Tetrapod Evolution

Australia: The Land Where Time Began

Tetrapod Evolution – Origin

About 200 million years before the Devonian, commonly called the Age of Fishes, about 380 Ma, the early vertebrates radiated to produce a profusion of fish-like animals that inhabited nearshore lagoons, the estuaries of rivers and lakes. From the Middle to the Late Devonian some of these fish-like animals developed limbs with digits – fingers or toes. These tetrapods gradually evolved into vertebrates that eventually moved onto land over the next 350 million years, where they thrived and have remained as the dominant life forms, at least life forms larger than the invertebrates, ever since. The modern descendants of these early tetrapods have been allocated into 2 major groups. The amphibians include the frogs (anurans-tailless amphibians), salamanders (urodeles – tailed amphibians). The other group, the modern amniotes, that include mammals, turtles, crocodiles and their related forms the birds and the lizards and their related forms, the snakes. The term “reptile” can be used to include all amniotes except mammals, but only if birds are included with the reptiles [they descended from dinosaurs and not synapsids [previously called mammal-like reptiles], the ancestors of mammals. In practice the tetrapods include any animal with 4 legs or whose ancestor had 4 legs. There are also many extinct related taxa of these 2 major radiations of vertebrates, such as the dinosaurs, the closest living relatives of which are the birds.

The move from the water to land was slow and difficult. Enormous evolutionary innovations have resulted from this development, with much of this change occurring by happenstance, as it was not a direct process, more a case of being in the right place at the right time.

The focus for the interaction of a number of disciplines, such as palaeontology, and the related studies of palaeoecology, taphonomy (how the animals died and became fossilised) and palaeobiogeography (where they lived and their distribution in space and time) for the study of the origin, early evolution and relationships of tetrapods. And also, modern zoology, anatomy, physiology, and developmental and molecular genetics.

The Phanerozoic is the interval of about 600 My in which there are abundant fossils. The Phanerozoic has been divided into 3 eras, originally named according to the proportion of its biota that resembled modern life. These divisions are the Palaeozoic (“ancient life”), the Mesozoic (“middle life”) and the Cainozoic (Cenozoic) (“recent life”). These ages have been subdivided into periods and these periods have been divided further into stages, and the boundaries of these divisions have been based on animal and plant fossils that have been dated to that time, though the names that are given to the divisions do not necessarily reflect this. The names of the stages are often based on the locations of the representative strata the fossils are first found in, or sometimes the locations they are seen most clearly.

It is usual for scientists to refer to an independently produced timescale to date their find, and the timescale produced by Gradstein et al., 2004 is one of the most recent of these timescales, and many different and complementary techniques were used in its construction. According to Clack¹ it is very detailed for the Phanerozoic.

The later part of the Palaeozoic, the Devonian, Carboniferous and the Permian are the periods that cover most of the interval when the tetrapods were evolving from their ancestral fish. The time at which the process of emerging from the water began is now believed to be about 380 Ma, though it has recently been suggested by further work that it may have actually began at an earlier date, suggesting that the move to the land was very slow and probably difficult. The earliest dinosaur has been recognised from about 225 Ma, with the last non-avian dinosaurs dropping out of the fossil record 65 Ma, which covers a similar time period to the rise of the tetrapods which took about 132 My from 380 Ma to about 248 Ma as the Permian was coming to a close.

Tetrapods – Their place in the family tree

It is necessary to have some understanding of the forms related to tetrapods, extinct and extant, to understand how tetrapods fit into the pattern of evolution. To determine which features that tetrapods have in common with other forms, extinct and extant, and those that are unique, that suggest their ancestral relationships and the basis on which evolution has acted to produce all the new features of tetrapods.

Tetrapods belong to the Sarcopterygii (sacropterygians), which are known as the “lobe-finned fishes”, a large group of vertebrates that in the past was very common. Clack¹ suggests it is necessary to consider the wider family tree of tetrapods to put them into context among related groups.

It is usual for most fishes, extinct and extant, to have 2 sets of paired fins, apart from groups where they have been lost by specialisation, the pectoral fins, a fore pair, and the pelvic fins, a hind pair, all their fins being attached to body supports, the girdles. Other fins, the midline fins, anal fins and dorsal fins and the tail, are single structures, not being found paired. These features are characteristic of vertebrates, a larger group that is more inclusive, and among the defining characters are jaws and teeth, these being the gnathostomes.

There is a whole suite of complex features that are shared among the gnathostomes that are not seen elsewhere in the animal kingdom, which Clack¹ says appear to be innovations that are related to an active predatory lifestyle. The possession of a series of paired, jointed gill bars that support a series of paired gills at gill slits which pierce the pharynx (throat) used in ventilation by gas exchange from the blood to the outside water that passes over the gills. The gill filaments, the structures that carry out the function of gas exchange, are located in pouches, each of which has a gill bar and a set of nerves, muscles and blood vessels to operate the system. The operation of the gill bars opens and closes the holes in the pharynx that connect the gill pouches to the outside. As the embryo grows dimples form in the body wall and grow inwards where they meet similar dimples in the throat with which they eventually coalesce to form complete tubes through pharyngeal region that allows the water to pass out.

The sharks, which are cartilaginous fishes, the chondrichthyans, have their skeletons formed of cartilage instead of bone, and osteichthyans (Osteichthyes) (bony vertebrates) such as carp, which have bony skeletons.

At the present most extant vertebrate species are comprised on animals with bony skeletons, the uniting factor is having a skeleton that at least in part, is composed of a special type of bone. This endochondral bone has formed by the replacement of a cartilaginous precursor. This kind of bone comprises human arms, legs, spinal column and most of the skull. This contrasts with bone that has formed in the skin that doesn’t have a precursor of cartilage, hence dermal bone. The most primitive of the known vertebrates have some form of dermal bone, though it is arranged into distinctive patterns in true osteichtyans that can be recognised throughout the group. Bones such as the maxillary and dentary, the jaw bones of humans, are dermal bones that have been retained.

The ability to form an extra pair of pouches behind the standard set of gill pouches in the throat appears to have been another feature that was shared among the early bony vertebrates that functioned as an air chamber; Clack¹ suggests they were essentially lungs in many early bony vertebrates. The lungs were retained in some of these animals in which cases they were used regularly for breathing air. When considering tetrapod evolution it is necessary to bear this in mind. Air-breathing has continued to be common among bony vertebrates since the early evolution of these animals, and it is not unique to terrestrial forms.

The bony vertebrates are comprised of 2 groups, the ray-finned fishes, the Actinopterygians or Actinopterygii, and the lobe-finned fishes, their differences from each other are quite straightforward, in most part, and can be recognised easily. The ray-finned fishes have paired fins that articulate with the shoulder girdle through a series of bones, the radials. The bony fin rays are in turn supported by these, lepidotrichia (the fin rays, modified rows of bony scales). The fins in some of the early fossil forms, at least, there was a broad base on the fins, which were not very maneuverable, though it is no longer the case in most modern forms. Almost all fishes are encompassed by this group.

The paired fins of lobe-finned fishes are attached to the shoulder girdle by a single radial, with the rest of the radials being strung in a chain growing outwards from the body. There are muscles running between the bones on the radials, with the result that the lobe of the fins is segmented and muscular, which gives it a base that is narrow and flexible that allows it to, be capable of being rotated as well as being raised and lowered. Humans and other tetrapods share this feature with the related lobe-finned fishes.

There are several fundamental characters that humans share with related forms among the fishes. A result of this is that in a classification of animals that reflects their phylogeny and relationships; it becomes inevitable that the tetrapods, including humans, are in this same grouping as other forms that possess these characters and share a common ancestor. Humans are mammals, which are tetrapods, which are sacropterygians (lobe-fins, a better translation being flesh-limbed tetrapod), which together with actinopterygians (ray-fins), their sister group, are osteichthyans (bony vertebrates), which also includes humans, though the word osteichthyans is sometimes translated as “bony fish”. This concept has been expressed as Your Inner Fish (Shubin, 2008). It looks at the transition fish-tetrapod, and also expands on the theme of nested sets of characters that are shared throughout and beyond the family tree of vertebrates.

Sources & Further reading