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.
See
also...
Azhdarchid Pterosaur cervical vertebra from the Late Cretaceous of the Haţeg Basin in Transylvania. Azhdarchids were long-necked,
toothless Pterosaurs, which came to dominate Pterosaur assemblages in...
The Loma del Pterodaustro lake deposits of Central Argentina have produced large numbers of the Pterosaur Pterodaustro guinazui...
Preserved ovarian follicles in Mesozoic Birds. The two surviving groups of Archosaurs, Crocodilians and Birds, show
very different reproductive strategies. Crocodilians have paired
ovaries and produce...
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