Study On The Biostratinomy And A New Anurognathid Pterosaur Of The Yanliao Biota

The Middle–Upper Jurassic Yanliao Lagerst(?)tte has yielded numerous exceptionally preserved fossils of aquatic and land organisms,including diverse plants,insects,fishes,salamanders,lizards,crocodylomorphs,dinosaurs,pterosaurs and mammaliaforms.Among these,many have been discovered as the early examples of their respective clades or have shed light on key evolutionary transitions.Despite of these extensive paleontological advances,the palaeoenvironmental setting and taphonomy of this exceptional preserved biota remain poorly understood.On the other hand,continuing discoveries of new fossils have been made from the biota,including a new anurognathid pterosaur(NJU–57003)studied here,preserved with a complete skeleton and extensive integumentary filamentous structures,yielded from the Upper Jurassic Tiaojishan Formation in Qinglong County,Hebei Province.Due to the scarcity and significant ontogenetic variation of the fossil record,the phylogenetic position of anurognathids as well as the relationship of the taxa within the clade,remain highly controversial.The new specimen provides valuable evidence for resolving the controversial anurognathid phylogeny and understanding the origin and evolution of pterosaurian pycnofibres.By means of intensive field investigation and high-resolution excavations,the palaeoenvironment and biostratinomic features of the Yanliao fossils in the Daohugou area are systematically studied for the first time.The results show that frequent volcanic eruptions generated extensive volcaniclastic aprons and palaeolake(s)in the area.Most terrestrial organisms are interpreted to have been killed near the lake(s)during volcanic eruptions.Their carcasses,together with influxes of fresh volcaniclastic materials,were transported into the lake(s)by meteoric runoff or distal pyroclastic flows,and were buried in lake deposits prior to significant decay occurring,probably under a combined interaction of convective sedimentation,ash fall and water turbulence.This timely burial may have prevented the further disarticulation,predation and decay of carcasses,thereby favored the subsequent exceptional preservation.These results provide evidence for understanding the exceptional preservation mechanism and reconstructing the Middle–Late Jurassic terrestrial ecosystems in East Asia.Allometric analysis is carried out on all twelve anurognathid specimens,including the new specimen NJU–57003,involving 23 skeletal dimensions from the skull,pectoral girdle and limbs.The results demonstrate for the first time that anurognathids share the same growth trajectory in most dimensions,confirming that some of the characters related to these dimensions in previous phylogenetic analysis reflect ontogenetic rather than phylogenetic variation.By correcting these characters that represent ontogenetic variation and updating all known diagnostic characters from newly discovered anurognathids,this study re-analyses the phylogeny of anurognathids.The results reveal that Anurognathidae diverges among relatively basal nonpterodactyloids and the relatively long-tailed forms are primitive in the family,and ten species are validated from the twelve specimens,including a new taxon,Cascocauda rong,based on the new specimen.The results demonstrate that anurognathids were rather diverse than previously thought,despite they retained a rather conservative Bauplan during growth that reflects the similar niches they occupied in their over-40-million-year evolution.Through intensive analyses of the morphology,ultrastructure and chemistry of the preserved pycnofibres in NJU–57003 and another anurognathid specimen,this study demonstrates,for the first time,that pterosaurian pycnofibres are morphologically diverse and bear diagnostic features of maniraptoran feathers,such as non-vaned grouped and bilaterally branched filaments,and preserved melanosomes with diverse geometries.Subsequent phylogenetic macroevolutionary analysis shows that pterosaurian pycnofibres and dinosaurian feathers are probably homologous structures that have deep evolutionary origins in ancestral archosaurs.The presence of feather-like structures suggests that anurognathids,and potentially other pterosaurs,possessed a dense filamentous covering that probably functioned in thermoregulation,tactile sensing,signaling and aerodynamics.These results add significant evidence for revealing the origin and evolution of anurognathids and featherlike integumentary filaments.