Live birth in an archosauromorph reptile

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.

intervertebral articulation between the 10th and 11th cervical vertebrae. The 8th cervical ribs extend posteriorly at least to the middle of the 12th cervical vertebra. The 9th cervical ribs extend posteriorly at least beyond the intervertebral articulation between the 12th and 13th cervical vertebrae. Posterior extension of left cervical ribs remains unclear because of weathering. Cervical ribs extend posteriorly parallel to the neck axis. They are dichocephalous, with anterior processes.
Dorsal ribs are short and stout compared with the cervical ribs. In LPV 30280, all of the visible dorsal ribs are holocephalous.
There are three caudal ribs preserved, associated with the relevant caudal vertebrae in LPV 30280. Caudal ribs are curved and separated from relevant caudal vertebrae.
Some gastral ribs are also preserved. Each gastral rib is composed of one angulated medial element and two lateral elements. Hind limb. Part of the hind limbs is preserved in LPV 30280. Only the distal end of one femur is preserved. Nothing else can be said about this bone. Both tibiae are preserved but with only the distal part exposed. The distal end of the tibia is much more delicate than the fibula. The fibula is distinctly curved. Compared with the proximal end, the distal end of the fibula is distinctly expanded.
There are two ossified tarsals preserved in each limb. Both are round elements. The larger one is identified as the astragalus, and the smaller one as the calcaneum.
All metatarsals are preserved. The first metatarsal is the shortest, while the fourth is the longest. The first metatarsal is different from other metatarsals in that its distal end is not expanded. The 5th metatarsal is straight without a hooked posterior margin, and is distinctly longer than the first metatarsal.
The phalangeal formula of the pes is unclear since the preservation is incomplete. However, the 4th digit preserves a complete sequence with six phalanges. This indicates mild hyperphalangy in the pes of LPV 30280, as in IVPP V13898. The length of the first phalanx of the 5th digit is subequal to the metatarsal, but significantly shorter than other metatarsals.

Supplementary Note 2. Character description
If a character is originally from Benton and Allen 1 , Jalil 2 or Dilkes 3 , it is noted as B/J/D plus the original character sequence in the relevant reference. If the coding for a character is different from Rieppel et al. 4 , a note is given in the text below. The character sequence has been changed to reflect the anatomical grouping. The character list was prepared using NDE Version 0.5.0.
Midline length less than maximum width 2.
Relative length of snout (D2, J64): Nasal, shape of cranial margin at midline (D13, J49): Boreopricea is recoded from 1 to ? since the reconstruction of the anterior region of the nasals is largely imaginative. 0. strongly convex with anterior process 1.
transverse with little convexity 16.
Nasals, relative length (B2, D18): The reconstruction by Carroll 10 contradicts with that by Gow 6 , so Youngina is recoded from 1 to ? temporarily. Macrocnemus is recoded from ? to 1 based on the GMPKU specimen. Boreopricea is recoded from 0 to ? since the anterior part of the nasal is incompletely preserved. 0. shorter than frontals 1.
Frontal, shape of dorsal surface next to sutures with postfrontal and parietal (D20): 0. flat to slightly concave 1.
longitudinal depression with deep pits 18.
large posterior process and smaller anterior process 2.
equally developed anterior and posterior processes 3.