Tetrapods can be loosely thought of as vertebrates that either have limbs or evolved from limbed ancestors. Among living vertebrates, the birds, mammals, reptiles and amphibians are considered tetrapods. Earlier in Earth's history, however, during the Devonian period, animals such as Ichthyostega and Acanthostega clearly possessed limbs while also retaining a finned tail and other fish-like anatomical features. The existence of these strange forms allows palaeontologists to indulge their passion for hair-splitting taxonomic debate by arguing over whether or not these animals should be considered "real" tetrapods.
By the following geological period, the Carboniferous, things were somewhat less ambiguous. Ichthyostega and Acanthostega had obligingly disappeared from the fossil record (although leaving some traces in the earliest Carboniferous of Nova Scotia), leaving a wide range of definite tetrapods that could be broadly classified into a relatively small number of major groups (or "taxa"). Throughout the Carboniferous, new taxa continued to appear and old ones became more diverse, a process that continued on into the Permian period. Fortunately, the evolutionary relationships between the different groups are somewhat controversial, so that Carboniferous and Permian researchers still have something to argue about. However, the membership of each major taxon is fairly well established.
The evolutionary tree depicted below shows one current set of ideas about the phylogeny of tetrapods during this time period. Among the oldest tetrapods were the highly diverse amphibians known as temnospondyls, which were close to the ancestry of both another amphibian group (the lepospondyls, which may have given rise to modern frogs and salamanders--although this point is highly contentious, even among current and former lab members!) and the amniotes - the taxon including reptiles, birds and mammals, originally characterised by the ability to produce large eggs in which the developing embryo is surrounded by a series of membranes. Among amniotes, there was a fundamental division between the ancestors of mammals (pelycosaurs, followed by therapsids) and those of birds and reptiles (parareptiles, which may have included the ancestors of turtles, and "true" reptiles, also known as eureptiles). To find out more about a particular group, click its name on the diagram.
It should be noted that not all Palaeozoic tetrapods fall into the groups depicted on the above diagram. Conspicuously absent are a number of minor and relatively primitive amphibian groups such as the seymouriamorphs and embolomeres, which were formerly grouped with the temnospondyls as the "labyrinthodont" amphibians. Labyrinthodonts were supposedly characterised by deep grooves on the teeth (labyrinthine infolding) and odd vertebrae composed of several small bony elements. However, it is now apparent that these features are primitive and therefore unreliable as indicators of evolutionary relationships. The labyrinthodonts have been broken up into an untidy assemblage of smaller clades which are no longer thought to be closely related to one another, and all except the temnospondyls have been left out of the above tree for the sake of clarity.
Temnospondyls
These amphibians retained the grooved teeth and complex vertebrae of the earliest tetrapods (and their ancestors among the fishes), but were quite diverse in their other features. While some Carboniferous and Permian temnospondyls attained very large sizes (the predatory Eryops, pictured here, had a body length of up to 2 metres) the majority were much smaller. Among the few completely terrestrial forms were dissorophids such as Cacops, which had an armadillo-like covering of bony plates overlying its backbone. Common (though not universal) temnospondyl features included a sprawling posture, a wide, stocky body, a long tail, heavily built limbs, large fangs on the roof of the mouth, and a pattern of shallow pits covering the skull bones. Temnospondyls survived well into the Mesozoic Era (or "Age of Dinosaurs"); many later forms were large, vaguely crocodile-like predators. Most scientist believe that temnospondyls, particularly species closely related to Cacops, gave rise to modern amphibians, or at least frogs. However, work done in this lab suggests that lepospondyls may actually be responsible for modern amphibian origins. Further work is necessary to clarify this issue.
Lepospondyls
The lepospondyls were small amphibians with vertebrae that formed as a single unit rather than as separate parts that later fused together (as was the case in the generally larger temnospondyls, and we'll ignore the intercentra present in some microsaurs to keep the story simple). Distinct types included the limbless, serpentine aistopods; the broadly salamander-like microsaurs; and the nectrideans, which had distinctive vertically symmetrical tail vertebrae and often (as in the illustrated genus, Diplocaulus) sported enormous flat skull projections that made their heads resemble miniature boomerangs. Recent studies have found that these animals form a unique evolutionary unit. It is possible that one of the lepospondyl groups was ancestral to some if not all the modern amphibians, which are very different from their Palaeozoic ancestors. In any case, this taxon should keep palaeontologists arguing (if not exchanging blows) for many years to come...
Pelycosaurs
Although rather reptilian and almost certainly cold-blooded, these animals were ultimately ancestral to mammals. Like their descendants, the therapsids and eventually the mammals, they display the "synapsid condition": a single opening, equivalent to a human's temple, in the side of the skull. Many were predators, a category incorporating both ferocious monstrosities such as Dimetrodon and smaller, more lightly built forms like Aerosaurus (see illustration), which were probably monitor-like predators. Others, such as the sail-backed Edaphosaurus and the peculiar Cotylorhynchus (an animal whose tiny head seems mismatched with its huge, stocky body) represent the earliest known herbivores. Another large genus, Ophiacodon, is sometimes considered to have been a semi-aquatic fish eater, though its high slender snout (which would be difficult to swing through the water) cannot easily be reconciled with this interpretation. Both Dimetrodon and Edaphosaurus are famous for the enormous spines growing upward from their vertebrae, which may approach a metre in length. These structures probably supported a membrane of skin and blood vessels; no one knows the exact function of this "sail", although a role in visual signalling and/or temperature regulation (by providing a large surface for heat loss or gain) seems likely. It was certainly a striking feature, and has secured a place for both Dimetrodon and Edaphosaurus in many popular books on palaeontology. Both animals look as though they would probably blow over in a stiff wind!
Therapsids
Therapsids probably evolved from a carnivorous pelycosaurian ancestor. They were the most diverse and ecologically important land vertebrates in the later part of the Permian Period, and many different groups are known from the fossil record. One, the dicynodonts, was made up of stocky herbivores that apparently fed using horny beaks similar to those of living turtles and birds; many dicynodonts (such as Pristerodon, pictured at left) were entirely toothless, and others had only large canine tusks that may have been used in fighting or digging rather than feeding. Other major therapsid groups, including dinocephalians, gorponopsians, therocephalians and cynodonts, were either entirely carnivorous or included a combination of carnivorous and herbivorous species. The cynodonts eventually gave rise to modern mammals, and show signs of typically "mammalian" features such as an upright posture, a hard secondary palate (separating the mouth cavity from the air passages) and a complex dentition with several distinct kinds of teeth. The name cynodont, which roughly means "dog-tooth", refers to the especially prominent canines. Therapsids are diverse and interesting, but they have their detractors: the famous evolutionist (and Jesuit priest) Teilhard de Chardin once referred to them as "clumsy and deformed" (The Phenomenon of Man, p. 128).
Parareptiles
Parareptiles have only recently been recognised as an important and distinct taxonomic assemblage. Just as paramedics are not exactly doctors, and paramilitary forces are not exactly military forces, parareptiles are not exactly "true" reptiles (or eureptiles); they almost certainly laid hard-shelled eggs and were basically reptilian in appearance, but differed from eureptiles in details of their skeletal anatomy. Although some parareptiles were small and rather lizard-like, this group also includes the procolophonids (such as the illustrated specimen, Owenetta) and pareiasaurs, taxa that have both been considered as possible ancestors for living turtles. Again, this relationship depends on skeletal details - it's a bit ironic that the only really obvious resemblance, between the shells of turtles and the heavy "dermal armour" of some pareiasaurs, may in fact be a very superficial and irrelevant point of similarity. Pareiasaurs are also unusual in that some of them grew to very large sizes, and attained an semi-upright posture, somewhat similar to that of mammals. Another interesting group of parareptiles are the mesosaurs, which were the first marine reptiles of any kind. Their mouths were filled with numerous slender teeth that would have formed an efficient trap for small fish and invertebrates.
Eureptiles
Eureptiles include modern lizards, snakes and crocodiles, as well as dinosaurs, pterosaurs, the large marine reptiles of the Mesozoic, and even (technically) birds! However, most of this diversity arose after the end of the Palaeozoic - some Permian eureptiles, such as Captorhinus (see illustration), were very widespread and abundant, but most were basically similar to small modern lizards in their general appearance. Captorhinus and its relatives were quite stocky and heavily built, with greatly widened neural arches (the upper part of the vertebra, enclosing the spinal cord) but certain other early eureptiles, such as Araeoscelis, were slender and long-limbed. In many early species (though not Araeoscelis) the skull was solidly constructed, lacking the temporal openings that are seen in all living eureptiles.
Further Reading
For reasons that aren't entirely clear to us, Palaeozoic vertebrates have never been a "hot topic" with the public (especially not in comparison to, for instance, a certain group of Mesozoic reptiles whose name begins with a "d"), and most publications that discuss them tend to be a bit technical and specialised. Vertebrate palaeontology textbooks such as Vertebrate Paleontology and Evolution by Robert Carroll (W.H. Freeman & Co., 1988) and Vertebrate Palaeontology by Michael Benton (Blackwell Science, 1997) provide some information and also contain references to papers in the scientific literature. Some of the more general web sites listed on our links page also might have a page or two on the Palaeozoic hidden away somewhere.
