| Cartorhynchus | |
|---|---|
 | |
| Holotype specimen (AGB 6257),Geological Museum of China | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | †Ichthyosauromorpha |
| Clade: | †Ichthyosauriformes |
| Clade: | †Nasorostra |
| Genus: | †Cartorhynchus Motaniet al.,2014 |
| Species: | †C. lenticarpus |
| Binomial name | |
| †Cartorhynchus lenticarpus Motaniet al., 2014 | |
Cartorhynchus (meaning "shortened snout") is an extinctgenus ofearlyichthyosauriformmarine reptile that lived during theEarly Triassicepoch, about 248 million years ago. The genus contains a single species,Cartorhynchus lenticarpus, named in 2014 by Ryosuke Motani and colleagues from a single nearly-complete skeleton found nearChaohu,Anhui Province,China. Along with its close relativeSclerocormus,Cartorhynchus was part of a diversification of marine reptiles that occurred suddenly (over about one million years) during theSpathiansubstage, soon after the devastatingPermian-Triassic extinction event, but they were subsequently driven to extinction by volcanism and sea level changes by theMiddle Triassic.
Measuring about 40 centimetres (16 in) long,Cartorhynchus was a small animal with a lizard-like body and a short torso; it probably swam in an eel-like manner at slow speeds. Its limbs bore extensive cartilage and could bend like flippers, which may have allowed it to walk on land. The most distinctive features ofCartorhynchus were its short, constricted snout, and its multiple rows ofmolar-like teeth which grew on the inside surface of its jaw bones. These teeth were not discovered until the specimen was subjected toCT scanning.Cartorhynchus likely preyed on hard-shelled invertebrates usingsuction feeding, although how it exactly used its inward-directed teeth is not yet known. It was one of up to five independent acquisitions of molar-like teeth among ichthyosauriforms.
In 2011, the only known specimen ofCartorhynchus was discovered in Bed 633 from the second level of the Majiashan Quarry near downtownChaohu,Anhui Province,China; the rock strata in this quarry belong to the Upper Member of theNanlinghu Formation.[1] The specimen consists of a nearly-complete skeleton missing only part of the tail[2] and some of the bones from the left part of the rear skull. The specimen's preservation likely resulted from it having been deposited in sediment right side down, thus leaving the left side exposed to the elements. It received a field number of MT-II, and later a specimen number of AGB 6257 at theAnhui Geological Museum.[3]
In 2014, the specimen was described by Ryosuke Motani and colleagues inNature as representing a new genus and species,Cartorhynchus lenticarpus. They derived thegeneric nameCartorhynchus from the Greek wordskartos (καρτός, "shortened") andrhynchos (ῥύγχος, "snout"), and thespecific namelenticarpus from the Latin wordslentus ("flexible") andcarpus ("wrist"). Both names refer to anatomical characteristics that it would have had in life.[1] The specimen was thought to be toothless until an isolated tooth was discovered during further attempts to remove rock from between the closed jaws. Since the specimen was too fragile to expose the interior of the jaws, Jian-Dong Huang, Motani, and other colleagues subsequently scanned and rendered the specimen in 3D usingmicro-computed tomography (micro-CT), performed at the Yinghua Testing Company inShanghai, China. In 2020, results from their follow-up work were published inScientific Reports.[3]
At the time of its discovery,Cartorhynchus was the smallest-known member of theIchthyosauriformes. The preserved specimen had a length of 21.4 centimetres (8 in); assuming that it had tail proportions comparable to close relatives, Motani and colleagues estimated a full body length of 40 centimetres (1 ft 4 in) and a weight of 2 kilograms (4.4 lb).[1][2] In 2021, Sander and colleagues produced a much lower weight estimate of 237 g (8.4 oz).[4]
Cartorhynchus had an unusually short and constricted snout, which only occupied half of the skull's length, and a deep jaw. The tip of the snout was only 6 millimetres (0.2 in) wide.[3] Unlike most reptiles, itsnasal bone reached the front of the snout. Due to its likewise elongatedpremaxilla, its bony nostrils were located relatively far back on the skull, and itsfrontal bone also lacked an expansion at its rear outer corner. All of these characteristics were shared with its close relativeSclerocormus. However, unlike the latter, the frontal bone did not contribute to the eye socket inCartorhynchus; theprefrontal and postfrontal bones did not meet above the eye socket;[5] and the location of the large hole for thepineal gland on the skull roof differed: it was at the contact between the frontal andparietal bones inSclerocormus, but solely on the parietals inCartorhynchus.Cartorhynchus also had a characteristically largehyoid bone.[1][6]
Initially, Motani and colleagues inferred thatCartorhynchus was toothless; however, micro-CT scanning subsequently revealed the presence of rounded,molar-like teeth that projected inwards nearly perpendicularly to the long axis of the jaw, therefore making them invisible externally. All of the teeth were either completely flattened or weakly pointed, and many of the teeth bore a constriction between the root and the crown. Unlike other ichthyosauriforms with molariform teeth, all of the tooth crowns were "swollen" to a similar extent. On themaxilla (upper jawbone) anddentary (lower jawbone), the teeth were arranged in three rows, with the outermost row having the most and largest teeth; the maxillae had seven, five, and probably one teeth each, while the dentaries had ten, seven, and four teeth each. Among ichthyosauriforms, onlyCartorhynchus andXinminosaurus have multiple rows of teeth.[7] The arrangement of the teeth meant that the front-most lower teeth would have had no corresponding upper tooth, and also that the two dentaries forming the lower jaw could not have been tightly fused. This characteristic would have been shared with its close relatives, the toothlesshupehsuchians.[3][8]
Cartorhynchus appears to have had 5neck vertebrae and 26back vertebrae, for a total of 31 pre-sacral vertebrae (vertebrae in front of itssacrum, or hip). Along withSclerocormus (with 34 pre-sacrals) andChaohusaurus (with 36 pre-sacrals),Cartorhynchus falls within the typical range for terrestrial animals, unlike the 40 to 80 pre-sacrals common among the morederived (specialized)ichthyopterygians.[1] UnlikeSclerocormus, theneural spines projecting from the top of the vertebrae inCartorhynchus were relatively narrow and inclined instead of broad and flanged.Cartorhynchus can also be distinguished by its parapophyses, vertebral processes that articulated with the ribs; their front margins were confluent with those of the vertebrae.[6]
BothCartorhynchus andSclerocormus had heavily-built ribcages, which were deepest near the shoulder, with broad, flattened, andthick-walled ribs, as is commonly seen in early members of secondarily-aquatic reptile lineages.[1][9] The Ichthyopterygia lost these flattened ribs with the exception ofMollesaurus.[10] On the underside of the chest, thegastralia ofCartorhynchus were thin and rod-like, unlike the flattened "basket" ofSclerocormus, but both lacked another pair of symmetrical elements at the midline of the body.[6]
The limbs ofCartorhynchus were poorlyossified (only three digits of the hand were ossified) with widely-spaced bones, particularly between thewrist bones (carpals) and the digits, suggesting the presence of extensive cartilage in the limbs. This would have made the limbs flipper-like. The forelimb flippers ofCartorhynchus were curved backwards, with the digits being tilted 50° relative to the axis of the long bones (zeugopodium), while the hindlimbs were curved forwards. Thefemur ofCartorhynchus was straight and not expanded at its bottom end.Sclerocormus had similar limbs, except they were better-ossified and their preserved curvature may not have been natural.[1][6]
The lack of complete fossil remains has resulted in a lack of clarity about the origins of the Ichthyopterygia, including theichthyosaurs. For many years, their fossils were considered to have abruptly appeared in theMiddle Triassic with strong aquatic adaptations. The discovery ofCartorhynchus andSclerocormus partially filled this gap.Phylogenetic analyses conducted by Motani and colleagues found that the two were closely related to each other — forming aclade (group) called theNasorostra — and to the Ichthyopterygia, to which nasorostrans formed thesister group.Cartorhynchus andSclerocormus were united by their short snouts, elongated nasals, deep jaws, frontals lacking expansions, rib-cages deepest near the shoulder, andscapulae (shoulder blades) wider at the bottom end than at the top end.[1][6]
Incorporating nasorostrans into phylogenetic analyses also provided evidence in support of the hupehsuchians as close relatives of the ichthyopterygians. In 2014, Motani and colleagues named the clade formed by Nasorostra and Ichthyopterygia as the Ichthyosauriformes, and the clade formed by Ichthyosauriformes and Hupehsuchia as theIchthyosauromorpha. Notably, the close relation between these different groups was recovered by their analyses regardless of whether characteristics linked to aquatic adaptations were removed from the analysis.[1][6] Such characteristics may have developedhomoplasiously (fromconvergent evolution) among multiple lineages due to similar lifestyles, which can bias phylogenetic analyses to reconstruct them ashomologies (derived from shared ancestry). The persistence of these reconstructed relationships even after the removal of aquatic characteristics points to their robustness.[11][12]
Below, thephylogenetic tree from the phylogenetic analysis published by Huang, Motani, and colleagues in 2020, in the description ofChaohusaurus brevifemoralis, is partially reproduced:[3]
| Ichthyosauromorpha |
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The appearance of ichthyosauromorphs was part of the recovery of marine ecosystems following the devastatingPermian–Triassic extinction event. It was commonly believed that marine ecosystems did not recover their full diversity until 5 to 10 million years following the mass extinction, and that marine reptiles recovered more slowly from the extinction than other lineages.[13][14] However, the discovery of multiple diverse faunas of marine reptiles occurring in the Early Triassic — includingCartorhynchus — subsequently showed that this was not the case.[8][15] In particular, it appears that ichthyosauriforms first appeared during the Spathian substage of theOlenekianstage and quickly attained highfunctional diversity in the first million years of their evolution.[6] They occupied a variety ofniches despite relatively low species diversity,[16] including bothdemersal (bottom-dwelling) species like hupehsuchians and nasorostrans, andpelagic (open-water) species like ichthyopterygians.[6]
Many ichthyosauriforms from the Early andMiddle Triassic had molariform teeth (shown in the phylogenetic tree above), includingCartorhynchus; such teeth indicate a diet at least partially based on hard-shelled animals.[17] In 2020, Huang and colleagues performed anancestral state reconstruction of teeth among ichthyosauromorphs. Probabilistic methods suggested that rounded or flat teeth most likely evolved independently five times, while methods based onparsimony suggested that they evolved independently three to five times. Huang and colleagues observed that the development of molariform teeth occurred independently many times in aquatic animals (including multiple lineages ofmonitor lizards,moray eels, andsparid andcichlid fish), and thus the frequency among ichthyosauriforms is not unusually high. They also observed that Early Triassic ichthyosauriforms generally had small, rounded teeth; the teeth of Middle Triassic ichthyosauriforms were more diverse in size and shape, which correlates with increased invertebrate diversity.[18] Thus, they suggested that the diversification of ichthyosauriforms was partially driven by the evolution of hard-shelled prey.[3]
However, hupehsuchians and nasorostrans ultimately went extinct at the boundary between the Early and Middle Triassic, producing a "taxonomic bottleneck". At the boundary, sea level changes[19] and volcanism[20] led to poorly oxygenated oceans, producing a characteristiccarbon isotope signature from decaying organic material in rock strata at the boundary.[21] Ichthyosauriforms did not recover in diversity after this turnover,[22] with theSauropterygia andSaurosphargidae driving a second wave of diversification lasting three to five million years.[6][16]
Motani and colleagues hypothesized in 2014 thatCartorhynchus was asuction feeder which fed by concentrating pressure in its narrow snout. This was further supported by the robustness of its hyoid and hyobranchial element (which would have anchored the tongue), and their incorrect observation of toothlessness.[1] Similar inferences were subsequently made forSclerocormus.[6]Shastasaurus andShonisaurus had previously been interpreted as suction-feeding ichthyosaurs,[23] but a quantitative analysis of Triassic and Jurassic ichthyosaurs by Motani and colleagues in 2013 showed that none of them had sufficiently robust hyobranchial bones nor sufficiently narrow snouts to enable suction feeding.[24]
The discovery of molariform teeth inCartorhynchus led Huang and colleagues to conclude in 2020 thatCartorhynchus wasdurophagous, feeding on hard-shelled prey. They noted that this did not contradict a suction-feeding lifestyle; some sparid fish are both durophagous and suction-feeding.[25] However, they suggested thatCartorhynchus would have been restricted to feeding on small prey. As for the horizontal orientation of the teeth, they observed wear surfaces which indicated that the sides of the teethoccluded with each other, instead of the crowns. However, they noted that teeth are structurally strongest at their tips, not on their sides, under the highstresses of crushing bites.[26] They suggested that the jaw may have been twisted during preservation, or soft tissues likecollagen which held up the teeth in life may have been lost, although they conceded that neither hypothesis would explain the wear patterns. Finally, they noted that the lower teeth without corresponding upper teeth were unusual; they show no wear patterns, and there is no evidence of muscular mechanisms which would have allowed the two jaws to be used against each other. Therefore, they inferred that these teeth were probably not used against other teeth.[3]
Cartorhynchus had poorly-ossified limbs in spite of its well-ossified skull and vertebrae. However, Motani and colleagues suggested in 2014 that it was still an adult because many early-diverging members of marine reptile lineages have poorly-ossified limbs throughpaedomorphosis (the retention of immature traits into adulthood), although they did not completely reject the possibility that it was a juvenile due to the existence of only one specimen.[1]
In the case ofCartorhynchus, Motani and colleagues proposed that the large, flipper-like forelimbs would have enabled it to move on land, thus making it amphibious. The extensive cartilage at the wrist joint would have allowed the flipper to bend without an elbow; juvenile sea turtles have similarly cartilaginous flippers that they use to move on land.[27] Although its flipper would not have been particularly strong,Cartorhynchus was relatively lightweight, with a body mass to flipper surface area ratio smaller than that ofChaohusaurus. The curved flippers would have allowed them to be kept close to the body. Motani and colleagues suggested that other traits ofCartorhynchus would also have aided an amphibious lifestyle, including the short trunk and snout, and the thickened ribs (which would have served as aballast, stabilizing the animal in near-shore waters).[1]
A 2019 study by Susana Gutarra and colleagues used computational simulations to estimate the energy cost of swimming in ichthyosauriforms. Early-diverging ichthyosauriforms with lizard-shaped bodies and elongate, flukeless tails, likeCartorhynchus, would have employedanguilliform (eel-like) swimming, while later ichthyosauriforms with deeper, fish-like bodies and well-defined tail flukes would have employedcarangiform (mackerel-like) swimming. It is generally thought that anguilliform swimming is less efficient than carangiform swimming.[28][29][30] Indeed, Gutarra and colleagues found that the energetic cost of swimming at 1 metre per second (3.3 ft/s) was 24 to 42 times higher inCartorhynchus than the ichthyosaurOphthalmosaurus (depending on whether swimming mode is accounted for), and itsdrag coefficient was 15% higher than that of thebottlenose dolphin. However,Cartorhynchus would have likely swam at slower speeds requiring less efficiency, and the advantages of carangiform swimming in later, larger ichthyosauriforms were also offset by increased body size.[31]
Bed 633 of the Majiashan Quarry, the locality whereCartorhynchus was found, is a layer of greyargillaceous (clay-bearing)limestone located 13 metres (43 ft) above the base of the Upper Member of the Nanlinghu Formation. It is defined above and below by layers of yellowishmarls. In terms ofammonitebiostratigraphy, this bed belongs to theSubcolumbites zone.[1][32] High-resolution date estimates have been produced for the Olenekian-aged strata exposed in the Majiashan Quarry based on isotopic records ofcarbon-13cycling and spectralgamma ray logs (which measure the amount of radiation in rocks of astronomical origin); Bed 633 in particular was estimated at 248.41Ma in age.[33][34][35] During the Middle Triassic, the Chaohu strata were deposited in anoceanic basin relatively far from the coast, which was bordered on the south by shallower waters andcarbonate platforms, and on the north by acontinental slope and deeper basins.[36][37]
The ichthyosauriformsSclerocormus andChaohusaurus are both found in Majiashan Quarry along withCartorhynchus;Sclerocormus is known from the younger Bed 719 (248.16 Ma), whileChaohusaurus is found in both beds.[33] The sauropterygianMajiashanosaurus is known from Bed 643.[38] Fish diversity in the Majiashan Quarry is poorer than other localities; the most common fish isChaohuperleidus, the oldest known member of thePerleidiformes, but a species of the wide-rangingSaurichthys and several undescribed fish are also known.[39][40] Potential invertebrate prey forCartorhynchus include small ammonites andbivalves, and thethylacocephalanarthropodAnkitokazocaris.[3][32]
