Box Phylogeny Of The Devonian Tetrapods

The relationships of the Late Devonian tetrapods (see cladogram), and their closest fish relatives, are controversial, not least because many of the specimens are incomplete and are currently under study. Panderichthyids seem to be the closest sar-copterygian relatives of tetrapods, sharing various features of the head and body that are not seen in osteolepiforms. The Tetrapoda, literally those vertebrates with 'four feet', certainly include Ichthyostega and Ventastega as basal forms. Their sister group, the elginerpetontids, may be the earliest tetrapods (Ahlberg and Milner, 1994) if their jaw and forelimb characters are confirmed.

There is disagreement about the relationships of the remaining Devonian tetrapods. In most analyses (e.g. Ahlberg and Milner, 1994; Coates, 1996; Laurin, 1998; Ruta et al., 2003a, b), Acanthostega is seen as more basal than Ichthyostega, as shown here (see cladogram), whereas Ahlberg and Clack (1998) reversed the order. A further dispute concerns the location of the major split between the ancestors of modern amphibians and of amniotes. Coates (1996) located the split in the Late Devonian, placing Tulerpeton on the line to amniotes, a view that is not robust (Laurin,1998). This simplifies our understanding of digital reduction: in Coates' (1996) scheme, reduction to five digits had to happen twice, once on the line to amphibians and once on the reptiliomorph line. In the scheme here, all the Devonian tetrapods are on the stem to later forms, and digital reduction happened once, between Tulerpeton and later tetrapods (Ruta et al., 2003a, b). Here, it is assumed that all Devonian tetrapods fall along the line to a single clade of more derived tetrapods, following Ahlberg and Clack (1998) and Ruta et al. (2003a, b).

> TETRAPODA

Cladogram showing postulated relationships of the basal tetrapods, based on Ahlberg and Clack (1998) and Ruta etal. (2003a, b),with some information from Ahlberg and Milner (1994) and Coates (1996). See Box 3.6 for context of Tetrapoda; see Box 4.5 for relationships of main post-Devonian tetrapod groups. The number of fingers/toes is indicated, where known. Synapomorphies include: A, flattened head with elongate snout, orbits on top of skull, external naris marginal, frontal bone present, body flattened, dorsal fin absent, enlarged ribs, humerus with anterior keel; B, large nasal bones, fang pair and tooth row on the parasymphysial plate, anterior coronoid narrow, Meckelian bone floors precoronoid fossa, rudimentary sacrum, pre- and postzygapophyses on vertebrae, ilium branches in two; CELGINERPETONTIDAE, deep furrow along dentary-splenial suture, humerus with thin flat entepicondyle continuous with humerus body and narrow tall ectepicondyle, tibia with articulation surfaces for intermedium and tibiale; D TETRAPODA, cheek with broad jugal-quadratojugal contact, large ornamented interclavicle, carpus, tarsus, up to eight digits, iliac blade extends dorsally and attached to vertebral column by sacral rib; E, coronoid fangs in tooth row; F, single pair of nasals meeting in midline, stapes, coronoid fangs absent, well-developed ventrally-directed ribs, pectoral girdle detached from skull, femur with adductor muscle crest, radius and ulna/ tibia and fibula parallel and both articulate with carpus/ tarsus, hand and foot with series of digits; G, postsplenial pit line in lower jaw absent, free ventral flange of the splenial absent, only symphysial and articular ends of the Meckelian element ossified, anocleithrum absent, olecranon process present and ulna as long as radius or longer, seven or fewer digits; H, open lateral line system on most or all dermal bones, elongate scapula and distinct cleithrum, six or fewer digits, tail fin absent; I, five or fewer digits.

Cetacea Order

> TETRAPODA

Liar Tail Tetra

Fig. 4.5 Silhouette diagrams oftetrapod outgroups (a, b) and tetrapods (c-e): (a) Eusthenopteron; (b) Panderichthys; (c) Acanthostega; (d)Ichthyostega; (e) Balanerpeton.(Courtesy ofMike Coates.)

Fig. 4.5 Silhouette diagrams oftetrapod outgroups (a, b) and tetrapods (c-e): (a) Eusthenopteron; (b) Panderichthys; (c) Acanthostega; (d)Ichthyostega; (e) Balanerpeton.(Courtesy ofMike Coates.)

Museum Silhouette

all tetrapods on each side; an ilium above, and a pubis and ischium below, the pubis lying to the front. The joint surface for the head of the femur, the acetabulum, is borne in part on all three of these bones. The pelvis is attached to the vertebral column by an elongate rib of the sacral vertebra,which meets the inner surface of the ilium on each side. The pubes and ischia also meet their opposite numbers in the midline ventrally, thus making the pelvic girdle a firm all-round basket that holds the acetabula in immovable positions, and supports the posterior part of the trunk and the tail. The glenoid and acetabulum face sideways and backwards, the charac teristic of tetrapods, instead of simply backwards as in sarcopterygians.

The limbs of Acanthostega and Ichthyostega are like later tetrapods in most features, but recent work has shown that they are startlingly different in others. The arm of Acanthostega, for example (Figure 4.6(e)), has all the major bones seen in later tetrapods (cf. Figure 4.1(b)), but Coates and Clack (1990) had a surprise when they prepared the hand region of one of their new specimens: they found that it had eight fingers. They then investigated the hindlimb (Figure 4.6(f)), and found that it had eight toes. Ichthyostega has seven toes,

Stegocephalia Bones CarboniferousAnatomy Acanthostega

anocleithrum scapulocoracoid clavicle interclavicle cleithrum arm in glenoid pubis

pubis

ilium ischium leg in acetabulum ilium ischium leg in acetabulum

Fig. 4.6 The anatomy ofAcanthostega: (a,b) skull in lateral view, with braincase (a) and dorsal view (b); (c) shoulder girdle and arm in lateral view; (d) pelvic girdle and leg in lateral view; (e) arm and hand in anterior view; (f) leg and foot of Ichthyostega in anterior view. [Figures (a, b), courtesy of Jenny Clack; (c—f), courtesy of Mike Coates.]

and Tulerpeton has six. Again, the remainder of the leg shows the standard tetrapod elements, although there are fewer elements in the ankle than in later tetrapods: femur, tibia and fibula in the leg, fibulare, intermedium, tibiale, perhaps one centralium, and at least five distal tarsals (1-5) in the ankle, and seven toes, each of which has a metatarsal and a number of phalanges. Counting outwards from toe 1, equivalent to our 'big toe', but in Acanthostega a small toe, the phalanges number 1,2,3,3,3,3,3,2. These observations have profound implications for our understanding of the development of the standard pentadactyl ('five-fingered') condition in all later tetrapods (see Box 4.2).

4.2.2 Modes of life

The Late Devonian tetrapods were clearly still aquatic, as is shown by the retention of a tail fin, a lateral line system and internal gills. The vertebral column was flexible, as in a fish, and Ichthyostega and Acanthostega could have swum by powerful sweeps of their tails. In addition, the orientation of the shoulder and pelvic girdles, and the shapes of the limb bones, show that the Late Devonian tetrapods used their limbs more in swimming than walking. The hand and foot, each with its extra digits, were broad and flat, and more use as paddles than feet (Figures 4.5 and 4.6(c, d)). These animals

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