Box Sarcopterygian Relationships

The relationships of the sarcopterygian groups to each other have been controversial, and this interest has been heightened by the assumption that the tetrapods, the land vertebrates, arose within the Sarcopterygii. The problems concern the relationships of the lungfishes, coelacanths, and the diverse Devonian groups (= 'Rhipidistia') to each other, and the identification of one of these as the sister group of the Tetrapoda.

The 'classic' view (e.g. Romer, 1966) has been that the Rhipidistia and Actinistia together form the Crossopterygii. This is treated as the sister group of the Tetrapoda, with the Dipnoi as outgroup (cladogram (a)). This consensus view was given a jolt by Rosen etal.'s (1981) suggestion that the lungfishes are the sister group of the tetrapods. In a revised version of this hypothesis (cladogram (b)), Forey et al. (1991) argued that lungfishes and coelacanths pair off, and that these two are the sister group of Tetrapoda. The 'Crossopterygii' and 'Rhipidistia' are split into their constituent parts, Osteolepiformes, Porolepiformes and Actinistia, as outgroups.

B CROSSOPTERYGII A SARCOPTERYGII

A SARCOPTERYGII (b)

A SARCOPTERYGII (c)

.JS1

A SARCOPTERYGII (d)

B CROSSOPTERYGII A SARCOPTERYGII

A SARCOPTERYGII (f)

A SARCOPTERYGII (f)

E TETRAPODOMORPHA

B CROSSOPTERYGII A SARCOPTERYGII

E TETRAPODOMORPHA

B CROSSOPTERYGII A SARCOPTERYGII

Panchen and Smithson (1988) presented a third view (cladogram (c)) in which the 'Crossopterygii' and 'Rhipidistia' are also split up, but the Osteolepiformes are regarded as the sister group of the Tetrapoda. In further analyses, Chang (1991) suggested that the Sarcopterygii as a whole are sister group of the tetrapods (cladogram (d)). Schultze (1994) accepted the traditional sequence of outgroups (cladogram (e)), with osteolepiforms as sister group of tetrapods, then porolepiforms as next outgroup, as in Panchen and Smithson's (1988) proposal, but he then considered that the coelacanths were closer to that clade than the lungfishes. Finally, Cloutier and Ahlberg (1996) found a similar cladogram, except that lungfishes and porolepiforms are paired, as Chang (1991) had also suggested (cladogram (f)). Hence, the majority verdict seems to be that some of the 'rhi-pidistians' are the closest relatives of tetrapods.

In a reanalysis (cladogram (g)), Zhu and Schultze (2001) find that the upper part of the tree is relatively stable, but they move Porolepiformes to lie above Dipnoi and below Actinistia, and split the Tristichopteridae (Eusthenopteron and relatives) from the remainder of Osteolepiformes. The onychodonts are placed as a sister group of coelacanths.

Molecular evidence has been equivocal. Meyer and Wilson (1990) found that lungfishes are more closely allied to tetrapods than are coelacanths, according to an analysis of 12S rRNA sequences, a view that would fit Panchen and Smithson's (1988) phylogeny (cladogram (c)). In a revised analysis, based on the 28S rRNA gene, Zardoya and Meyer (1996) found that the coela-canth and the lungfishes form a clade, separate from tetrapods, hence matching the cladograms of Forey et al. (1991) and Chang (1991) (cladograms (b, d)). In the end, however, Zardoya and Meyer (2001a) were unable to discriminate relationships among lungfishes, the coelacanth and tetrapods for a variety of reasons. Lungfishes show significantly higher rates of evolution of the 28S rRNA gene sequences than coelacanths, other fishes and tetrapods, and this makes it hard to discriminate their correct position in the tree. In addition, the mitochondrial tRNA and nuclear rRNA sequences seem to give erroneous results overall because long branches pair with each other, as do short branches. Brinkmann et al. (2004) confirmed the close relationship of lungfish to tetrapods, based on nuclear genes.

Cladograms showing four competing theories for the relationships of the sarcopterygian fishes and tetrapods, according to (a) Romer (1966) and other 'classic' sources; (b) Forey et al. (1991); (c) Panchen and Smithson (1988); (d) Chang (1991); (e) Schultze (1994); (f) Cloutier and Ahlberg (1996); (g) Zhu and Schultze (2001). See Figure 3.12 for context of Sarcopterygii; see Box 4.1 for relationships of basal tetrapods and Figure 7.7 for relationships of Actinopterygii. Synapomorphies (selected, from Cloutier and Ahlberg (1996) and Zhu and Schultze (2001): A SARCOPTERYGII (including Tetrapoda), muscular pectoral and pelvic limbs with substantial limb bones, true enamel on teeth, sclerotic ring composed of more than four plates, tectal bone in skull, one or more squamosals, splenial in lower jaw, triradiate scapulocoracoid; B CROSSOPTERYGII (sensulato), extratemporal present, squamosal present, preopercular does not contact maxilla or postorbital, tusk on vomer, double-headed hyomandibular, single bone (humerus) in pectoral fin contacts girdle, folded enamel and dentine (plicidentine) in teeth; C, entepicondylar foramen in humerus; D, more than two supraorbitals, branchiostegal rays absent; ETETRAPODOMORPHA, single external naris, posterior naris absent, two supraorbitals, palatal opening ('choana'), vomers articulate with each other, narrow parasphenoid, proximal articular surface of humerus convex, deltoid and supinator processes on humerus; F OSTEOLEPIDIDA, one branchiostegal ray per side, vomer has a posterior process, supraneural processes on only a few anterior vertebrae or absent; G, anterior palatal fenestra present, well ossified ribs; H, flattened head and dorsally-placed orbits, snout elongate, paired frontals, spiracle large and open posteriorly, preopercular canal reduced, fang pair on anterior end of dentary, scapular blade large, dorsal and anal fins absent, labyrinthodont plicidentine in teeth; I CROSSOPTERYGII (sensu stricto), fleshy lobed pectoral and pelvic fins with asymmetrical arrangement of bones; J RHIPIDISTIA, many narrow submandibular bones, four infradentaries, three coronoids; K, many supraorbital bones, reduction of dorsal elements of gill arches; L, short hyomandibular free from palate, pectoral and pelvic fins equal in size; M, short cheek region, biting possible between teeth on palate, upright jaw suspension, short dentary, angular bone dominant, ball-and-socket joint between paired fins and girdles; N, preopercular does not contact maxilla; O, many narrow submandibular bones, four infradentaries, three coronoids, plicidentine; P, three or more tectals, posterior branched radial complex associated with posterior dorsal fin, same structure of the cosmine, leaf-shaped pectoral fins, predominant palatal bite; Q SARCOPTERYGII (sensustricto), cosmine (specialized scale tissue), intracranial joint, anocleithrum in shoulder girdle; R, double-headed hyomandibular, intracranial joint, three extrascapulars.

Lepidosiren Fin

Fig. 3.21 The living lungfishes Neoceratodus from Australia (a), Lepidosiren from South America (b), and Protopterus from Africa (c). (Based on various sources.)

100 mm

Fig. 3.21 The living lungfishes Neoceratodus from Australia (a), Lepidosiren from South America (b), and Protopterus from Africa (c). (Based on various sources.)

Protopterus Fossil

10 mm

Fig. 3.22 Coelacanths,fossil (a) and living (b): (a) the Triassic Osteopleurus from North America; (b) the living Latimeria from the modern seas of the Indian Ocean. (After Andrews, 1973.)

10 mm

Fig. 3.22 Coelacanths,fossil (a) and living (b): (a) the Triassic Osteopleurus from North America; (b) the living Latimeria from the modern seas of the Indian Ocean. (After Andrews, 1973.)

season. When the monsoonal rains fall, the lungfishes come to life again, and creep out of their cocoons. Large fossilized burrows in Devonian and Triassic rocks suggest that early lungfishes also aestivated.

3.9.3 Actinistia: the coelacanths

The coelacanths arose in the Mid-Devonian, and are represented by fossils up to the Late Cretaceous, when it was thought they had died out. Typical coelacanths, such as Osteopleurus from the Triassic of eastern North America (Figure 3.22(a)), have short bodies with large dorsal, anal and paired fins, all of which are lobed except for the anterior dorsal. The tail is characteristically divided into three parts—a dorsal and ventral portion separated by a small middle lobe at the end of the notochord. The skull is short overall, although the snout portion is longer than in the osteolepiforms.

The coelacanths are particularly well known as living fossils.In 1938, an unusual large lobefinned fish was hauled up in the Indian Ocean, and brought ashore in South Africa. Eventually, the fish was identified as a coelacanth, and named Latimeria (see Figure 3.26(b)). Latimeria (Forey, 1988,1998) is called a living fossil because it belongs to a group that was long thought to be extinct, and its morphology is very like that of its ancestors of more than 100 million years ago. Since 1938, about 200 specimens of Latimeria have been fished up from the deep oceans off the coast of the Comoro Islands, and Sulawesi in Indonesia, where it feeds on fishes. Latimeria swims slowly by beating its paired fins in a pattern like the locomotion of a tetrapod, and sculling with its muscular dorsal and anal fins. It can achieve fast thrust by beating its tail, a standard escape response primitive to all bony fishes.

3.9.4 Diverse Devonian sarcopterygians

At one time, the Devonian sarcopterygians that were not lungfishes were grouped together as 'Rhipidistia'. This assemblage, including the Porolepiformes, Onychodontida, Rhizodontida, Osteolepiformes, Tris-tichopteridae and Panderichthyida, is paraphyletic (see Box 3.6).

The porolepiforms, represented by Holoptychius (Figure 3.23(a)), generally have large rounded scales, and long pointed pectoral fins with more extensive lobed portions than in the osteolepiforms. Porolepi-forms have deep bodies and a short skull. Their strongly folded teeth are welded into the jaws by a plug of attachment bone inserted into the pulp cavity.

Fig. 3.23 Diversity ofDevonian sarcopterygians: (a) lateral view of the porolepiform Holoptychius,(b, c) lateral views of the osteolepiform Osteolepis, with and without scales, (d) lateral view of the tristichopterid Eusthenopteron, (e) lateral view of the panderichthyid Panderichthys. [Figure (a) after Andrews, 1973; (b, d) after Moy-Thomas and Miles, 1971; (c) after Andrews and Westoll, 1970a; (e) modified from Vorobyeva and Schultze, 1991.]

Fig. 3.23 Diversity ofDevonian sarcopterygians: (a) lateral view of the porolepiform Holoptychius,(b, c) lateral views of the osteolepiform Osteolepis, with and without scales, (d) lateral view of the tristichopterid Eusthenopteron, (e) lateral view of the panderichthyid Panderichthys. [Figure (a) after Andrews, 1973; (b, d) after Moy-Thomas and Miles, 1971; (c) after Andrews and Westoll, 1970a; (e) modified from Vorobyeva and Schultze, 1991.]

Sarcoperygian Fishes

The onychodontids were a small group of probably predatory fishes with long, hooked teeth at the front of the lower jaw. They ranged in length from 50 mm to over 1 m. They have not been known in detail up to now, but new specimens from Gogo (see Box 3.2) show many features in common with Psarolepis (see Box 3.5), and hint that the Onychodontida might turn out to be basal sarcopterygians (Long, 2001).

The rhizodontids from the Early Carboniferous were large hunters. One massive rhizodont jaw from Scotland suggests that its owner must have reached a length of 6-7m (Jeffery, 2003). This was probably a fearsome hunter of some of the early tetrapods (see Chapter 4).

The osteolepiforms had their heyday in the Devonian, although certain forms survived through the Carboniferous and into the Early Permian. Osteolepis from the Mid-Devonian of Scotland and elsewhere (Andrews and Westoll, 1970b) has a long slender body with large midline fins (two dorsals, one anal), and lobed paired fins (pectoral and pelvic). The tail is hete-rocercal, with fins above and below (Figure 3.23(b, c)). Some Late Devonian osteolepiforms were larger.

Eusthenopteron from the Upper Devonian of Canada (Figures 3.23(d) and 3.24) has generally been called an osteolepiform, although it seems likely that it, and its relatives, collectively the Tristichopteridae, are a closer sister group to the tetrapods (see Box 3.6). Eusthenopteron reached a length of 1 m, and it has a characteristic three-pointed symmetrical tail. The outer portions of the head, gill region, and attached shoulder girdle are covered by a complex of thin dermal bone plates (Figure 3.24(a, b)). Small teeth are borne on the maxilla, premaxilla and dentary, as well as on several bones of the palate (Figure 3.24(c)). Some of the palatal teeth are large and fang-like, and they have complex, or labyrinthine, internal patterns of infolding (Figure 3.25(e)), the so-called labyrintho-

Sclerotic Ring Osteolepis

Fig. 3.24 The skull of the tristichopterid Eusthenopteron in (a) lateral, (b) dorsal, and (c) ventral views; (d) lateral view of the braincase, showing the postulated range of movement about the middle joint; (e) cross-section of a tooth to show the labyrinthine infolding of the enamel (tooth diameter, 5 mm). (After Moy-Thomas and Miles, 1971.)

Fig. 3.24 The skull of the tristichopterid Eusthenopteron in (a) lateral, (b) dorsal, and (c) ventral views; (d) lateral view of the braincase, showing the postulated range of movement about the middle joint; (e) cross-section of a tooth to show the labyrinthine infolding of the enamel (tooth diameter, 5 mm). (After Moy-Thomas and Miles, 1971.)

dont type of tooth, found also in early tetrapods. The skull is highly kinetic, being jointed in order to allow the mouth to open wide. Even the braincase (Figure 3.25(d)), deep within the skull, is jointed in order to permit greater flexibility, a feature retained in Latimeria, but otherwise unknown in other living sarcopterygians.

The panderichthyids, or elpistostegids, of which Panderichthys from the Upper Devonian of Latvia (Figure 3.23(e)) is the best known, are rather stout fishes with long snouts. The skull is flattened, and the eyes are located partly on top of the head. Panderichthyids have only the paired pectoral and pelvic fins, as well as a tail fin, and they lack the midline fins seen above and below the body in their relatives (Vorobyeva and Schultze, 1991).

+1 0

Post a comment