Terrestrial Vertebrates

Modified from Carroll (1995), Laurin and Reisz (1997), and Laurin (1998a-c). The position of Whatcheeria follows Lombard and Bolt (1995). The position of the poorly known Devonian taxa (those in which the presence of digits is uncertain: Elginerpeton, Metaxygnathus, Ventastega, and Hynerpeton) follows Ahlberg (1998). Temnospondyls are often thought to be early amphibians. However, notice that in this phylogeny, they are not closely related to lissamphibians.

Introduction

This page discusses terrestrial vertebrates and other vertebrates that bear limbs with digits rather than fins. The muscular limb characteristic of this clade generally has well-defined joints and digits (fingers and toes) and is called a chiridium. This group includes about 21100 extant species and a probably much greater number of extinct species. Tetrapods are only one of the several groups of chiridium-bearing vertebrates (see the section on classification below).

Terrestrial vertebrates have a worldwide distribution. The earliest members of this group were moderately large (1-2.5 m body length). The oldest known skeletal remains of terrestrial vertebrates were found in the Upper Devonian of East Greenland (Clack, 1994). The presence of Lower to Middle Devonian trackways in Australia has led to suggestions that this group may have originated in the Lower Devonian, at least 400 million years ago (Warren et al., 1986), but digits are not visible in these impressions, so these trackways may have been left by other sarcopterygians.

The largest group of terrestrial vertebrates is Tetrapoda (see the section "Classification of Terrestrial Vertebrates", below). Tetrapoda means "four feet", and the group was so-named as its members primitively had four limbs, as opposed to fins. This taxon includes about 3000 extant species of amphibians (frogs, salamanders, and caecilians) and approximately 18100 extant species of amniotes (mammals, reptiles, and birds). The number of extinct species of tetrapods is of course unknown, but about half of the currently known species of tetrapods are extinct (Carroll, 1988).

Tetrapods originated no later than the Mississippian (about 350 million years ago), the period from which the oldest known relatives of living amphibians are known. Relatives of amniotes must have been present at that time, but they have not been found so far. The fossil previously reported (Smithson et al., 1994; Carroll, 1995) as an early Mississippian amniote or anthracosaur (Westlothiana) is probably either a stem-tetrapod or an early amphibian (Laurin, 1998a). Stegocephalians (see section on classification below for a definition of this group) originated no later than the Upper Devonian.

Tetrapods range from 9.8 mm (in the frog Psyllophryne didactyla) to 30 m (in the blue whale) in overall length. They have a worldwide distribution and inhabit all major habitats. Most are terrestrial, but several have returned to the aquatic environment in which our distant ancestors lived. Aquatic tetrapods include various salamanders (sirenids, cryptobranchids, proteids, etc.), frogs (pipids), some caecilians (typhlonectids), leatherback turtles, sea snakes, pinnipeds (seals and walruses), and whales. Some tetrapods are capable of flight (birds and bats), while others glide, such as flying squirrels, dermopterans (sometimes called "flying lemurs", even though they are not primates), and the flying dragons (Draco volans).

The page Life History of stegocephalians contains information on this complex topic.

The main breathing organ of most stegocephalians is the lung, but other respiratory organs exist in many groups. More detail is available on the Breathing in stegocephalians page.

Many stegocephalians have a tympanum for hearing high-frequence, air-borne sounds, and a lateral-line organ is found in many aquatic amphibians. For more information, see the Hearing in stegocephalians page.

Characteristics

Classification of Terrestrial Vertebrates

In the past, most terrestrial choanates were included in Tetrapoda (Gaffney, 1979). Recently, Tetrapoda was formally defined as a crown-group (Gauthier et al., 1989). A crown-group is a clade that includes the last common ancestor of two or more extant taxa, and all its descendants. In this case, Tetrapoda was defined as the clade that includes the last common ancestor of lissamphibians and amniotes, and all its descendants. 

According to Gauthier et al. (1989), Tetrapoda included most known fossil terrestrial vertebrates because temnospondyls were thought to be the stem-group of amphibians, whereas embolomeres, gephyrostegids, and seymouriamorphs were thought to be more closely related to amniotes than to lissamphibians. Therefore, only a few very early terrestrial vertebrates, such as Ichthyostega and Acanthostega, were excluded from Tetrapoda.

The choanate phylogeny presented here suggests that temnospondyls, embolomeres, gephyrostegids, and seymouriamorphs are not part of the crown-group. If it is accurate, these taxa are not tetrapods and the origin of the "tetrapod limb" predates the origin of Tetrapoda. In the first versions of this page, all sarcopterygians bearing digits were simply called terrestrial vertebrates because no formal phylogenetic taxonomy of this whole clade had been proposed. Such a taxonomy was recently published (Laurin, 1998a). The taxon Stegocephali (that included for a long time all the vertebrates bearing a chiridium, except for lissamphibians and amniotes) was defined as all choanates more closely related to Temnospondyli than to Panderichthys (the closest relative of tetrapods known to have retained paired fins). Therefore, Stegocephali includes all taxa that bear digits, and a few (Elginerpeton, Metaxygnathus, Ventastega, and Hynerpeton) that may retain paired fins. Contrary to the old usage of this term, Stegocephali now refers to a clade. The term stegocephalian will be used below because it does not convey the hypothetical and probably somewhat erroneous interpretation that all digit-bearing vertebrates are terrestrial. As explained below, the earliest members of that clade were probably primitively aquatic.

Note about the node marked Amphibia: Others restrict the name Amphibia to descendents of the most recent common ancestor of extant amphibians (the terminal taxon Living Amphibians in this tree). The author prefers the usage indicated in the tree above, and the definition of Amphibia as all tetrapods more closely related to extant amphibians than to amniotes has historical precedence (Laurin, 1998a), but the second usage has been fairly widespread and cannot be ignored. In this page, the term Amphibia always refers to the node indicated in the tree above, and extant amphibians are referred to as lissamphibians.

Note about the node marked Tetrapoda: Others expand the use of this name to include all vertebrates that possess limbs with digits (Laurin and Anderson, 2004). However, in this page, Tetrapoda always refers to the node indicated in the tree above.

Discussion of Phylogenetic Relationships

The phylogeny of stegocephalians is controversial. Nearly all systematists agree that extant amphibians form a monophyletic group (Lissamphibia) that is closely related to amniotes, but the origin of lissamphibians and amniotes is controversial. Laurin and Reisz (1997, 1999), Laurin (1998a, b), and Vallin and Laurin (2004) suggest that lepospondyls are the stem-group of amphibians, and that diadectomorphs are more closely related to amniotes than to lissamphibians. Consequently, many groups of Paleozoic stegocephalians, such as temnospondyls and seymouriamorphs, are not closely related to amniotes or to lissamphibians:

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However, most early studies and a few recent ones have suggested that lissamphibians are derived from temnospondyls, and that diadectomorphs, seymouriamorphs, and embolomeres are more closely related to amniotes than to lissamphibians (Bolt, 1969; Gaffney, 1979; Gauthier et al., 1988, 1989; Milner, 1988, 1993; Panchen and Smithson, 1988; Trueb and Cloutier, 1991; Lombard and Sumida, 1992; Ruta et al., 2003a, b; Ruta and Coates, 2007). Lepospondyls have often been viewed as a paraphyletic group that included early relatives of temnospondyls and lissamphibians. Therefore, most known stegocephalians were viewed as related to either lissamphibians or amniotes. Only Ichthyostega and other Devonian taxa (Acanthostega, Tulerpeton) were not believed to be on one of these two main evolutionary lineages:

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A few paleontologists still propose a polyphyletic origin of extant amphibians (Anderson, 2007, 2008; Carroll, 2007; Anderson et al., 2008), with gymnophionans nested within "lepospondyls" and anurans and urodeles nested in temnospondyls. This implies that gymnophionans are more closely related to amniotes than to anurans and urodeles. This phylogeny is incompatible with all published molecular and most morphological phylogenies, which suggest that extant amphibians form a clade which excludes amniotes (Laurin, 2002). Furthermore, reassesment of the coding of the main supporting matrix shows that it actually supports a monophyletic origin of lissamphibians among "lepospondyls" (Marjanovic and Laurin, 2009). Developmental data have also been proposed to support extant amphibian polyphyly (Schoch and Carroll, 2003; Fröbisch et al., 2007), but these arguments have been refuted (Schoch, 2006; Germain and Laurin, 2009; Marjanovic and Laurin, 2009). The developmental data do not discriminate between various hypotheses on lissamphibian origins.

The most recent phylogeny advocating lissamphibian polyphyly (Anderson et al., 2008) is based partly on the recently-described temnospondyl  Gerobatrachus hottoni, which was  initially  interpreted as the sister-group of Batrachia, which is deeply nested in temnospondyls; Gymnophiona is nested in "lepospondyls". That conclusion is based partly on the interpretation of various morphological characters of Gerobatrachus, such as pedicely of the teeth (a lissamphibian synapomorphy), a basal commune (fused distal tarsals 1 and 2; a batrachian synapomorphy), and a narrow, laterally-directed palatine (a salientian synapomorphy). However, the presence of synapomorphies of several nested clades, and of other, much more primitive characters, such as the retention of postparietal, tabular and supratemporal, suggests extensive homoplasy. Other interpretations of the morphology of Gerobatrachus hottoni are possible, and incorporation of that form in the first data matrix which supported lissamphibian polyphyly (McGowan, 2002) still supports a monophyletic origin of Lissamphibia within lepospondyls (Marjanović and Laurin, 2008b).

A few molecular studies have proposed very early dates for the beginning of lissamphibian diversification, ranging from about 337 Ma ago (Zhang et al., 2005) to 362 Ma ago (San Mauro et al., 2005; Roelants et al., 2007).  Zhang et al. (2005) also suggested that this iimplied that lissamphibians derived from temnospondyls, but Lee and Anderson (2006) pointed out that such early diversification dates were more compatible with a polyphyletic hypothesis, in which extant amphibians form a clade which excludes amniotes, but derive from both "lepospondyls" and temnospondyls. However, new molecular and paleontological dating of the diversification of lissamphibians suggests a much more recent diversification for this group, in the Permian (Marjanović and Laurin, 2007). Calculation of the confidence interval on the true stratigraphic range of Lissamphibia, based on further development of the most sophisticated method developed by Marshall (1997) also suggests a Permian, monophyletic origin of the group (Marjanović and Laurin, 2008a).

Additional discussion and more detailed phylogenies can be found on the Phylogeny of stegocephalians page.

Origin of Stegocephalians

Several scenarios have been proposed to explain the origin of the chiridium and why vertebrates ventured onto dry land. A theory found in most popular books is that the arid climate that was once thought to have prevailed in the Devonian had forced our sarcopterygian ancestors to crawl out of seasonally drying ponds to reach larger, deeper bodies of water (Romer, 1933). However, a problem with this scenario is that the Devonian is no longer viewed as having been seasonally dry. Furthermore, recent studies on functional morphology suggest that the limbs may have evolved to allow our early ancestors to walk at the bottom of shallow ponds or swamps (Edwards, 1989). Discovery of fairly complete specimens of the Devonian choanate Acanthostega (Coates and Clack, 1990) confirms that its limbs were poorly suited to walk on dry land (Zimmer, 1995). The inferred presence of internal gills in Acanthostega also confirms that this animal was still mainly aquatic (Coates and Clack, 1991). The transition from the aquatic to the terrestrial environment appears to have taken longer than previously thought.

Early Evolution of the Limbs

The presence of five fingers and toes has generally been believed to be primitive for tetrapods and to have been the highest number of digits that our early ancestors had. Five digits were probably present in the earliest tetrapods, but not in the earliest stegocephalians. Recent discoveries have demonstrated that the earliest stegocephalians had more digits; Acanthostega had eight fingers, Ichthyostega had seven toes, and Tulerpeton had six fingers (Lebedev, 1986; Gould, 1991). These "extra" digits were quickly lost, however, because no post-Devonian stegocephalian is known to have had more than five digits.

Coates (1991, 1996) suggested that the pentadactyl condition arose twice (at least in the fore limb) because in his proposed phylogenies, the Devonian polydactylous stegocephalian Tulerpeton was believed to be more closely related to amniotes than to lissamphibians. Therefore, he argued that the reduction from a polydactyl to a pentadactyl condition occurred convergently in anthracosaurs (amniotes and their extinct relatives) and in amphibians (although the pentadactyl condition in amphibians is only know from a few specimens; most lissamphibians have only four digits in the manus). However, reanalysis of these data, and incorporation of Tulerpeton into a data matrix that incorporates more taxa suggests that the pentadactyl condition appeared only once, and well before the divergence between amphibians and anthracosaurs (Laurin, 1998c). This conclusion results from the placement of Tulerpeton in a more basal position in the phylogeny, outside the clade that includes all post-Devonian (and all pentadactyl) taxa.

Four Devonian taxa tentatively included in the tree presented above are known from fragmentary remains. The fragmentary nature of the remains of these genera precludes a confident assessment of their affinities, although Coates (1996) and Ahlberg (1998) have suggested that they are more closely related to tetrapods than to panderichthyids (the closest known relatives of tetrapods known to have retained paired fins). These problematic taxa include Elginerpeton (Ahlberg, 1995), Ventastega (Ahlberg et al., 1994), Metaxygnathus (Campbell and Bell, 1977), and Hynerpeton (Daeschler et al., 1994). Their appendicular skeleton is poorly (or not) known, and they are excluded from the smallest clade which includes limbed vertebrates. Therefore, they may have retained fins (Laurin et al., 2000). Only further discoveries of more complete specimens will solve this question.

Conquest of Land by Vertebrates

The first limbed vertebrates, such as Acanthostega and Ichthyostega, were clearly aquatic taxa (Clack, 2002) because they appear to have retained internal gills, as shown by grooves on the ceratobranchials, which may have accommodated afferent branchial arteries. They also retained a postbranchial lamina of the cleithrum, which may have delimited the branchial chamber posteriorly (Coates and Clack, 1991). In addition, they retained lepidotrichia in the caudal fin and a well-developed lateral-line organ, as shown by canals for its cephalic portion.

The internal gills may have disappeared fairly early in the Carboniferous, and there is no trace of lepidotrichia in stegocephalians after the Devonian, and this may indicate a slightly less aquatic lifestyle. However, the lateral-line organ persisted in most Carboniferous and many Permian stegocephalians, which suggests that many species were still primarily aquatic. The absence of grooves for the lateral-line organ is not a reliable criterion to infer a terrestrial lifestyle because the organ may be present without leaving any traces on the skeleton. Since the body shape of early stegocephalians is fairly conservative, aquatic species do not necessarily differ drastically from terrestrial ones in body shape (Laurin, 2008). Thus, additional criteria, such as bone microanatomy, have been studied to determine when vertebrates became terrestrial (Laurin et al., 2004; Germain and Laurin, 2005). Preliminary results suggest that a terrestrial lifestyle appeared in stegocephalians in the Early Carboniferous (Kriloff et al., 2008), but many more species will have to be studied to yield reliable results.

Atmospheric oxygen concentration may conceivably have played a role in enabling vertebrates to move onto land. The first, aquatic stegocephalians appeared while the atmospheric oxygen was present in low concentration. Romer's gap, a period during which very few stegocephalians are known, represents the final phase of this episode of low oxygen concentration. Soon after, as atmospheric oxygen concentration reached and even surpassed its present level, stegocephalians underwent a spectacular evolutionary radiation, which included the first terrestrial vertebrates. That evolutionary radiation was paralleled in arthropods. Thus, Ward et al. (2006) suggest that the oxygen concentration triggered or facilitated terrestrialization in arthropods and vertebrates, and that an oxygen concentration of about 20% in the atmosphere was the lower threshold which allowed a switch from gill breathing in water to air-breathing in vertebrates and arthropods.

Other Names for Terrestrial Vertebrates

• Stegocephalians

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