The Pterosaurs were a group of flying animals that appeared in the Late Triassic and persisted till the end of the Cretaceous. They were Diapsids, members of the group that also includes Squamates (Lizards and Snakes), Archosaurs (Crocodiles and Dinosaurs) and Chelonians (Turtles and Tortoises), with most experts in the field considering them to be Archosaurs more closely related to Dinosaurs than Birds. While in many ways Pterosaurs were quite Bird-like, in others they were very different and often quite puzzling. For example their reproductive strategy is thought to have been quite different from that of Birds, with the animals taking several years to reach maturity and having been able to fly and probably reproduce before reaching full size, unlike Birds which reach their maximum size in the first year of life, before leaving the nest. Unfortunately no Pterosaur nest has ever been recovered, and very few eggs are known either, so that most of what we know about very young Pterosaurs is inferred from examining the skeletons of older specimens.
In 2011 a team of palaeontologists led by Lû Junchang of the Institute of Geology of the Chinese Academy of Geological Sciences described the fossil of a female Wukongopterid Pterosaur from the Late Jurassic Tiaojishan Formation of Liaoning Province, China, preserved with an egg adjacent to the pelvis in a paper in the journal Science. This was interpreted as being a parchment egg (egg with an uncalcified shell) that had been expelled from the body post-mortem rather than laid. This was of particular interest as previous Pterosaur eggs discovered had all had calcarous shells, though the egg was preserved as an impression only, preventing detailed study of the specimen.
In a paper published in the Anais da Academia Brasileira de Ciências on 3 July 2015, Xiaolin Wang of the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, Alexander Kellner of the Laboratory of Systematics and Taphonomy of Fossil Vertebrates at the Department of Geology and Paleontology at the Universidade Federal do Rio de Janeiro, Xin Cheng of the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences and the Laboratory of Systematics and Taphonomy of Fossil Vertebrates at the Universidade Federal do Rio de Janeiro, Shunxing Jiang and Qiang Wang, also of the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, Juliana Sayào of the Centro Acadêmico de Vitória at the Universidade Federal de Pernambuco, Taissa Rodrigues of the Departamento de Biologia at the Universidade Federal do Espírito Santo, Fabiana Costa of the Department of Biological Sciences at the Universidade Federal de São Paulo, Ning Li of the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences and the University of Chinese Academy of Sciences and Xi Meng and Zonghe Zhou, again of the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, describe the recently discovered counterpart to the slab on which the fossil described by Lû et al. in 2011, which reveals a second egg within the body of the Pterosaur as well as details of the structure of the eggs.
The specimen is a subadult Wukongopterid Pterosaur which Lû et al. assigned to the genus Darwinopterus but which Wang et al reassign to the genus Kunpengopterus, as the new material is more complete and shows additional details of the skeleton, and our understanding of the anatomy and diversity of Wukongopterid Pterosaurs has improved considerably in the last four years. There are two eggs preserved on the slab, one to the left of the skeleton and immediately to the posterior of the pelvis, the other within the body cavity behind the pelvis and associated with the gastrula and ribs.
Gravid pterosaur of Kunpengopterus sp. with two eggs, one inside the body cavity and a second posterior to the pelvic region. (a) Counter slab. (b) Composite drawing of slab and counter slab indicating the position of the eggs and the bones (in grey) preserved in the counter slab. The arrow indicates the part of the femur removed for histological sections. Scale bar is 100 mm. Wang et al. (2015).
The eggs are flattened, and whitish in colour, they show concentric folding around the edges, but not crazing or cracking which would be associated with a crushed calcified egg. Wang et al. attempted to detect traces of calcium in the shells of the eggs using Energy Dispersive Spectroscopy under a Scanning Electron Microscope, but were unable to do so, from which they conclude that the eggs were either never calcified or have lost all their calcium, either by re-absorbtion by the female prior to death or during the preservation process.
The eggs measure 27.8 by 19.8 mm and 27.9 by 18.2 mm respectively. Following attempts to reconstruct these flattened dimensions with hatched, waterlogged snake eggs leads Wang et al. to conclude the original eggs were between 3.19 g and 4.50 g in mass (considerably smaller than Lû et al.'s estimates), and would have produced hatchlings in the range 2.75 g to 3.75 g.
The presence of two eggs within and closely associated with the specimen strongly suggests that the female was capable of carrying two eggs at the same time, which in turn strongly suggests that the Pterosaur had two functioning ovaries (no living animal is capable of producing two large eggs from a single ovary in this way). This is different to the situation in modern Birds, where one ovary has been lost, presumably as part of the general trend to lose weight and improve flight performance in Birds.
Wang et al. also examined the femur bone of the specimen for traces of a medullary bone layer. This is a tissue produced in female Birds which stores calcium, allowing this to be rapidly deposited into developing eggs and foetuses, and which has also been found in some Dinosaurs. Were medullary bone to be produced in Pterosaurs then is would be expected to be found in a gravid female. However no trace of medullary bone could be found, suggesting that this tissue arose after the split between Dinosaurs and Pterosaurs.
Unfortunately the early life history if Pterosaurs is largely unknown. A number of Pterosaur eggs have previously been recovered, but no nests or hatchlings, and it is unclear to what extent young Pterosaurs relied upon parental care. It is known that Pterosaurs took several years to reach maturity, and that they were capable of flying and even breeding before reaching maturity. The new specimen cannot shed any direct light upon the early life of Pterosaurs, in other Diapsids small parchment shelled eggs tend to be associated with large clutch sizes, suggesting a reproductive strategy closer to Crocodiles or Turtles rather than Birds.
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