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Fossil range: Early Triassic-Late Triassic
Life restoration of Hyperodapedon.
| Scientific classification
Rhynchosaurs (meaning "beaked lizards") were a group of unusual herbivorous quadrupedal archosauromorphs that lived during the Triassic period. Rhynchosaurs ranged in size from the 50 cm long Rhynchosaurus to the 2 meter (6 feet) long Hyperodapedon, with the average size being 1 meter (3.3 feet). Rhynchosaurs were a widespread and worldwide taxon, being found all across the supercontinent of Pangaea. Rhynchosaur fossils have been found in Britain, South Africa, Zimbabwe, Tanzania, Madagascar, India, Brazil, Argentina, Canada and the United States, although they are poorly represented in the Northern Hemisphere fossil record. 15 species are currently regarded as valid, and another five are valid taxa still in need of a name. In some fossil assemblages, several taxa lived alongside one another, as evidenced by the four contemporaneous species were contemporaneous in the Upper Triassic Santa Maria Formation of Brazil. Rhynchosaurs went extinct during the Permian-Triassic extinction event that marked the end of the Carnian stage of the Late Triassic.
Rhynchosaur fossils are very abundant in some assemblages (in some fossil localities accounting for 40 to 60% of specimens found) and the anatomy and ontogeny of a few species is comparatively well known. Early primitive forms like Mesosuchus and Howesia were more typically lizard-like in build, and had skulls rather similar to the early diapsid Younginia, except for the beak and a few other features.
Rhynchosaurs can be classified into three groups: one clade and two grades. Basal rhynchosaurs, including Mesosuchus, Howesia, and Noteosuchus from the Early Triassic, share with all later forms a united naris, down-turned premaxillae and the presence of several maxillary tooth rows, but they were less 'extreme' in morphology than the more derived rhynchosaurs, all of which are united in Rhynchosauridae. Basal rhynchosaurids form the second group. These animals had deeper, shorter skulls than basal rhynchosaurs, a more robust body, stockier limbs that were more similar in length than those of basal rhynchosaurs, and a much shorter tail. The several Rhynchosaurus species belong here, as does the African Stenaulorhynchus and the English Fodonyx. Traditionally, these taxa were classified together as the Rhynchosaurinae. However, this group is evidently paraphyletic and has now been abandoned.
Hyperodapedontines form the third and final group. These rhynchosaurs were the largest ones, and had broad, short, and deep skulls with the most complicated tooth fields. Their fore- and hindlimbs were nearly equal in length, and their bodies were bulkier than those of other rhynchosaurs. Hyperodapedontinae is defined as a branch-based taxon that includes all rhynchosaurs closer to Hyperodapedon than to Fodonyx. The branch-based definition of Hyperodapedontinae means that Ammorhynchus and 'Scaphonyx' sulcognathus are both part of the clade. Several hyperodapedontine species previously regarded as representing distinct genera, including Scaphonyx from Brazil and Paradapedon from India, were found to be nested within Hyperodapedon.Isalorhynchus from Madagascar has also been shown to be referable to Hyperodapedon.
Since the first rhynchosaur, Rhynchosaurus articeps, was named in 1842 by Richard Owen, there has been debate as to where they belong on the family tree. Richard Owen initially described some rhynchosaur material as that of a 'labyrinthodont' (= temnospondyl). He later realised this mistake and came to regard rhynchosaurs as close relatives of dicynodonts, as the tusk-like caniniforms of the latter were wrongly thought to be homologous with the paired premaxillary beaks of rhynchosaurs. An affinity with Sphenodon (tuatara) was also suggested, as the beak-like premaxillae of the tuatara was, again, thought homologous with the beaks of rhynchosaurs. Thomas Huxley also supported an affinity between rhynchosaurs and Sphenodon when he described Hyperodapedon in 1869, and he proposed the name Sphenodontina for this group.
While numerous different classification schemes were proposed during the course of the 20th century, it was mostly thought by the 1960s and 70s that tuatara and rhynchosaurs were close relatives, that they formed a group termed Rhynchocephalia, and that Rhynchocephalia was part of Lepidosauria (the group that also includes lizards and snakes).
Numerous studies published since the 1980s have shown that rhynchosaurs and the members of the tuatara clade were not really closely related. Pedro R. Burckhardt in 1900 even argued that the supposedly similar premaxillary beaks of tuatara and rhynchosaurs were actually completely different. Tuatara and kin - now termed the Sphenodontia - are lepidosaurs (and hence close to squamates), while rhynchosaurs are archosauromorphs: part of the same group as archosaurs and their relatives. Archosauromorpha has been defined as a branch-based taxon that includes all taxa closer to Protorosaurus than to Lepidosauromorphap. 528 and autapomorphies of the clade include slender cervical ribs, a notch on the leading margin of the interclavicle, and an ilium with a small anterior and large posterior process.
Within Archosauromorpha, the distribution of characters indicate that rhynchosaurs are closest to the Prolacerta and archosaur clade, with trilophosaurs and protorosaurs being successively more distant. Not all studies agree on this topology: Prolacerta was recovered as the sister taxon to a rhynchosaur and archosaur clade. Rhynchosauria and Archosauria are both node-based.
The basal members of all archosauromorph clades were long-tailed quadrupeds, superficially similar to living monitor lizards, and generally reahed about 50 cm in length. After appearing in the Late Permian, these basal forms had evolved and branched out into a diverse group by the end of the Early Triassic. In fact, the development of so many 'extreme' morphologies among Triassic archosauromorphs suggests that these animals faced heavy selection pressure.
Rhynchosaurs had stocky bodies and a powerful beak.
The most distinctive rhynchosaur characters are found in the skull, and their teeth and jaws are unlike those seen in any other animal. Seen from above, the rhynchosaur skull is vaguely triangular, with a narrow snout and broad posterior region. In later and more advanced genera the skull is short, broad, and triangular, becoming much wider than long in the most advanced forms like Hyperodapedon (= Scaphonyx), with a deep cheek region, and the premaxilla extending outwards and downwards to form the upper beak. In the most derived taxa, like Upper Triassic Hyperodapedon, the skull is nearly twice as wide as it is long. In these wide-skulled forms, gigantic supratemporal fenestrae virtually meet along the midline. This implies that rhynchosaurs had large jaw muscles and a powerful bite. The lower jaw was also deep, and when the mouth was closed it clamped firmly into the maxilla (upper jaw), like the blade of a penknife closing into its handle. This scissors-like action would have enabled rhynchosaurs to cut up tough plant material.
In all rhynchosaurs, the nostrils are united to form a large aperture on the midline. Another rhynchosaurian peculiarity is that the premaxillae are strongly down-turned relative to the maxilla. In the most basal known rhynchosaur, Mesosuchus browni from the Early Triassic of South Africa, there were two large, conical teeth in each premaxilla. In Howesia browni, an Early Triassic basal rhynchosaur, the premaxillae are unknown, and may also have had teeth. In all members of the mostly Middle and Late Triassic rhynchosaur clade Rhynchosauridae the premaxillae are toothless and form paired, tusk-like structures. When the jaws are closed, these tusk-like premaxillae fit in between the dorsally curved anterior tips of the dentaries. The 'tusks' are striated and were presumably horn-covered in life; distinct 'tide marks' show that the tissues that covered them were not continuous with those on the rest of the face. Large hyoid bones indicate the presence of a large, muscular tongue.
The short-faced, wide-skulled appearance of rhynchosaurid rhynchosaurs was less developed in the basal rhynchosaurs Mesosuchus and Howesia. These basal taxa did possess all of the key rhynchosaur autapomorphies, but they had longer, narrower skulls, longer, slimmer limbs and much longer tails than rhynchosaurids (50 vertebrae as opposed to 25-30). Mesosuchus and Howesia would have superficially resembled big lizards or tuataras in proportions appearence.
The large orbits of rhynchosaurs demonstrate large eyes and hence good eyesight. A large cavity within the nasal region suggests that the animals had good olfactory abilities, but the lack of a tympanic crest in the quadrato-squamosal area suggests that a tympanum was absent. It has been speculated that bone-conducted hearing might have been employed, and that skin membranes behind the quadrate may have permitted the detection of airborne sounds.
The teeth of rhynchosaurs were unusual, with the maxilla teeth and palate modified into broad tooth plates. Both the maxilla and dentary in rhynchosaurs possessed more than one row of teeth. The occlusal surfaces of these bones are vaguely rectangular (the longest axis being the one parallel with the long axis of the body) and covered on their surfaces with parallel lines of conical tooth crowns (the tooth rows are also parallel to the long axis. In basal rhynchosaurs, the teeth were relatively low in number and spaced out, but they were abundant and closely packed in the most derived taxa. The teeth themselves were deep-rooted cylinders with large pulp cavities. They were firmly fixed to the jaw bones; therefore, rhynchosaurs exhibited ankylothecodonty or ankylosed thecodonty. This is where distinct sockets are absent, and the deeply rooted teeth are held in place by both soft tissues and the surrounding jaw bone. The tooth crowns were either conical or pyramidal. The Late Triassic rhynchosaurid Hyperodapedon typically possessed both crown shapes, whereas other taxa, such as Middle Triassic Ammorhynchus, had conical crowns only.
Rhynchosaurid rhynchosaurs all possess a groove extending along the length of the maxillary tooth field. When the jaws were closed, the blade-like tooth row of the dentary fitted into this groove, thereby indicating that rhynchosaurids used precise shearing when feeding. Some rhynchosaurids had two maxillary grooves, matched by two dentary blades. In some taxa (such as Fodonyx), the medial maxillary groove only developed as the animal matured, while in others (such as Stenaulorhynchus), two maxillary grooves were present even in juveniles. However, most species of Hyperodapedon (the most derived rhynchosaurid) lacked a medial maxillary groove at all stages of growth, and–in the dentary–had a reduced number of tooth rows medial to the main tooth row. There are two interpretations of this:
- Hyperodapedon was pedomorphic compared to Middle Triassic rhynchosaurids, and that its single maxillary groove and simple medial part of the dentary tooth field were secondarily simplified relative to the double-grooved, more complex tooth fields of more basal rhynchosaurids.
- The double-grooved maxillary tooth field is derived relative to the single-grooved condition, and that the single-grooved species of Hyperodapedon are simply plesiomorphic.
Ammorhynchus from the Middle Triassic of Arizona is similar to basal rhynchosaurs and Hyperodapedon in being single-grooved, yet was found to be positioned between the double-grooved taxa Fodonyx and 'Scaphonyx' sulcognathus. Furthermore, while most Hyperodapedon species were single-grooved, H. huenei is double-grooved, and it was determined to be the most basal Hyperodapedon species.
The distribution of the double-grooved condition would therefore appear to be complicated, and it is possible that there were several convergences and reversals. There seems to be some ambiguity as to whether all specimens referred to some taxa - like Fodonyx - exhibit the same condition. Some have noted that more work is needed to better determine the distribution and phylogeny of the double-grooved condition.
Tooth replacement and wearEdit
The teeth were not replaced in the conventional reptilian fashion. Instead, new teeth were added to the back of the toothrow during ontogeny: as the animal grew, the size of the occlusal regions increased (The tooth-bearing platforms on the maxillae and dentaries became steadily larger). As this occured, the new teeth at the back moved forward and into the active tooth field. As the teeth moved through the field, they became more solid as dentine was depositing within their pulp chambers, and they also became worn down by wear, until eventually their surfaces were flush with those of the bone surface. Old teeth also became absorbed into their bases. The dual effects of wear and resorption mean that the oldest, most anterior teeth are frequently tiny or even absent.
Several tooth rows were also operating in parallel at any one time. The tooth field did not just expand posteriorly, but lingually as well (lingually = on the side of the tongue, the opposite of labially), so new tooth rows were added to the tooth field during growth. When teeth from different rows came into close contact, the younger teeth eroded the older ones. The teeth did not 'drift' through the tooth field: rather, the tooth field itself expanded backwards and sideways, all the while remodelling its bone and resorbing its older, worn teeth.
The hind feet were equipped with massive claws, presumably for digging up roots and tubers by backwards scratching of the hind limbs.
The rhynchosaur humerus is stout, with large crests for muscle attachment and a wide, flaring distal end. The rest of the forelimb appears fairly typical for a diapsid reptile. Many sources state that the rhynchosaur hindlimb appears well suited for scratch-digging (this is where curved claws are used, in a raking motion, to shift sediment), but few elaborate on it. While illustrations usually depict rhynchosaurs as ground-hugging animals with short, fully sprawling limbs, the details of their limbs and limb girdles show that their limbs were semi-erect and that their bodies were typically held well up off the ground.
In the shoulder girdle, the glenoid (= socket) is located far posteriorly and faces backwards and sideways (rather than being located closer to the middle of the girdle and facing fully sideways). These features and others indicate that a semi-erect forelimb gait could be adopted. In the hindlimb, the acetabulum (= hip socket) is broad and shallow, and faces downwards and backwards as well as sideways. When the in-turned head of the femur is articulated with the acetabulum, a semi-erect (rather than fully sprawling) posture seems to work best. The location of the distal condyles on the distal end of the femur (rather than on its ventral surface) indicate that the femur normally functioned in a near-vertical (rather than near-horizontal) pose, and the straight, column-like tibia also indicates a semi-erect stance. Rhynchosaur hands and feet were not close together and directly underneath the body, as they are in erect-limbed tetrapods, but they were closer together than those of truly sprawling tetrapods.
The hindlimb bones are thin anteroposteriorly, but broad mediolaterally. Large attachment areas were available for well developed flexors and extensors of the foot. Combined with the near-parasagittal (parasagittal = parallel to the long axis of the body) movement of the femur, this shows that the long, strongly clawed feet could have functioned well in digging.
The foot claws of rhynchosaurids were not long and pointed, but very deep and narrow, and borne on digits that allowed a wide range of flexion and extension. It is inferred from these details that rhynchosaurids used their feet to break up and shift sediment, perhaps while they were foraging for roots, tubers and other plant structures. Longer, slimmer unguals were present in Mesosuchus, so this basal rhynchosaur may not have indulged in the same behavior.
The tusk-like premaxillae have typically been imagined as plant-gathering organs and the powerful, shearing jaws and occluding dentitions look well suited for a diet of tough plant material. Seed-ferns, conifers, cycads, ginkgos and ferns have all been suggested as possible food items. Rhynchosaurs were limited to plants growing at or near ground level due to the fact that they could not climb.
An alternative suggestion to herbivory is that rhynchosaurs ate unionid bivalves (the group that includes swan mussels). Some paleontologists favor this possibility both because unionid shells were common in rhynchosaur-bearing sediments, and because snails are sometimes eaten by some living reptiles. However, others have argued that the precision 'blade and groove' rhynchosaur jaw morphology does not compare at all well with the jaw and tooth morphology of extant shell-cracking animals. Furthermore, the unique teeth and jaws of rhynchosaurs were clearly specialized for shearing, not cracking.
See also Edit
- ^ a b c Hone, D. W. E. & Benton, M.J. 2008. A New Rhynchosaurian Genus from the South-West of England. Journal of Palaeontology 51: 95-115)
- ^ a b Langer, M. C. & Schultz, C. L. 2000. A new species of the Late Triassic rhynchosaur Hyperodapedon from the Santa Maria Formation of south Brazil. Palaeontology 43, 633-652.
- ^ a b Chatterjee, S. 1969. Rhynchosaurs in time and space. Proceedings of the Geological Society of London 1658, 203-208.
- ^ Chatterjee, S. 1974. A rhynchosaur from the Upper Triassic Maleri Formation of India. Philosophical Transactions of the Royal Society of London B 276, 209-261.
- ^ a b c d Langer, M. C., Boniface, M., Cuny, G. & Barbieri, L. 2000. The phylogenetic position of Isalorhynchus genovefae, a Late Triassic rhynchosaur from Madagascar. Annales de Paléontologie 86, 101-127.
- ^ Langer, M. C. & Schultz, C. L. 2000. A new species of the Late Triassic rhynchosaur Hyperodapedon from the Santa Maria Formation of south Brazil. Palaeontology 43, 633-652.
- ^ a b c Dilkes, D. W. 1998. The Early Triassic rhynchosaur Mesosuchus browni and the interrelationships of basal archosauromorph reptiles. Philosophical Transactions of the Royal Society of London B 353, 501-541.
- ^ Benton, M. J. 1985. Classification and phylogeny of the diapsid reptiles. Zoological Journal of the Linnean Society 84, 97-164.
- ^ a b c d e Benton, M. J. 1990b. The species of Rhynchosaurus, a rhynchosaur (Reptilia, Diapsida) from the Middle Triassic of England. Philosophical Transactions of the Royal Society of London B 328, 213-306.
- ^ Sphenodontians are closely related to Gephyrosaurus from the Early Jurassic and both form a clade that needs a name. Gauthier et al. (1988) decided to co-opt Rhynchocephalia for this purpose. This has led to a confusing history of this name (in addition to sphenodontians and rhynchosaurs, 'Rhynchocephalia' of old authors might also include choristoderes, thalattosaurs and/or younginiforms).
- ^ Müller, J. 2003. Early loss and multiple return of the lower temporal arcade in diapsid reptiles. Naturwissenschaften 90, 473-476.
- ^ There is a distinction between 'rhynchosaur' and 'rhynchosaurid'.
- ^ a b Benton, M. J. 1983b. The Triassic reptile Hyperodapedon from Elgin: functional morphology and relationships. Philosophical Transactions of the Royal Society of London B 302, 605-718.
- ^ a b Nesbitt, S. J. & Whatley, R. L. 2004. The first discovery of a rhynchosaur from the upper Moenkopi Formation (Middle Triassic) of northern Arizona. PaleoBios 24, 1-10.
- ^ a b Benton, M. J. 1984. Tooth form, growth, and function in Triassic rhynchosaurs (Reptilia, Diapsida). Palaeontology 27, 737-776.
- ^ Dilkes, D. W. 1995. The rhynchosaur Howesia browni from the Late Triassic of South Africa. Palaeontology 38, 665-685.
- Benton, M. J. (2000), Vertebrate Paleontology, 2nd ed. Blackwell Science Ltd
- Carroll, R. L. (1988), Vertebrate Paleontology and Evolution, W.H. Freeman & Co.
| Primitive Archosauromorphs — Euparkeriidae • Erythrosuchidae • Proterochampsidae • Proterosuchidae • Choristodera • Prolacertiformes • Rhynchosauria • Trilophosauria
Crurotarsi Archosaurs — Ornithosuchidae • Aetosauria • Phytosauria • Rauisuchia • Crocodylomorpha • Crocodilia
Avemetatarsalia and Ornithodira Archosaurs — Scleromochlus • Pterosauria • Dinosauromorpha • Dinosauria • Ornithischia • Saurischia • Aves
Avian Archosaurs — Avialae • Archaeopteryx • Confuciusornis • Ichthyornis • Enantiornithes • Hesperornithes • Neornithes • Paleognathae • Neognathae