Wednesday, February 22, 2017

[Paleontology • 2017] Live Birth in An Archosauromorph Reptile, Dinocephalosaurus



Abstract
Live birth has evolved many times independently in vertebrates, such as mammals and diverse groups of lizards and snakes. However, live birth is unknown in the major clade Archosauromorpha, a group that first evolved some 260 million years ago and is represented today by birds and crocodilians. Here we report the discovery of a pregnant long-necked marine reptile (Dinocephalosaurus) from the Middle Triassic (∼245 million years ago) of southwest China showing live birth in archosauromorphs. Our discovery pushes back evidence of reproductive biology in the clade by roughly 50 million years, and shows that there is no fundamental reason that archosauromorphs could not achieve live birth. Our phylogenetic models indicate that Dinocephalosaurus determined the sex of their offspring by sex chromosomes rather than by environmental temperature like crocodilians. Our results provide crucial evidence for genotypic sex determination facilitating land-water transitions in amniotes.




Figure 3: Skeleton of the new Dinocephalosaurus specimen LPV 30280.
(a) Photograph. The three separate blocks are arranged following their original positions in the field. (b) Interpretive drawing. Dotted line indicates the rough course of the vertebral column of the adult. The different colour in the cervical region aims to facilitate the association of cervical ribs with corresponding vertebrae. (c) Photo showing a close-up of the embryo preserved in the stomach region of LPV 30280. (d) Interpretive drawing of the embryo. (e) Photo showing a close-up of the perleidid fish preserved in the stomach region of LPV 30280. (f) Artist's reconstruction of Dinocephalosaurus showing the rough position of the embryo within the mother. ax, axis; car, caudal rib; crh, cervical rib head; cv, cervical vertebrae; d4, fourth digit; f, perleidid fish; fe, femur; fi, fibula; h, humerus; ha, haemal arch; m, mandible; mt1, metatarsal 1; mt5, metatarsal 5; poz, postzygapophysis; prz, prezygapophysis; ra, radius; ti, tibia; ul, ulna. Scale bar, 20 cm. 

Jun Liu, Chris L. Organ, Michael J. Benton, Matthew C. Brandley and Jonathan C. Aitchison. 2017. Live Birth in An Archosauromorph Reptile. Nature Communications. 8, 14445. DOI:  10.1038/ncomms14445

Everyone thought this ancient reptile only laid eggs. They were wrong. http://ti.me/2l7PeKT
  

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