Ichthyosaurs were marine reptiles that patrolled the oceans of the Mesozoic era (251 million to 65.5 million years ago). Similar in appearance to modern dolphins, with pointed snouts and streamlined bodies, they were apex predators. The largest reached 25 metres in length.
A new fossil discovery indicates that some species had enormous litters of babies. A paper published in the journal PeerJ in July 2026 describes an ichthyosaur (Mixosaurus panxianensis) found in Panzhou County, Guizhou Province in southwestern China.
The specimen dates to the Anisian age (247-242 million years ago) of the Triassic period. In addition to the 1.1 metre adult, the fossil also includes at least 11 foetuses in a late stage of development - they were nearly ready to be born.
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“Whenever you hear about such a find, there is a fear that it has been over-sensationalised and the actual work won't support headline claims,” says Bethany Burke Franklin, a vertebrate palaeontologist with the Heritage Museum of the Texas Hill Country who was not involved in the research. However, she considers the analysis of the remains to be thorough.
Ichthyosaurs were viviparous - that is, they gave birth to live young. Viviparity was noted in ichthyosaur specimens as early as 1749 and fossilised remains of 11 genera contain foetuses.
“Gravid specimens of the basal ichthyosauriform Chaohusaurus indicate that live birth was already established in ichthyosaurs in the Early Triassic. Live birth was likely a key adaptation of the clade towards a fully marine lifestyle, as it removed the need to return to land to lay eggs,” says Cole Counts, who focuses on ichthyosaurs in his doctoral research at Southern Methodist University. Counts was not involved with the research either.
Early researchers believed that these fossil inclusions may have been prey animals - maybe even young ichthyosaurs that had been cannibalised. Later analysis found that specimens that included foetal remains lacked evidence of digestion, such as disarticulation of skeletons and acid etching that would be typical of an animal that had been eaten.
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The foetuses in the Mixosaurus fossil are remarkably well-preserved, indicating that they too were gestating rather than simmering in gastric juices. They all appear to be in a similar stage of development and are all roughly the same size. Some remained within the uterine cavity but others were ejected - possibly due to a gas explosion as the mother’s body decayed. Scattered bones around the mother indicate that the litter may have been even larger.
“Other species of Mixosaurus are preserved with two to three pups. Mixosaurids are among the most cosmopolitan reptile families of the Triassic Period, and this variation in litter size reflects a degree of flexibility in reproductive strategies. This versatility may have helped ichthyosaurs successfully proliferate across a wide range of ecosystems throughout the Middle Triassic,” Counts says.
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While the large litter sizes of this species are remarkable, Burke Franklin cautions that its fecundity may not have contributed to the overall abundance of the group, as the authors of the paper suggest.
“While having loads of young is helpful for survival, it is not my understanding that this is an established approach taken by ichthyosaurs,” she notes.
Still, the fossil stands out as probably the largest preserved litter of ichthyosaur babies – one other specimen from the Jurassic contains an equivalent litter, but the newly described Chinese fossil is far better preserved.
Top image credit: Shengxiao Gu et al. PeerJ 2026, under open access license CC BY 4.0
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