Note

Life History in Stegocephalians

Michel Laurin

The life history cannot be reconstructed with confidence for all the extinct terrestrial vertebrates, but a few taxa exceptionally well represented in the fossil record give us a glimpse at the life history of these taxa. Some temnospondyls and seymouriamorphs are known to have had larvae with external gills, even though the adults of at least some of these taxa were terrestrial. The external gills, along with grooves for the lateral-line organ (a structure composed of ciliated cells that detects motion in the water) indicate that these larvae were aquatic. Therefore, early stegocephalians presumably reproduced in the aquatic environment, probably in streams and ponds.

The life history of tetrapods is much better known. Most tetrapods are oviparous (frogs, salamanders, some caecilians, the Platypus, and most reptiles), although some are ovoviviparous (most species of the salamander Salamandra, two species of the toad Nectophrynoides, some squamates) or viviparous (some caecilians, one species of Nectophrynoides, most mammals, and some squamates). In ovoviviparous taxa, the egg remains in the body of the mother until hatching, but the embryo is nourished entirely by the yolk of the egg, whereas in viviparous forms, at least part of the nourishment is provided by the mother. Oviparous tetrapods may lay their eggs in streams and ponds (many amphibians), near or above freshwater bodies (many amphibians, some amniotes), near the ocean (leatherback turtles), or on dry land (a few amphibians, many amniotes). Fertilization may be internal (amniotes, caecilians, most salamanders, and four genera of frogs) or external (hynobiid and cryptobranchid salamanders and most frogs). Development may proceed via a larval stage (many frogs and salamanders) or be direct (amniotes, some caecilians, some salamanders, leiopelmatid frogs).

Amphibian eggs do not fossilize well because they have no shell, but even amniote eggs are relatively uncommon. The oldest known amniotic eggs are from the Upper Triassic (Bonaparte and Vince, 1979), but they are very rare in the fossil record until the Cretaceous (Mikhailov, 1991). No Paleozoic tetrapods are known to have had larvae, but this could be an artifact of preservation because little is known about the ontogeny of lepospondyls and diadectomorphs. Many lissamphibians still reproduce in freshwater environments, and this could be a primitive character. Early amniotes probably laid eggs on dry land.

References

Bonaparte, J. F. & M. Vince. 1979. El hallazgo del primer nido de dinosaurios Triasicos, (Saurischia, Prosauropoda), Triasico Superior de Patagonia, Argentina. Ameghiniana. Revista de la Asocioción Paleontolóngica Argentina 16: 173-182.

Mikhailov, K. E. 1991. Classification of fossil eggshells of amniotic vertebrates. Acta palaeontologica polonica 36: 193-238.

About This Page

Michel Laurin
Muséum National d'Histoire Naturelle, Paris, France

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