| After the end-Permian mass extinction,the earth entered a new era-the reptile era.Land vertebrates started recolonizing the sea rapidly evolved into many ecomorphs during the Triassic period,forming marine reptile taxa represented by Ichthyosauria,Thalattosauria and Saurpterygia(including Placodongtia Pachypleurosauria,Nothosauria and Pistosauroidea).Pachypleurosaurs had been taken as the most primitive representatives of sauropterygians(Storrs,1993;Rieppel,1993),which diversified in the Triassic epicontinental seas and intraplatform basin of the Tethyan Ocean.The evolutionary lineage shows that the Chinese pachypleurosaurus Keichousaurus and all European Pachypleurosaurus constitute a monophyletic group,meaning that they are more closely genetic relationship than any species outside the group.Keichousaurus hui,as the earliest assigned to the Pachypleurosauridae(Young,1958),is one of the most abundant Triassic marine reptiles in South China.A large quantity of well-preserved specimens of Keichousaurus hui have provided the basis for a more comprehensive scientific study,including skeletal system,functional morphology,ontogenesis,sexual dimorphism,allometric growth,reproductive mode,paleoecological environment.Because of extremely specific taphonomic conditions that usually limit soft parts preservation,there is little documented direct evidence about soft tissue preservation of Keichousaurus hui.For most Mesozoic marine reptiles,dietary inference relies primarily on indirect evidence from dentition and jaw architecture.Here we show the occurrence of spectacularly preserved nine specimens with soft part fossilization of the alimentary canal in Keichousaurus hui from the Triassic of South China.These remarkably well-preserved specimens not only provide first direct and unequivocal evidence of prey preference and feeding strategy,but also help in the reconstruction of the alimentary canal of this species.Through the relationship between the remains and the skeleton of Keichousaurus hui,it is confirmed that these remains were preserved in the coelom of Keichousaurus hui.Most of the Keichousaurus hui preserved these residues were male,which excluded the relationship between residues and reproductive system.The membrane-like remains in the abdominal cavity show fold characteristics,similar to the soft tissue of Jurassic ichthyosaur and modern dolphin.The X-ray computed tomography(CT)results of the "J" shaped mass remains preserved in the abdominal cavity of an exquisitingly preserved Keichousaurus hui showed that a whole articulated individual fish was wrapped in it,and a complete fish scale fossil was found in the left pelvic region of Keichousaurus hui.Various evidences not only show that the residual material in the body cavity is a collection of gastrointestinal soft tissue and gastrointestinal contents of Keichousaurus hui.This suggests that Keichousaurus hui was a frequent piscivore,and often swallowed its prey whole without the process of chewing.The preservation position of these digestive tract residues in the abdominal cavity was almost the same,which provided favorable conditions for the recovery of the digestive tract of Keichousaurus hui.Based on the analysis of the preserved form of digestive tract soft tissue in the right abdominal cavity of the exceptionally well-preserved specimen and the overlapping area of digestive tract residue in the left abdominal cavity of the posterior trunk of 9 specimens,combined with the comparison of modern reptiles,the distribution position and digestive organ composition of Keichousaurus hui in the body cavity were determined.As a carnivorous animal,the digestive tract of Keichousaurus hui is simpler than that of herbivorous animal,which is composed of five digestive organs: esophagus,stomach,small intestine,large intestine and cloaca.The elongated esophagus extends from the throat along the main axis of Keichousaurus hui and turns into the stomach at the 8th to 9th dorsal vertebra.The stomach distributes in the right abdominal cavity in a nearly "J" shape from the 10 th to 15 th dorsal vertebra.Then the stomach narrows into the pylorus and turns left into the intestinal region at the 16 th to 17 th dorsal vertebra.The intestinal tract is distributed in the left posterior abdominal cavity and pelvic region,divided into two parts,a relatively short annular small intestine and a short straight large intestine,which then enters the cloaca from the large intestine and ends at the caudal vertebrae of the 2nd to 3rd segment.A large number of gastrointestinal contents were preserved in the body cavity of Keichousaurus hui,and the fish were completely hinged,indicating that the mucosal wall of esophagus and stomach had a strong ability to expand and contract to adapt to the size of food.Several short feeding intervals indicate that Keichousaurus hui had strong digestive capacity.Stomach muscular,have good mechanical digestion ability.The pylorus has a strong sphincter,which can well prevent the intestinal food from returning to the stomach,and the secretions can neutralize the acidity of the digestion in the stomach,providing a stable environment for further digestion and absorption in the small intestine.The reconstruction of the gastralia of Keichousaurus hui indicated that the presence of the gastralia provided anchoring attachments for the peritoneal muscle tissue of Keichousaurus hui.From the end of the skeleton in the chest region,the gastralia composed of five elements gradually decreased in angle surrounded by the central V-shaped skeleton,and were arranged along the axis of the Keichousaurus hui to the position of the 11 th vertebra.This combination of structures makes the gastralia in the front of the trunk fan out and play a supporting and protective role for the liver,lung and other weak organs in the front of the trunk.After the 11 th vertebrae,the central V-shaped skeleton disappeared,the lateral skeleton could extend outwards,which enlarged the abdominal cavity space for the mass feeding and embryonic development of Keichousaurus hui. |
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