestrial eusuchian from thePaleogene.[2][3]
Another possible method of determining the paleoecology ofTrilophosuchus would be to compare it with other, better understood forms. In the original description ofTrilophosuchus, Willis briefly touches upon the similarities betweenTrilophosuchus and the modern dwarf caimans and crocodiles, but notes that at that time information was inconsistent. Due to this Willis did not pursue the comparison any further. Instead, Willis looked to older forms like notosuchians andprotosuchids, which are generally considered to be terrestrial animals based on not only their skulls but also their limb proportions. As Willis consideredTrilophosuchus to have been more similar to these forms than to modern dwarf species, he tentatively suggested thatTrilophosuchus was a terrestrial animal. This hypothesis aligns with what other authors have suggested for the mekosuchines most closely related toTrilophosuchus. Molnar, Worthy and Willis suggested thatVolia from Fiji was terrestrial based on the lack of other land predators and the anatomy of the ulnare, while terrestrial or semi-terrestrial habits are widely inferred forMekosuchus based on a variety of factors.[2]
Willis also comparesTrilophosuchus toBoverisuchus andSebecus, two terrestrial crocodylomorphs from the early Cenozoic, noting major differences in the shape of the rostrum. While the latter have deeper and more elongated jaws and grow notably larger,Trilophosuchus was small with a boxy skull, a sign that it did not inhabit the same niche even if it was terrestrial. Willis proposes that the difference between these large terrestrial crocodylomorphs andTrilophosuchus may be analogues to the differences between largemonitor lizards like theKomodo dragon and smallagamas andiguanas. This would suggest thatTrilophosuchus was restricted to small prey, fitting its small body size.[2]
Beyond the overall morphology, clues towardsTrilophosuchus' ecology have also been drawn from the shape of its brain. Several researchers for instance have previously found a correlation between the cranial pneumaticity and aspects of the respective animal's lifestyle, which is briefly discussed by Ristevski and colleagues while describing the neuroanatomy ofTrilophosuchus. Uniquely,Trilophosuchus displays a general mix of features found in Crocodylia and features found in much more basal groups. The overall shape of the endocast has been compared most closely to the terrestrial notosuchianAraripesuchus, while the ratios between the elements of the inner ear shows similarities to some baurusuchids and sphenosuchians among others. The highly pneumatized skull ofTrilophosuchus once again mirrors the condition ofPaleosuchus,Osteolaemus and various notosuchians, while poorly pneumatized skulls on the other hand appear prominently in highly aquatic forms with slender snouts and powerful abductor muscles, such as thalattosuchians and dyrosaurids. Ristevski concludes from this thatTrilophosuchus may have been ecologically similar to today's dwarf crocodilians while being overall more terrestrial than any extant crocodilian.[4][5]
As a species native to what is now the Riversleigh World Heritage Area,Trilophosuchus was part of a highly diverse crocodilian fauna. Both the Hiatus Site and the Ringtail Site have yielded multiple crocodilians of different size and shape, whose coexistence is generally easily explained byniche partitioning. For instance, the Ringtail Site also preserves the bones ofBaru, a large semi-aquatic ambush predator that is unlikely to have competed for the same resources as the much smallerTrilophosuchus. Not as easily explained is the apparent coexistence ofTrilophosuchus andMekosuchus sanderi. Although known primarily from fragmentary remains,Mekosuchus is also thought to have been a terrestrial animal with an altirostral (high) skull. It is thus unclear how the two would have differed in their ecology. Although it is possible that both genera inhabited different areas and only ended up in the same deposits during preservation (thanatocoenosis), this possibility is deemed less likely due to the composition of the mammal fauna indicating a singular origin for the different fossils.[10] The alternative explanation would be thatTrilophosuchus andMekosuchus may have differed significantly enough in parts of their anatomy that are not currently known, allowing them to exploit different niches.[11][3]
The Ringtail Site is currently interpreted as a preserving what was once a pool located in a forestedkarst terrain, preserving primarily aquatic animals that, aside from the crocodilians, includeslungfish,chelid turtles and the extinct platypusObdurodon dicksoni. Terrestrial animals are known in the form ofpossums, which gave the site its name and are indicative of trees being present around the pool.[12]
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