Terrestrial origin of viviparity in Mesozoic marine reptiles indicated by Early Triassic embryonic fossils

Maternal specimen AGM I-1 directly documents pregnancy in Chaohusaurus through two embryos and an adjacent neonate, while head-first delivery and a terrestrial origin of viviparity remain orientation- and topology-based interpretations.

The Ichthyosauria navigation envelope starts at the approximately 248 Ma Chaohusaurus specimen from the Procolumbites Zone and retains 90 Ma as a rounded younger composite; the sampled occurrence is not an exact global first appearance, complete lineage duration or direct ancestor.

Chaohusaurus is anchored by maternal specimen AGM I-1 from the Procolumbites Zone at Majiashan; the locality record does not establish a global genus range, an exact first appearance or ancestral status.

Chaohusaurus is assigned to the Early Triassic ichthyopterygians in Motani et al.'s study using delayed ossification of carpals and tarsals, dental morphology, vertebral count and hypophalangeal flippers; species-level taxonomy may require revision as more specimens are prepared.

The profiled Chaohusaurus record is limited to south Majiashan Quarry at Chaohu, Anhui, China, where the Spathian Nanlinghu Formation is exposed; this named collection does not establish a complete geographic distribution.

Maternal specimen AGM I-1 preserves an articulated Chaohusaurus skeleton with two embryos in the body and a neonate adjacent to the pelvis, while the genus assignment uses its documented dental, vertebral and flipper characters.

AGM I-1 directly documents pregnancy in an Early Triassic ichthyosauromorph; the head-first embryo orientation and a proposed terrestrial origin of viviparity are comparative interpretations, not directly observed behaviour or an ancestral state for all ichthyosaurs.