468 FOSSIL VERTEBRATES/Palaeozoic Non-Amniote Tetrapods Shu D G, Conway Morris S, Han J, et al (2003) Head and backbone of the Early Cambrian vertebrate Haikouichthys Nature 421: 526 529 Vorobyeva E and Schultze H P (1996) Description and systematics of panderichthyid fishes with comments on their relationship to tetrapods In: Schultze H P and Trueb L (eds.) Origins of the higher groups of tetrapods: controversy and consensus, pp 68 109 Cornell: Cornell University Press Wilson MVH and Caldwell MW (1998) The Furcacaudi formes: a new order of jawless vertebrates with thelodont scales, based on articulated Silurian and Devonian fossils from northern Canada Journal of Vertebrate Palaeon tology 18: 10 29 Young GC (1997) Ordovician microvertebrate remains from the Amadeus Basin, central Australia Journal of Vertebrate Paleontology 17: 25 Young GC, Karatajute Talimaa VN, and Smith MM (1996) A possible Late Cambrian vertebrate from Australia Nature 383: 810 812 Zhu M, Yu X B, and Janvier P (1999) A primitive fossil fish sheds light on the origin of bony fishes Nature 397: 607 610 Palaeozoic Non-Amniote Tetrapods J A Clack, University of Cambridge, Cambridge, UK ß 2005, Elsevier Ltd All Rights Reserved Introduction The Late Palaeozoic saw the emergence of land vertebrates from their aquatic ancestors Vertebrates with legs and digits are usually referred to as ‘tetrapods’, meaning ‘four-legged’ In fact, the term not only refers to ancient extinct forms, but also embraces all modern forms which either possess legs (or arms, wings, or flippers) or whose ancestors possessed them (such as snakes and others that have lost them) This great ‘family tree’ of animals has its roots in the Late Palaeozoic, specifically the Late Devonian, when the first legged vertebrates evolved Because of the nature of evolution, it is, in practice, sometimes difficult to draw the distinction between a ‘fish’ and a ‘tetrapod’, especially when there are no limbs or fins preserved in the fossils we have; therefore, to understand the emergence of tetrapods, it is also necessary to look at the ‘fish’ relatives from whom the ‘tetrapods’ evolved This article therefore starts by describing a few ‘tetrapod-like fish’ and their relationships to the ‘fish-like tetrapods’ that evolved from them, before going on to see what kinds of animals emerged from the swamps and began their colonization of the land during the Carboniferous and Permian periods It was during the Late Palaeozoic that the foundations were laid for fully terrestrial living vertebrates, when they adapted their skeletons and physiologies for a very different environment from that in which they first evolved Breathing mechanisms, feeding mechanisms and strategies, sensory systems, reproductive modes, and locomotory techniques all had to be modified during this greatest of evolutionary transitions, and many of these changes are reflected in skeletal changes that can be picked up in the fossil record The differing means by which this was achieved is also reflected today in the legacy of modern tetrapod anatomy and physiology The general reader may be aware that modern tetrapods fall into a number of major groups, namely the amphibians, reptiles, birds, and mammals, but in fact these can be clumped into two coherent but biologically divergent groups, with the amphibians (known as ‘lissamphibians’) forming one lineage and the rest forming the other, known as the amniotes Amphibians and amniotes each trace their origins to the Late Palaeozoic and, within the amniotes, each of the subgroups can also be traced back to a Carboniferous or Permian origin By contrast, one of the least understood episodes of tetrapod evolution is the origin of modern lissamphibian groups (of which there are only three: frogs, salamanders, and caecilians) None of these can be found in the fossil record before the Early Jurassic, and the earliest stem member is Triadobatrachus, an animal with both frog- and salamander-like features from the Late Triassic To be called an amphibian in the strict sense, an animal would be more closely related phylogenetically to these modern forms than to any amniote group Amongst Palaeozoic forms were also many lineages that have left no modern descendants, and whose origins not seem to fall into either the amphibian or the amniote lineages, and these simply have to be called ‘early tetrapods’ (They may have had an ‘amphibious’ life style, laying eggs in water and emerging onto land only as adults, but this describes their mode of life, not their relationships, and in many cases there is no evidence of what their reproductive mode might