| Phytosaurs | |
|---|---|
 | |
| Skeleton ofRedondasaurus at theCarnegie Museum of Natural History | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Archosauromorpha |
| Clade: | Archosauriformes |
| Clade: | Eucrocopoda |
| Clade: | Crurotarsi |
| Order: | †Phytosauria von Meyer, 1861 |
| Subgroups | |
| Synonyms | |
ParasuchiaHuxley, 1875 | |
Phytosaurs (Φυτόσαυροι inGreek, meaning 'plant lizard') are anextinct group of large, mostlysemiaquaticLate Triassicarchosauriform orbasalarchosaurian reptiles. Phytosaurs belong to theorderPhytosauria and are sometimes referred to asparasuchians. Phytosauria, Parasuchia, Parasuchidae, and Phytosauridae have often been considered equivalent groupings containing the same species. Some recent studies have offered a more nuanced approach, definingParasuchidae andPhytosauridae as nested clades within Phytosauria as a whole. The clade Phytosauria was defined by Paul Sereno in 2005 asRutiodon carolinensis and all taxa more closely related to it than toAetosaurus ferratus,Rauisuchus tiradentes,Prestosuchus chiniquensis,Ornithosuchus woodwardi, orCrocodylus niloticus (the Nile crocodile).[3] Phytosaurs were long-snouted and heavily armoured, bearing a remarkable resemblance to moderncrocodilians in size, appearance, and lifestyle, as an example ofconvergence orparallel evolution.
The name "phytosaur" means "plant lizard", as the first fossils of phytosaurs were mistakenly thought to belong to plant eaters.[4]
For many years, phytosaurs were considered to be the mostbasal group ofPseudosuchia (crocodile-linearchosaurs), meaning that they were thought to be more closely related to the crocodilians than to birds (the other living group of archosaurs). Some studies of the evolutionary relationships of early archosauriforms have suggested that phytosaurs evolved before the split between crocodile- and bird-line archosaurs and are asister taxon of Archosauria. The most recent study retains the former way of classifying phytosaurs as pseudosuchians.[5]
Phytosaurs had a nearly global distribution during the Triassic. Fossils have been recovered fromEurope,North America,India,Morocco,Thailand,Brazil,Greenland[6] andMadagascar. Fossils attributed to phytosaurs have been found in Early Jurassic rocks, possibly extending their temporal range beyond the Triassic-Jurassic boundary.[2] They may have also been present in rock layers dating to the Middle Triassic of China as evidenced byDiandongosuchus, however it is not known if this is truly a member of the clade.
Phytosaurs are known from many different morphologies, specifically with vastly different skull forms. These changes relate to the feeding and habits of the animals, not completely evolutionary modifications. Dolichorostral ("long snouted") phytosaurs have a long, slender snout with manyconical teeth that arehomodont (all the same). These taxa were most likelypiscivores that were well adapted to capture fast aquatic prey, but not terrestrial animals.Paleorhinus,Rutiodon andMystriosuchus are dolichorostral phytosaurs, but do not form a distinct group of taxa (named Mystriosuchinae ofFriedrich von Huene) as other morphotypes such asPseudopalatus are more closely related toMystriosuchus than it is to the other long-snouted taxa. Brachyrostral ("short snouted") forms are the opposite, having a massive, broad snout, and very strong skulls and jaws. They areheterodont, as the front teeth are prominent fangs, and the rear teeth are blade-like for slicing food into chunks that can be swallowed easily. Taxa like this, such asNicrosaurus andSmilosuchus, were powerful taxa that fed on stronger prey, such as terrestrial animals that came to the water to drink. Altirostral ("high snouted") animals are intermediate between the two distinct types. They had heterodont dentition but not as extremely developed as the brachyrostral type.Pseudopalatus is an altirostral phytosaur, and was most likely a generalist feeder. Modern crocodilians exhibit a similar morphological diversity, for example the broad snouted altirostralalligator and the long snouted dolichorostralgavial.[7]
Various phytosaurs have crests and similar ornamentions in their snouts.Nicrosaurus has a ridge along the snout that would have supported a keratinous crest in life, whileMystriosuchus westphali has several bony crests.[7]
Despite their great similarities in appearance and lifestyle, there are still a number of minor differences that distinguish phytosaurs from true crocodiles. For one thing, the phytosaurankle structure is much more primitive than that of any crocodile. Also, phytosaurs lack the bonysecondary palate that enables crocodiles to breathe even when the mouth is full of water. However, it is possible that phytosaurs had a fleshy palate, as many Mesozoic crocodiles are presumed to have had. Phytosaurs were even better armoured than crocodiles, protected by heavy bonyscutes (often found as fossils), and the belly reinforced with a dense arrangement ofgastralia (abdominal ribs). Finally, and most noticeably, phytosaurs had nostrils placed near or above the level of the eyes, in contrast to crocodiles where the nostrils are near the end of the snout. This adaptation may have developed to allow them to breathe while the rest of the body was submerged.
Unlike most crocodilians, phytosaurs have tooth serrations.[8]
In a 2001 study of the biomechanics of the dinosaurAlbertosaurus's teeth,William L. Abler also examined a phytosaur's teeth, finding that it has had serrations so fine that they resembled a crack in the tooth.[9]Albertosaurus had similarly crack-like serrations, but, at the base of each serration Abler discovered a roundvoid, which would have functioned to distribute force over a largersurface area.[9] This void, termed an ampulla, would hinder the ability of the "crack" formed by the serration to propagate through the tooth.[9] The phytosaur was found to lackadaptations for preventing its dental "cracks" from propagating.[9] Abler examined another sort of prehistoric predator,Dimetrodon, and found that it also lacked adaptations for guarding against crack propagation.[9] Based on their teeth, most phytosaur genera are carnivorous, piscivorous, or a combination of the two. However, two taxa show slight adaptations towards hunting and consuming harder invertebrates.[10]
A study on phytosaur microwear patterns has foundMystriosuchus to line with soft invertebrate consumers,Nicrosaurus with hard invertebrate consumers andSmilosuchus andMachaeroprosopus with carnivores and piscivores.[11]
Phytosaurs have been traditionally held as rather "primitive" animals in regards to terrestrial locomotion, particularly in regards to archosaurs such ascrocodilians, lacking the erectgait seen in these, otherpseudosuchians,dinosaurs andpterosaurs. However, theApatopusichnofossil shows that the animals did in fact have an erect gait like their archosaur relatives.[12][13]
Most phytosaurs are thought to be aquatic animals, and indeed most do show adaptations for such a lifestyle; swim tracks attributed to phytosaurs, for example, are known.[14] However, at leastNicrosaurus seems to have evolved towards a secondarily terrestrial lifestyle, developing longer limb bones, straighter femora and a deeper pelvis, and indeed occurs in terrestrial or marginal lacustrine settings. Combined with its deep upper jaw, it probably led a similar lifestyle to terrestrial predatorycrocodylomorphs likesebecians.[15]
Inversely, some dolichorostral forms likeMystriosuchus have become further specialised to life in the water, and occurred in marine environments.[16] A skeleton ofMystriosuchus planirostris, found in a marine setting and with evidence of little post-mortem transportation – indicating that it died either at sea or in a freshwater environment nearby – shows that this animal had paddle-like limbs, less adapted for terrestrial locomotion than in most other phytosaurs.[17] Furthermore, the tail ofMystriosuchus was laterally compressed and could have been used in propulsion.[18]
Scans on various phytosaur braincases suggest that these animals generally had long olfactory tracts, weakly demarcated cerebral regions, dorsoventrally short endosseous labyrinths and various sinuses, including large antorbital and dural venous ones; the general bauplan is vaguely similar to that of crocodilians, but differs significantly in the presence of multiple sinuses, smaller cerebral hemispheres and smaller endosseous labyrinths. The similarities are considered to be plesiomorphic in relation to the ancestral archosauriform design, lacking many features seen inavemetatarsalians, though convergence in terms of lifestyle might also play a role.[19]
No phytosaur eggs have been found so far. There are pits associated with footprints in theChinle Formation, but these "nests" are apparently the result ofsandstone weathering.[20] A recent study suggests they might have had parental care.[21]
When the first phytosaur fossils were found, it was not immediately obvious what kind of animal/species they were. The first phytosaur species known to science was namedPhytosaurus cylindricodon – "plant lizard with cylindrical teeth" – by G. Jaeger in 1828 because he mistakenly believed that petrified mud fillings in the jaw were herbivore teeth. The specimen is too poor to be diagnostic, and this species name is no longer valid. The name of the group – Phytosauria – was coined by the German paleontologistHermann von Meyer in 1861, on the basis of this first species.
The next species to be described wasBelodon plieningeri by von Meyer in von Meyer and Plieninger 1844. The altogether more appropriate name Parasuchia ("alongside the crocodiles", as they resembled crocodiles to a great degree) was coined byThomas Huxley in 1875 along with his discovery and naming of the Indian speciesParasuchus hislopi (Chatterjee, 1978), on the basis of a partial snout. The specimen also is usually considered non-diagnostic, and the nameParasuchus replaced byPaleorhinus. Although the names Parasuchidae and Phytosauridae are variously still used by different specialists, "phytosaur" is the standard generic name for these animals, despite the fact that these animals have been clearly shown to becarnivorous.
Phytosaurs first appeared during theCarnian orLadinian age, evolving from an unknowncrurotarsan ancestor. There are no clear intermediate forms, as even the earliest known phytosaurs are highly specialized aquatic animals, unlike most contemporary archosauriforms that were terrestrial. However, a recent study has suggested thatDiandongosuchus is a basal phytosaur. If this is the case, this taxon offers more of a bridge between phytosaurs and earlier Archosauriformes.[22]
The earliest phytosaurs are traditionally classified in the genusPaleorhinus, now thought to bepolyphyletic.Parasuchus and related basal species were widely distributed, meaning that phytosaurs dispersed acrossPangea early on and there were probably few geographical barriers for their distribution; only in the southernmost regions are they rare, possibly due to increased aridity.[23]
A somewhat more advanced and larger form,Angistorhinus appears at the same time or soon after. Later in the Carnian, both these animals were replaced by more specialised forms likeRutiodon,Leptosuchus, and the hugeSmilosuchus (Lucas 1998). TheCarnian-Norian extinction meant that these animals died off, and the EarlyNorian sees new genera likeNicrosaurus andPseudopalatus, both of which belong to the most derived clade of phytosaurs, thePseudopalatinae. Later in the middle Norian the advanced and specialised fish-eaterMystriosuchus appears. Fossil remains of this widespread animal is known from Germany, northern Italy, and Thailand. Finally the largeRedondasaurus in southwest North America and the long-snouted (altirostral)Angistorhinopsis ruetimeyeri in Europe continued the group into theRhaetian. Phytosaur footprints (theichnotaxonApatopus) are also known from the latest Rhaetian of the East Coast of USA (theNewark Supergroup) (Olsenet al. 2002). This indicates that phytosaurs continued as successful animals until the very end of the Triassic, when, along with many other large crurotarsan reptiles, they were killed off by theend Triassic extinction event, about 200 Ma ago.
There have been reports of phytosaur remains found in lowermost Jurassic rocks. Several teeth from Early Jurassic deposits in France have been identified as phytosaur teeth, but other studies argue they have either been misidentified or were reworked from Late Triassic into Early Jurassic deposits. In 1951, a partial upper jaw was discovered in the Early JurassicLower Lufeng Series inChina and described as a new genus of phytosaur,Pachysuchus, but a study in 2012 reinterpreted the fossil as asauropodomorph dinosaur.[24]
A fragment of a lower jaw from a longirostrine archosaur has been described from earlyHettangian strata in the town ofWatchet inSomerset, England. Whileteleosauridthalattosuchians had similar longirostrine jaws to phytosaurs and were common in the Jurassic, they do not appear in the earliest Jurassic rocks. The mandible is more similar to those of known phytosaurs than to thalattosuchians, and likely belongs to a phytosaur closely related to the genusMystriosuchus. The presence of phytosaurs in the earliest Jurassic may have prevented thalattosuchians from occupying similar ecological niches at that time.[25] However, more recent work suggests that the jaw fragment came from a pre-Hettangian rock unit, and is therefore Late Triassic in age.[24] Also, if the age of theMagnesian Conglomerate does extend into theEarly Jurassic (Hettangian), then it is possible thatRileyasuchus survived into the Early Jurassic.[26]
| Genus | Status | Age | Location | Unit | Notes | Images |
|---|---|---|---|---|---|---|
| Junior synonym | Junior synonym ofNicrosaurus | |||||
| Valid | LateCarnian |  | ||||
| Valid | MiddleCarnian | |||||
| Junior synonym | Junior synonym ofMachaeroprosopus | |||||
| Nomen dubium | Middle Norian | Many remains have since been attributed to other animals or given their own genera |  | |||
| Valid | Carnian | "Pre-Tecovas Horizon" (Dockum Group) | ||||
| Nomen dubium | Norian | Named from several teeth in 1856 | ||||
| Valid | Norian | |||||
| Colossosuchus | Valid | Carnian-Norian |  India | Tiki Formation | ||
| Valid | Ladinian | Possible basalmost taxon. |  | |||
| Valid | Late Carnian | |||||
| Nomen dubium | Late Carnian | |||||
| Valid | Carnian | Tecovas Formation and unknown formation (Dockum Group) | ||||
| Valid | Carnian-Norian |  | ||||
| ? Valid | Triassic, ? early Olenekian | |||||
| Valid | Middle Norian | |||||
| Nicrosaurus | Valid | Late Norian – Rhaetian |  | |||
| Junior synonym | Late Carnian | Hassberge Formation | Polyphyletic. | |||
| Valid | Late Carnian – Early Norian | India |  | |||
| Valid | Norian | |||||
| Valid | ||||||
| Valid | Norian |  | ||||
| Junior synonym | Junior synonym ofMachaeroprosopus | |||||
| Valid | Norian – Rhaetian |  | ||||
| Valid | Carnian |  | ||||
| Valid | Carnian |  | ||||
| Valid | Carnian | India | Tiki Formation | |||
| Valid | Carnian-Norian |
Phytosaurs are generally regarded as the most basal group of Crurotarsi, aclade of archosaurs that includes crocodilians and their extinct relatives.[27][28][29] Phytosaurs are often excluded from a clade calledSuchia, which usually encompasses all other crurotarsans, includingaetosaurs,rauisuchians, andcrocodylomorphs.[29] Some studies have foundpolytomies between phytosaurs and other groups, likeOrnithosuchidae and Suchia. In these cases, it is unclear whether phytosaurs are the most basal crurotarsans.[30] In one of the earliest studies of crurotarsanphylogeny, Sereno and Arcucci (1990) found Crurotarsi to be amonophyletic grouping consisting of phytosaurs, ornithosuchids, and the morederived suchians, but produced a trichotomy between the three groups in their tree.[31] In resolving this trichotomy, Parrish (1993) placed ornithosuchids, not phytosaurs, as the most basal crurotarsans. However, most other studies, such as Sereno (1991) and Bentonet al. (2010), recover phytosaurs in a basalmost position among crurotarsans.[32] Below is acladogram modified from Bentonet al. (2010) showing the widely accepted phylogenetic relationships of phytosaurs:[29]
A phylogenetic analysis of early archosaurs by paleontologistSterling Nesbitt (2011) found strong support for asister taxon relationship between phytosaurs and Archosauria.[3] If this is the case, phytosaurs would be placed outside Pseudosuchia in a more basal position among archosauriforms. Phytosaurs would be considered closely related to the ancestors of both crocodilians anddinosaurs. Furthermore, the definition of the clade Crurotarsi would change, as it is often defined by the inclusion of phytosaurs. Thus, Crurotarsi would include phytosaurs and all other archosaurs —including dinosaurs— under this phylogeny.[33] Below is a cladogram showing the placement of phytosaurs from Nesbitt (2011):[3]
The phylogenetic analysis of Stocker (2010) placedPaleorhinus outside Phytosauridae as a basal phytosaur. Under this phylogeny, Phytosauridae and Phytosauria are not synonymous. Stocker also erected the clade Leptosuchomorpha for derived phytosaurs, includingLeptosuchus andSmilosuchus.[34]
Ezcurra (2016) updated Nesbitt's analysis and found that Phytosauria was once again a group of basal pseudosuchian archosaurs. His study analyzed the ten phylogenetic traits which Nesbitt claimed were lacking in phytosaurs but not archosaurs, thus excluding phytosaurs from Pseudosuchia. Four of the traits (well-developedpalatal processes of the maxilla which meet at the midline, an elongated cochlear recess, a tuber on the lateral side of theulna, and a particular orientation of thecalcaneal tuber) were confirmed to support Nesbitt's placement of Phytosauria. However, one of the ten traits was found inEuparkeria (anabducens nerve exit foramen only present in the prootic) and another was found inproterochampsians (a swollenbiceps tubercule), so their lack in phytosaurs may be reversals rather than basal traits. Another one of the traits (anantorbital fossa contacting the horizontal process of the maxilla) was found in the basal phytosaurParasuchus. One trait (shortmetacarpals compared tometatarsals) was difficult to analyze in anycrurotarsan, and another (a medial tuber on thefemur) was found in bothproterochampsids andParasuchus. One trait (a divided tibial facet of theastragalus) was also lacking inMarasuchus andNundasuchus, and therefore had a variable existence in Archosauria. This reanalysis, along with the observance of many traits linking Phytosauria with pseudosuchians, concluded that it was more likely that phytosaurs were pseudosuchians than non-archosaur archosauriforms. The following cladogram is a simplified version the fourth strict reduced consensus tree of Ezcurra's third phylogenetic analyses within his study. This cladogram only shows taxa from the groupEucrocopoda.[35]
In the Late Triassiccoprolite which could belong to a phytosaur, eggs ofnematodes and probablyprotozoancysts were found.[36]
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