An exceptionally preserved gravid female of the aigalosaur Carsosaurus contains at least at least four advanced embryos […] Their orientation suggests that they were born tail-first […] to reduce the possibility of drowning, an adaptation shared with other other highly aquatic amniotes” M.W. Caldwell & M.S.Y. Lee (2001) Proceedings of the Royal Society of London B, vol. 268, p.2397

Oviparity and viviparity

Forms of vertebrate reproduction can be divided into either ‘oviparous’ (egg-laying) or ‘viviparous’ (live-bearing), as first described by Aristotle in his Historia Animalium. In egg-laying species, the developing embryo obtains nutrients from a store of yolk (termed ‘lecithotrophy’), whereas viviparity is a broad term encompassing varied divergence from simple egg retention in the female oviduct to elaboration of complex placental structures between fetal and maternal tissues. During viviparous gestation, embryos may acquire nutrients by digestion of ovum yolk (‘lecithotrophy’), intake of nutrients transferred from the mother (‘matrotrophy’), or – in certain viviparous amphibians and fish only – by consuming sibling ova or embryos in utero. Current evidence suggests that genetic and phenotypic changes essential to acquiring viviparity strongly (but not absolutely) resist reversal to oviparity. Accordingly, in this case of evolution, not only is the direction of the “tape of life” very much determined, but it is very difficult to rewind.

Viviparity in reptiles and other vertebrates

Viviparity has evolved more than 120 times in vertebrates, and surprisingly almost all of the transitions from oviparity to viviparity have occurred in the reptiles – the remaining occurrences being restricted to a few fish (sharks and rays), amphibians (certain salamanders and caecilians) and the well-known eutherian mammals, which are characterised by highly specialised placental structures for fetal nutrition. Over 85% of reptile species are oviparous, and yet viviparity has evolved convergently over 100 times. Most of these independent events have occurred in squamates (lizards, snakes and amphisbaenians), but the fossil record of ichthyosaurs, sauropterygians (best known, perhaps, for the plesiosaurs) and mosasaurs clearly indicates that live birth also evolved in several disparate lineages of Mesozoic marine reptiles. Here we focus on the mosasaurs…

Viviparity in mosasaurs

Mosasaurs were serpentine marine predators that lived in warm, shallow seas during the Cretaceous period, dominating the niche previously occupied by plesiosaurs and ichthyosaurs. They evolved from semi-aquatic primitive forms termed aigialosaurs (e.g. Aigialosaurus, Dallasaurus), which resembled monitor lizards and were up to a few metres long. The transition from aigialosaur to true mosasaur involved evolution of short, paddle shaped limbs, a long, streamlined tail for highly efficient swimming and an enlarged body size (up to 17m long). The aquatic habitat of mosasaurs suggests that they gave birth to live young (lacking anywhere to lay eggs!), but more compelling still is fossil evidence for viviparity that was discovered in Mid-Cretaceous limestone deposits in Slovenia in the form of a primitive ‘mosasauroid’ belonging to species Carsosaurus marchesetti. Its skeleton shows that Carsosaurus was well adapted for swimming as well as movement on land, and proof that it was viviparous comes from a pregnant (gravid) female specimen holding four embryos within paired oviducts that run along her posterior trunk. As aigialosaurs were the ancestors of the ocean-bound mosasaurs, we may assume from the Carsosaurus finding that later mosasaur lineages were indeed also viviparous.

Although their relationship to other reptiles is not completely certain, mosasauroids (aigialosaurs and true mosasaurs) appear to be closely related to the group of squamates known as varanoid, or monitor, lizards. Accepting a link between mosasaurs and squamates places them in the larger category known as lepidosaurs (‘reptiles with overlapping scales’), notably separating them from other marine reptiles which, as a group termed the euryapsids, diverged from the lepidosaurs more than 250 million years ago. The wide evolutionary separation between mosasaurs and other major groups of marine reptiles such as ichthyosaurs and sauropterygians makes the emergence of viviparous gestation in these taxa a striking example of convergent evolution. In addition to this, the likely origin of mosasaurs from within the Squamata highlights the remarkable propensity for independent squamatan lineages to acquire adaptations leading to viviparity, and finally, clarifies the status of reptilian viviparity as a prime case study in convergent evolution.