ALBANERPETONTIDAE, Fox & Naylor, 1982

Carroll, Robert L., 2007, The Palaeozoic Ancestry of Salamanders, Frogs and Caecilians, Zoological Journal of the Linnean Society 150, pp. 1-140 : 75-77

ALBANERPETONTIDAE

The Albanerpetontidae ( Estes & Hoffstetter, 1976; Fox & Naylor, 1982; McGowan & Evans, 1995; McGowan, 2002; Gardner, Evans & Sigogneau-Russell, 2003) are the most enigmatic of salamander-like animals. They are known from the Middle Jurassic to the Upper Miocene (a period of approximately 150 million years), from North America, Europe, Asia, and North Africa. Three genera with 11 species are currently recognized, but all retain a highly conservative morphology ( Fig. 50 ). The complete skeleton of the Early Cretaceous Celtedens ibericus has a broadly salamander- like body form, with 22 presacral vertebrae and relatively long, slender limbs. The skull–trunk region is approximately 60 mm in length, but even in smaller specimens, the skeleton is highly ossified, including the braincase, carpals, and tarsals.

Superficially, the skull resembles that of salamanders in having a large orbitotemporal opening and anteriorly placed jaw articulation. The squamosal articulates dorsally with the braincase, and lacks a posterior embayment. There is a large bone in the position of the prefrontal and lacrimal in hynobiids that has been referred to as a lacrimal, but could be either a prefrontal or the result of fusion between these two bones. However, other features of the skull indicate divergence from salamanders from at least the Middle Jurassic. Unlike in any salamander, the frontals are fused at the midline, without a trace of suture. In contrast to the absence of a bony link between the maxilla and the jaw suspension in salamanders, a jugal is present in this position in the only well-articulated skull, that of the Lower Cretaceous Celtedens ibericus ( McGowan, 2002) . The paired bones of the palate are poorly known, but the cultiform process of the parasphenoid is unique among extant amphibians in being in the shape of either a short narrow spike or a long, very slender rod, rather than forming a broad supporting surface beneath the braincase.

The braincase is best known in the Miocene genus Albanerpeton inexpectatum ( Estes & Hoffstetter, 1976) . The posterior portion is fused into a solid unit, superficially resembling that of caecilians. It differs, however, in the fusion of the two sides of the occipital arch above the foramen magnum, but the retention of a suture between the otic region and the area of the basisphenoid and pleurosphenoid. The sphenethmoid portion of the braincase has not been described. There is a huge fenestra ovalis, described as showing separate areas for articulation with the head of the stapes and the operculum, although neither of these bones has been recognized. The occipital condyles and corresponding cotylar surfaces of the atlas are located primarily beneath the foramen magnum rather than more laterally, as in primitive salamanders, strongly resembling the position common to Palaeozoic microsaurs.

Well-preserved upper and lower jaws, known from the Middle Jurassic into the Miocene, bear teeth without a trace of pedicellate structure. The crown is chisel-shaped and typically bears three cusps. The lower jaws of the Miocene Albanerpeton , and apparently all earlier albanerpetontids, are highly derived in a manner distinct from that of frogs, salamanders, or caecilians. As in these groups, the number of bones is much reduced from that of Palaeozoic tetrapods, but their specific configuration is highly distinct. Almost all the external surface is composed of the dentary, which occupies much of the area of lateral expression of the angular and surangular in Palaeozoic amphibians. The joint-forming surface of the articular is unique among all amphibians in being sharply concave and facing primarily posteriorly to receive the convex surface of the quadrate. The medial surface of the articular extends forward over much of the area occupied by the angular in early tetrapods. Presumably, these bones have become fused. The other major element exposed medially is a large, chevron-shaped prearticular that extends dorsally into the area primitively occupied by the coronoid bone, where its rugose texture suggests that it functioned as a major site of attachment for the adductor jaw musculature. The front of the jaw has an asymmetrical peg-and-socket joint that would have allowed some degree of movement between the right and left halves, in contrast to the essentially flat surface in salamanders. Mentomeckelian bones have not been recognized. The unique modes of articulation at the symphysis of the lower jaws and between the quadrate and articular suggest a very different mode of feeding from that of any salamanders.

An impression between the jaws of Celtedens ibericus ( McGowan, 2002) has been tentatively identified as a hyoid element, but no details can be distinguished.

The cervical vertebrae, known from the Middle Jurassic into the Miocene, have a pattern that is unique among all terrestrial vertebrates ( Fig. 50G–L ). The atlas–axis complex (not recognized as a functional complex in any other putative lissamphibians) consists of a large bicotylar atlas and a small axis centrum lacking a neural arch. The clearly bicondylar articulation with the skull would have restricted movement to the vertical plane, as in frogs, salamanders, and caecilians. However, according to McGowan (1998), the configuration between the atlas and axis would have allowed movement in the mediolateral plane, which is not possible in any of the extant amphibian orders. Several authors (e.g. Gardner et al., 2003) have referred to the atlas–axis complex of albanerpetontids as being structurally similar to those of mammals or amniotes in general, but this is not an appropriate comparison, as this implies rotational movement, provided by a very different configuration of these bones from that seen in albanerpetontids ( Jenkins, 1971). Estes & Hoffstetter (1976) described a further unique character in Albanerpeton inexpectatum , in which the second cervical centrum is fused to the third. However, this fusion is not recognized among the many specimens of the axis that have been collected from the Jurassic and Cretaceous ( McGowan, 1998).

Neither the atlas nor the axis bears ribs. The more posterior vertebrae of albanerpetontids are also distinct from those of any salamanders in the shortness of the transverse processes and the absence of bifurcated surfaces for articulation with the ribs. In fact, the trunk vertebrae of Albanerpeton inexpectatum closely resemble those of many microsaurs (pers. observ.), although they lack intercentra, which are found in several elongate microsaurs.

The girdles and limbs superficially resemble those of salamanders. No dermal or medial elements of the shoulder girdle have been recognized. The humerus bears conspicuous surfaces for articulation with the radius (a large spherical structure) and the ulna. The carpals and tarsal are all ossified, even in animals of very small size, and appear to retain most of the elements present in Palaeozoic tetrapods. However, they are notable in lacking the fusion of distal carpals and tarsals 1 and 2 that forms the basale commune in salamanders. In the fully articulated specimen of Celtedens ibericus , the phalangeal formula of the manus is 2,3,3,3, and that of the pes is 2,3,4/5,3,3; this is common to neither salamanders nor frogs.

Celtedens ibericus has a continuous covering of hexagonal dermal scales over the trunk and head, totally unlike the condition in frogs and salamanders ( McGowan & Evans, 1995). Caecilians have scales, but not of a comparable pattern. This species is also unique in having patches of about 100 tiny spherules lying just above each femur. McGowan (2002) interprets these as being associated with glands concerned with courtship. In addition to the extensive scalation, the high degree of ossification of the entire skeleton, even in tiny specimens, and the well-developed appendicular skeleton suggest that all albanerpetontid species known were fully terrestrial animals.

The earliest discovered albanerpetontid material was originally assigned to previously named salamander taxa by Costa (1864), Estes (1964, 1969), Seiffert (1969), and Estes & Hoffstetter (1976). Only in 1982 did Fox & Naylor recognize that the configuration of the cervical vertebrae and the nature of the symphysis of the lower jaws were unique among all known terrestrial vertebrates ( Fox & Naylor, 1982). They also listed several other features in which they were distinct from salamanders: nonpedicellate teeth; the parasphenoid a narrow midline spike; a highly convex quadrate fitting into a deeply concave articular; a labyrinthodont-like sculpture on the skull bones; frontals fused; and unicipital rib-bearers on trunk vertebrae. To these, we can now add the absence of fusion of distal carpals and tarsals 1 and 2, and retention of a large, ossified pubis.

Albanerpetonids have been classified as a separate clade within the Lissamphibia, as a doubtful sister taxon of either salamanders ( Milner, 2000), or both salamanders and frogs ( McGowan & Evans, 1995), or all other extant orders ( Milner, 1988; Ruta et al., 2003). Although much more has been learned of albanerpetontids since the paper by Fox & Naylor (1982), their conclusions regarding the probable ancestry of this family still apply today: ‘The family may be derived from lepospondyls, from primitive lissamphibians, or even from labyrinthodonts, but available information provides no resolution of ultimate ancestry.’ Whatever their specific relationships, they provide little if any evidence regarding the ancestry of the Caudata , whose first appearance in the fossil record is identical in time and place ( Gardner et al., 2003).

Figures

Figure 50. Albanerpetontids, an enigmatic clade known from the Middle Jurassic into the Miocene that has been suggested as being related to the extant amphibian orders. A, skeleton of Celtedens megacephalus from the Lower Cretaceous of Spain. Reproduced from McGowan & Evans (1995). B–L, Albanerpeton inexpectatum from the upper Miocene of France. Reproduced from Estes & Hoffstetter (1976). B, C, skull in lateral and dorsal views. D, E, lateral and medial views of lower jaw. F, single tooth (much enlarged). G–I, atlas vertebra in anterior, lateral, and ventral views. J–L, fused second and third vertebrae in anterior, lateral, and ventral views.