The persistence of egg-laying in modern Monotremes has led evolutionary biologists to conclude that this is likely to have been the ancestral state in the Synapsids, the group from which the Mammals arose. However, fossil evidence for this has been surprisingly absent. The earliest known potential fossil amniotic egg comes from the Permian of South America, and has been attributed to a Mesosaurid Sauropsid (a group not closely related to Synapsids of Mammals). This specimen preserves an immature skeleton curled in a position consistent with having been in an egg at the time of death, but no actual eggshell (not altogether surprising, as the earliest amniote eggs are not predicted to have been mineralised). The earliest amniotic egg fossils with both embryonic remains and eggshell come from Sauropodomorph dinosaurs from the Early Jurassic of Gondwana. Some potential eggs associated with Synapsid Pelycosaurs from the Early Permian of North America are not considered to be reliable, as neither embryos nor shell structures are preserved.
The Late Triassic-Early Jurassic Elliot Formation of South Africa's Karoo Basin has produced numerous Dinosaur egg fossils with embryos, as well as the skeletal remains of many non-Mammalian Cynodonts, something which has led to questions about whether Permo-Triassic Synapsids laid eggs at all. This is a serious consideration; Synapsids, particularly groups such as Lystrosaurus and Diictodon, are extremely common in the Permian and Triassic of the Karoo, with perinate specimens (specimens thought to have died around the time of birth or hatching) being found here and elsewhere, but no eggs are known. The preservation of Dinosaur eggs in the Karoo suggests there was no taphonomic process here producing a bias against the preservation of eggs, and palaeontologists have been active in the Karoo Basin for over 180 years, suggesting that if such eggs were present, there should have been a good chance of their being found. Egg-laying and bearing live young are found in closely related Snakes and Lizards, and it appears that this group has been able to switch back-and-forth between these conditions fairly easily. It is therefore conceivably possible that Synapsids developed the ability to bear live young very early in their history, and that Monotremes have secondarily switched back to egg-laying.
However, this has wider implications than Synapsid palaeontology. Current theories on the origin of lactation in Mammals have been built on the assumption that this preceeded the switch to live-birth (largely because Monotremes produce both eggs and milk). It is now generally accepted that the purpose of lactation was not originally to feed the young, but rather started as skin secretions used to either moisturise the eggs, provide nutrients, protect them against fungi and bacterial infections, or for hormonal signalling through the egg membrane. Should it be found that the Synapsids from which Mammals evolved bore live young, then these theories would have to be abandoned.
In a paper published in the journal PLoS One on 9 April 2026, Julien Benoit of the Evolutionary Studies Institute at the University of the Witwatersrand, Vincent Fernandez of the European Synchrotron Radiation Facility, and Jennifer Botha of the Evolutionary Studies Institute and Centre of Excellence in Palaeosciences at the University of the Witwatersrand, describe three perinate specimens of the Dicynodont Synapsid Lystrosaurus from the Early Triassic of Xhariep Municipal District in Free State Province, South Africa, one of which appears to have been preserved within an egg.
The specimens examined are the three smallest specimens attributed to Lystrosaurus. They include BP/1/4011, an isolated skull measuring 43.0 mm, discovered by James Kitching in the upper Palingkloof Member of the Balfour Formation at Orangia on Tweefontein 508, BP/1/9332, an almost complete articulated skeleton with a skull length of 44.0 mm, discovered by Brandon Stuart in the upper Palingkloof Member of the Balfour Formation at Nooitgedacht 68 Farm near Spitskop, and NMQR 3636, a complete skeleton with a skull length of 34.5 mm, found by John Nyaphuli at Rheeboksfontein 5 Farm in 2008, probably from the upper Palingkloof Member of the Balfour Formation or the lower Katberg Formation.Each of these fossils was a scanned at the European Synchrotron Radiation Facility in Grenoble, France, with three dimensional models being reconstructed with the Avizo Software Package.
The isolated skull BP/1/4011 was described by Kitching as the smallest known skull attributed to Lystrosaurus in 1964, and attributed to either Lystrosaurus murrayi or Lystrosaurus curvatus by a study in 2006. Benoit et al. are more cautious, attributing it to Lystrosaurus sp. but suggesting it shows affinities to Lystrosaurus curvatus.
The first of the articulated skeletons, BP/1/9332, is considered to be an early juvenile of Lystrosaurus sp., with affinities to Lystrosaurus murrayi. It is preserved in a splayed out position, similar to that of most larger Lystrosaurus specimens from the Karoo Basin, with most bones perfectly articulated, and synchrotron images show that no loose elements are preserved in the surrounding matrix. It appears to be the most developmentally advanced of the three specimens, because its splenials are co-ossified at the mandibular symphysis, although its occipital and basicranial bones remain loose. From the splayed out position in which it was found, Benoit et al. determine that it had hatched before dying, probably moving some distance from its hatching site before death.
The final specimen, NMQR 3636, is also considered by Benoit et al. to be an early juvenile of Lystrosaurus sp., with affinities to Lystrosaurus murrayi. However, unlike BP/1/9332, this specimen is curled into a fetal position, consistent with having been within an egg at the time of death. It also appears to be the most developmentally immature of the specimens, lacking tusk buds in its maxillary alveolae, something present in both the other specimens, or a mesethmoid bone, the structure that supports the olfactory bulbs in life, which is again present in the other two specimens.
Most notably, the lower jaw of NMQR 3636 has an incompletely co-ossified symphyseal suture between the two paired bones in the lower jaw. This is completely co-ossified in both the other specimens, as well as in modern beaked Amniotes such as Turtles and Birds at the time of hatching. Modern Monotremes do hatch with an unfinished intermandibular symphysis, but these feed on milk provided by their mothers for some time after hatching, something Lystrosaurus is not thought likely to have been able to produce.
Based upon this, Benoit et al. conclude that the early developmental stage of the skeleton, combined with a posture which would be expected of a perinate prior to hatching and a jaw which had not developed to the stage where it could feed on the hard foodstuffs likely to have been consumed by juvenile Lystrosaurus. is indicative of an Animal which died within the egg and was subsequently preserved, albeit without preservation of the egg itself.
Based upon the position of the embryo, it is estimated that the original egg was 3.65 cm long and 2.75 cm in diameter, with an internal mass of 115 cm³ and a mass of 115 g. While size estimates for adult Lystrosaurus vary, this is clearly larger compared to the size of an adult than either living Monotremes or most non-Avian Reptiles, although comparatively smaller than the eggs of Birds. This is probably indicative of a large yolk, which can feed the embryonic Animal for longer, allowing it to develop further within the egg.
Modern Monotremes produce small eggs compared to the size of an adult, which contain comparatively little yolk material. This is possible because the young hatch at an early developmental stage, and are then nourished with milk. Interestingly, the Jurassic Tritylodontid Cynodont Kayentatherium produced eggs which were even smaller compared to the size of an adult. While Kayentatherium has been reconstructed as being quite Reptile-like in physiology, the small egg size could be a sign that it was capable of a form of lactation. It has also been suggested that Kayentatherium probably had hair, something which is known to be linked genetically to the formation of mammary glands (which produce milk), and it has also been shown that there is a genetic link between the reduction in egg yolk production and the ability to produce milk. All of which suggests that Kayentatherium may have been more Mammal-like than previously reconstructed, and that the appearance of the ability to produce milk may have been closely linked to the emergence of the Mammaliamorpha.
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