| Liaodactylus | |
|---|---|
 | |
| Holotype skull ofLiaodactylus | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Order: | †Pterosauria |
| Suborder: | †Pterodactyloidea |
| Family: | †Ctenochasmatidae |
| Genus: | †Liaodactylus Zhouet al., 2017 |
| Type species | |
| †Liaodactylus primus Zhouet al., 2017 | |
Liaodactylus is a genus of filter-feedingctenochasmatidpterosaur from theJurassic ofChina. The genus contains one species,L. primus, described by Zhouet al. in 2017. As an adaptation to filter-feeding,Liaodactylus had approximately 150 long, comb-like teeth packed closely together. It is both the earliest known ctenochasmatid and the first filter-feeding pterosaur from the JurassicYanliao Biota. Later and more specialized ctenochasmatids differ fromLiaodactylus in having longer snouts, smaller openings (or fenestrae) in the skull, and more teeth. Within the Ctenochasmatidae,Liaodactylus was most closely related to the EuropeanCtenochasma.
There is one specimen ofLiaodactylus known, namely theholotype PMOL-AP00031, which is stored at the Palaeontological Museum of Liaoning. It consists of a complete skull and lower jaws, along with the first twocervical vertebrae. It originates from outcrops located about 500 metres (1,600 ft) west of the village of Daxishan, inJianchang County,Liaoning,China. These outcrops are considered to belong to theLate JurassicTiaojishan Formation, which has been dated to range from 161.8 ± 0.4 to 159.5 ± 0.6 million years ago (Oxfordian) based onargon-argon dating.[1][2][3] However, a 2023 study instead suggested that these deposits pertain to theHaifanggou Formation instead.[4]
Liaodactylus was described in 2017 by Zhouet al. The genus name combinesLiao, a shortened prefix form of "Liaoning", with Greekdactylos ("finger"), a standard pterosaurian suffix. Meanwhile, the specific nameprimus, from Latin, refers to the early age ofLiaodactylus relative to otherctenochasmatids.[1]
The head ofLiaodactylus is slender and long, with a skull length of 13.3 centimetres (5.2 in) and a jaw length of 11.7 centimetres (4.6 in).[1] Out of this, the snout is 49.1% of total skull length, and thenasoantorbital fenestra (the fusion of the nostril and theantorbital fenestra, seen in all members of theMonofenestrata[5][6]) is 31% of skull length. In morederived ctenochasmatids, the nasoantorbital fenestra is much smaller relative to the skull, being only 10-12% of skull length inPterodaustro; the snout is also longer, being over 85% of skull length inPterodaustro.[7] Also shorter than other ctenochasmatids is thedentary symphysis, the fused portion of the lower jaw, which is only 30.5% of jaw length. Morphologically, the sides of the snout are parallel (unlike the spoon-tipped snouts ofGnathosaurus), most of themandible is composed mainly of the dentary, and the well-developed retroarticular process is formed by theangular bone like in other ctenochasmatids.[1]
There are many long, needle-like, outward-projecting, tightly packed, equally-spaced teeth in the jaws ofLiaodactylus, totalling to 152 teeth across both jaws. This is more thanGnathosaurus (128–136), about the same asGegepterus (150),[8] but much less thanCtenochasma (200-552)[9] andPterodaustro (almost a thousand).[7] The teeth become shorter towards the back of the jaws, eventually becoming short, peg-like structures at the back of the tooth row; the back of the tooth row corresponding to the front 1/3 of the nasoantorbital fenestra, which is unusual among ctenochasmatids (where the teeth usually stop entirely before the fenestra). The first tooth at the front of each jaw is also somewhat shortened, being only half the length of the second. Collectively, the teeth, which would have interlocked when the jaw was closed, form a comb-like complex that would have been used for filter-feeding.[1][10]
Among ctenochasmatids, thejugal bone ofLiaodactylus is quite narrow, especially below the large eye socket. It splits into three branches: the very long and tapering anterior process, which forms the back two-thirds of the bottom border of the nasoantorbital fenestra; the vertical orbital process; and the angled temporal process, which is very long compared to the orbital process. The bone is bent slightly such that the margin of the jaw below the eye socket is curved. At the back, the temporal process of the jugal has fused rather extensively with the neighboringquadratojugal bone. Further back on the skull, thequadrate bone articulates with thesquamosal bone above and the lower jaw below; the long shaft at the rear of the quadrate is very strongly angled, forming an angle of 160° with the jaw margin, like other ctenochasmatids. The jaw joint is directly underneath the eye socket.[1]
At the back of themaxilla, thepalate form a wide shelf which is deeply notched at the back, forming in part the suborbital fenestra. Small spikes along the midline of the suborbital fenestra represent thepalatine bones, which contact the maxilla. The ectopterygoid bones are three-pronged; the lateral processes close off the back of the suborbital fenestra, the anterior processes join the process of the palatal shelf, and the posterior process overlaps thepterygoid bone and forms part of the pterygo-ectopterygoid fenestra, which is separated from the suborbital fenestra by a bar of the pterygoid; the same bar is seen inGnathosaurus, albeit much shorter.[11][12] As for the pterygoid itself, it was expanded along the midline and projected backwards to connect to thebasisphenoid bone and quadrate.[1]
The top of thebraincase consists of the strongly-fusedfrontal bone andparietal bone, which curve sharply downwards near the back so as to produce a rounded back of the skull. At the back of the skull, the basisphenoid is short, wide, and platelike, and has a wide U-shaped notch at the front end. At either side of the notch is a stout basisphenoid process, which contacts thepterygoid bone and quadrate. Meanwhile, at the back, the basisphenoid becomes narrower and is overlapped by thebasioccipital bone. On the basioccipital, theoccipital condyle, which connects to the vertebrae, is spherical; consequently, the hole for the spinal cord, theforamen magnum, is hidden when the vertebrae are articulated. There is a roughened ridge extending along the midline of the basioccipital, forward from the occipital condyle.[1]
As for thecervical vertebrae themselves, theatlas andaxis are fused, albeit with a visiblesuture. In front of the atlas is a small, rudimentary proatlas, which is symmetrical, platelike, and has a projection at the front. Corresponding to the spherical occipital condyle is the cup-like joint of the atlas. The atlas had a prominentneural spine, but the neural spine on the axis is not exposed. At the back of the axis, on theneural arch, is thepostzygapophysis, which would have connected to the next cervical vertebra.[1]
Zhouet al. assignedLiaodactylus to theCtenochasmatidae in 2017 based on aphylogenetic analysis. It shares with other ctenochasmatids the relative proportions of its skull height at thesquamosal bone, its skull height at the jaw joint, and the length of the snout at the level of thenasoantorbital fenestra; additionally, in terms of morphological characters, it shares a retroarticular process that is pointed backwards and downwards. Within the Ctenochasmatidae,Liaodactylus was found to be closest toCtenochasma, with which it shares the ratio of jaw-to-skull length and the teeth being angled out to the sides. The phylogenetic tree recovered by the analysis is reproduced below.[1][5]
Liaodactylus is known from theYanliao Biota, a diverse assemblage of well-preserved Jurassic animals containing at least 40 species. A number of other pterosaurs have been found from the Yanliao Biota, including therhamphorhynchidsJianchangnathus robustus andQinglongopterus guoi; theanurognathidsDendrorhynchoides mutoudengensis andJeholopterus ninchengensis; thedarwinopteranPterorhynchus wellnhoferi; and thewukongopteridsDarwinopterus modularis,Wukongopterus lii, and possiblyArchaeoistiodactylus linglongtaensis,[13] among many others.[1][14]Liaodactylus represents the first Yanliao filter-feeding pterosaur.[1][10] Dinosaurs from the Yanliao Biota include thescansoriopterygidsEpidendrosaurus ninchengensis,Epidexipteryx hui, andYi qi; theparaviansAnchiornis huxleyi,Pedopenna daohugouensis, andXiaotingia zhengi.[14][15]
Non-archosaurs are also present in the Yanliao Biota.Mammaliaforms includeLiaotherium gracile,Manchurodon simplicidens,Pseudotribos robustus,Volaticotherium antiquum,Castorocauda lutrasimilis,Docofossor brachydactylus,Arboroharamiya jenkinsi,Megaconus mammaliaformis,Xianshou linglong andX. songae,Shenshou lui, andJuramaia sinensis.[16][17] Additionally, there are lizards, including"Yabeinosaurus" youngi;[18] an undescribedcrocodylomorph; salamanders, includingChunerpeton tianyiensis,Jeholotriton paradoxus,Liaoxitriton daohugouensis, andPangerpeton sinensis;[19] and fish, includingLiaosteus hongi and a member of thePtycholepidae.[14][20]
In terms of environment, the Tiaojishan Formation represents a forested woodland with conifers, cycads, and ferns. Plant species present include thehorsetailsNeocalamites carcinoides,N. narthosi, andEquisetum sp.; thetree fernsConiopteris hymenophyloides andC. margaretae; thecycadZamites gigas; the enigmaticgymnospermsCzekanowskia ketova,C. ridiga, andPhoenicopsis speciosa; theginkgoSphenobaiera kazachstanica; thecypressProtaxodioxylon jianchangense; and the indeterminateconiferXenoxylon peidense.[20][21] Collectively, they indicate a cool, temperate, wet, and seasonal climate with a mean temperature below 15 °C.[20] This is consistent with a trend of global cooling around this time,[22] but contrasts with the warm, dry climate of the earlier Haifanggou.[23]
