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Inhaltsverzeichnis

Frontmatter

Abstract

The phylogenetic relationships of anguinomorph lizards are reconsidered on the basis of the structure of the head musculature, of the skull, and of the atlas - axis complex. A basal dichotomy separates the Anguinomorpha into the Anguinoidea and Platynota. The most primitive Anguinoidea are gerrhonotiform lizards. They share common ancestry with a hypothetical common ancestor of the Anguinae and Diploglossinae. Anniella is most closely related to the Anguinae among the Anguinoidea. The relationships of Xenosaurus and Shinisaurus are poorly determinable on the basis of present evidence. For reasons of parsimony, the two genera are included within the same family, the Xenosauridae, which is tentatively derived from the transitional field leading from the Gerrhonotinae to the hypothetical ancestor of the Anguinae and Diploglossinae.The Helodermatidae share many similarities with the Gerrhonotinae which indicate their primitive status. However, the Helodermatidae show diagnostic platynotan features and hence cannot be included in the Anguinoidea. On the basis of shared derived characters, the Lanthanotidae and the Varanidae are considered to share a common ancestry after the Helodermatidae split off from the lineage of platynotan evolution. Structurally, Lanthanotus is intermediate between Heloderma and Varanus.

Olivier Rieppel

Introduction

Anguinomorph lizards have attracted the attention of various workers for several reasons. They form a lacertilian infraorder well defined by the subdivision of their tongue into a retractile fore-portion and a broad, elastic hind-portion. The long, independent evolutionary history is documented by a rich fossil record (Hoffstetter, 1962 a). The lizards referred to the Anguinomorpha are subdivided into two suprafamilies, the Anguinoidea and the Platynota. The Anguinoidea comprise three families, the Anguidae, Xenosauridae and Anniellidae. The Platynota comprise seven families, the Helodermatidae, Parasaniwidae, Lanthanotidae, Dolichosauridae, Aigialosauridae, Mosasauridae and Varanidae. The Anguinomorpha exhibit a wide range of adaptation from the burrowing Anniellidae to the Dolichosauridae, Aigialosauridae and Mosasauridae, inhabitants of the Cretaceous seas. Since the days of Nopsca (1903,1908,1923), Janesch (1906) and Fejervary (1918) they stand as ancestors of snakes, a view which was accepted by Camp (1923) and McDowell and Bogert (1954, see also McDowell, 1972).

Olivier Rieppel

Material and Methods

The trigeminal jaw musculature as well as the muscles of the neck and of the throat were dissected in representatives of every subgenus of anguinomorph lizards. Each subgenus was represented by at least two specimens, preferably of two different species, with the exception of varanids where the lack of material restricted the investigation to a single specimen of many subgenera. After the dissections were completed, the skulls were prepared in a commercial bleaching solution prior to examination. A number of dried skulls from a variety of museum collections were also used for comparison. A list of the specimens examined is given in Appendix I.

Olivier Rieppel

The Procedure of Classification

The taxonomic concepts used for the following study are the evolutionary ones as discussed by Mayr (1969,1974).

Olivier Rieppel

Problems of Terminology and the General Structure of Lacertilian Jaw Musculature

Species names referred to in the present study are brought into correspondence with Wermuth (1969) and Mertens (1942 c).

Olivier Rieppel

The Head Musculature of the Gerrhonotinae

The gerrhonotiform stock was selected to start the discussion of the head musculature of anguid lizards. There is continuing contrevesy concerning the subdivisions of gerrhonotiform lizards into taxonomic units (Waddick and Smith, 1974), and an analysis of variation within different species will be given after the description of the head musculature of Gerrhonotus (Gerrhonotus) liocephalus.

Olivier Rieppel

Patterns of Variation within Gerrhonotiform Lizards

As Gerrhonotus liocephalus is considered to be a primitive representative of the gerrhonotiform stock (Bogert and Porter, 1967, and below), it was chosen to serve as a basis for the discussion of variations within the group.

Olivier Rieppel

The Head Masculature of the Diploglossinae

This subfamily is considered to include the genera Ophiodes, Diploglossus, Wetmorena, Celestus and Sauresia, the latter two of questionable validity (Underwood, 1959). Such arrangement will be justified by similarities in skull structure and in the anatomy of the head musculature, as will become clear from the following descriptions. The musculature of the members of this subfamily is known by the study of Fürbringer (1870) on Ophisaurus striatus only. The trigeminal jaw musculature has never been described for any species included within this subfamily. In the following descriptions, the head musculature of G. liocephalus will serve as a basis for comparison which will help to avoid repeated statements about similar or identical structures.

Olivier Rieppel

Patterns of Variation within Diploglossine Lizards

Some of the species currently assigned to the genus Diploglossus Wiegmann (Wermuth, 1969) are occasionally found to be referred to Celestus Gray. Underwood (1959) placed this latter genus into synonymy with Diploglossus. The proposed (Underwood, 1959) synonymy of the monotypic genus Sauresia Gray from Hispaniola and from the Gonave Islands with Diploglossus appears somewhat more conjectural, however. A fourth genus to be included within the Diploglossinae is Wetmorena Cochran from Hispaniola. Cochran (1941) considers Sauresia to be structurally intermediate between Cuban and Puerto Rican Celestus and Wetmorena: it lacks a fifth toe as does Wetmorena but shows a laterally exposed tympanum which is covered with skin in Wetmorena.

Olivier Rieppel

The Head Musculature of the Anguinae

Of the Anguinae, both Anguis fragilis and Ophisaurus apodus have already been studied quite extensively (Bradley, 1903; Zavattari, 1910; Lakjer, 1926; Richter, 1933; Säve-Söderbergh, 1945; Poglayen-Neuwall, 1954). However, these studies stand rather isolated as none of them aimed neither at a detailed comparison of Anguis with Ophisaurus nor of these two genera with the other anguid lizards. Such comparison revealed a number of common specialisations of the head musculature of the Anguinae which justify the inclusion of Anguis and Ophisaurus within a single subfamily (contra McDowell and Bogert, 1954). The species dissected were Anguis fragilis, Ophisaurus apodus, O. attenuatus, O. compressus, O. ventralis and O. gracilis, including ophisaurs from Europe, Northern Africa, North America and a specimen of gracilis from Burma.

Olivier Rieppel

The Evolution of the M. Intermandibularis Anterior in the Anguidae

The presence of an anterior superficial portion of the intermandibularis anterior is a constant feature throughout the Anguidae and needs no further discussion.

Olivier Rieppel

The Phylogeny of the Anguidae based on their Head Musculature

An extensive review of the history of anguid classification has been given by Meszoely (1970) and needs no further comments here. Meszoely (1970) was able to show that McDowell and Bogert (1954) placed too much emphasis on the presence of a lateral fold when classifying Ophisaurus with the gerrhonotines. Meszoely (1970) groups the extant Anguidae within three subfamilies, the Gerrhonotinae (Gerrhonotus, Abronia and Coloptychon), the Anguinae (Ophisaurus, Anguis and the fossil Pancelosaurus) and the Diploglossinae (Diploglossus, Wetmorena and Ophiodes). To these he adds a fourth subfamily, the Glyptosaurinae, to encompass the Eocene and Oligocene fossils such as Xestops, Peltosaurus, Melanosaurus, Arpadosaurus and Glyptosaurus, all probably derived from a Pancelosaurus-like ancestor. In contrast to McDowell and Bogert (1954: Anguinae including Anguis only which is considered to be a modified genus), Meszoely (1970: Anguinae including Anguis and Ophisaurus, Anguis interpreted as a degenerate genus) considers the Anguinae to be the most primitive anguid subfamily on the basis of the following evidence: a)extensive palatal dentition is retained in Ophisaurus (vomerine teeth present in 0. apodus).b)Pancelosaurus, the oldest known anguid from the Upper Cretaceous Lance Formation is referable to the Anguinae.

Olivier Rieppel

The Head Musculature of Shinisaurus crocodilurus

Shinisaurus crocodilurus, a rare anguinomorph lizard from China, is still a very poorly known species. The original description of its skull has been given by McDowell and Bogert (1954) who considered the species to be more closely related to Xenosaurus than to any other anguinomorph lizard on the basis of seemingly superficial resemblences. Such conclusion is in marked contrast to Nopsca–s proposition (1932) that Shinisaurus is closely related to Lanthanotus.

Olivier Rieppel

The Head Musculature of Xenosaurus grandis

Xenosaurus grandis is another rather poorly studied anguinomorph species. The first comprehensive description of the skeleton was given by Barrows and Smith (1947). Camp (1923) studied the throat musculature, and Haas (1960) described the trigeminal jaw musculature. McDowell and Bogert (1954) added little new information to the account of Barrows and Smith (1947), but allocated their family Xenosauridae (including Xenosaurus and Shinisaurus) to a tentative phylogenetic position derived from the Gerrhonotinae, based mainly on the presence of fused frontals and of an elongated suture between the nasals and the prefrontals. Such an arrangement has later been accepted by Estes (1963).

Olivier Rieppel

The Xenosauridae, a Diphyletic Family?

The throat musculature shows some similarities in Shinisaurus and Xenosaurus which might be indicative of a relationship of the two genera. The single interdigitating portion of the intermandibularis anterior separates a wide mesial from a narrower lateral portion of the geniohyoideus, a marked contrast to the Anguidae, and the genioglossus lateralis and medialis are of equal strength. No genioglossus lateralis fibres reach the hypobranchial skeleton in Shinisaurus.

Olivier Rieppel

The Head Musculature of Anniella pulchra

The jaw musculature of Anniella is very poorly known. It has been incompletely described and figured by Gomes (1973). The throat musculature has been described in detail by Camp (1923).

Olivier Rieppel

The Phylogenetic Position of the Genus Anniella

The genus Anniella was first described by Gray (1852) and grouped within the Scincidae. Cope (1864) established a separate family, the Anniellidae, and placed it, together with the Anelytropsidae and Acontiidae, within his tribe Typhlophthalmi.

Olivier Rieppel

The Head Musculature of Heloderma

The helodermatidae are generally considered to represent primitive platynotan lizards (McDowell and Bogert, 1954). Their precise phylogenetic position is still subject of considerable controversy, however (c.f. Camp, 1923; Hoffstetter, 1962 a; Russell, 1967). So far, the structure of head muscles has not been used as a test of helodermatid relationships, although a number of workers have dealt with it (Shufeldt, 1890; Poglayen-Neuwall, 1954; Gomes, 1973). The throat musculature is known through the work of Camp (1923) and McDowell (1972), but here again additional details useful for comparative purposes could be found. Of the two species, only H. suspectum has been dissected so far (Haas, 1973). A comparison of H. suspectum with H. horridum provided very few dissimilarities, however.

Olivier Rieppel

Heloderma, a Primitive Platynotan Lizard

Many features of the head musculature of Heloderma demonstrate the primitive status of the genus.

Olivier Rieppel

The Head Musculature of Varanus

The varanid head musculature has been extensively studied by a number of workers already among which are Bradley (1903), Watkinson (1906), Adams (1919), Camp (1923), Lakjer (1926), Richter (1933), Kesteven (1944), Säve-Söderbergh (1946), Poglayen-Neuwall (1954), Frazzetta (1962), Langebartel (1968) and Haas (1973). The throat musculature is known in great detail through Sondhi’s (1958) studies.

Olivier Rieppel

Varanus, an Advanced Platynotan

The features enumerated to characterise the platynotan status of Heloderma are also found in Varanus. They are the narrowly based bodenaponeurosis, the restriction of the origin of the 3b-layer to the mesial surface of the supratemporal and the insertion of the episterno-cleido-mastoideus into the supratemporal process of the parietal.

Olivier Rieppel

The Head Musculature of Lanthanotus borneensis

The jaw musculature of the rare Lanthanotus has so far been described by Haas (1973) only. The throat musculature has been somewhat incorrectly described and figured by McDowell (1972). The relationships of Lanthanotus are still a matter of debate (McDowell and Bogert, 1954; Russell, 1967; McDowell, 1972). Moreover, the species is of great interest as it is sometimes thought to be close to the ancestry of snakes (McDowell, 1972; Bellairs, 1972). It was therefore considered to be desirable to review the head musculature of Lanthanotus in comparison with other anguinomorph lizards.

Olivier Rieppel

The Phylogenetic Position of Lanthanotus based on its Head Masculature

McDowell and Bogert (1954:51) concluded “that Lanthanotus is not a primitive pre-platynotan intermediate between the Helodermatidae and the Varanidae but rather is an advanced and specialised platynotan lizard…”. They considered Lanthanotus to be more closely related to varanoids than to Heloderma, and derived him from the closely related Cretaceous families Mosasauridae, Dolichosauridae and Aigialosauridae. This view has been maintained by McDowell in 1967 and in 1972. Russell (1967) on the other hand believes Lanthanotus to be derived from a varanid ancestor, and finds it to approach the fossil saniwid varanids most closely in a number of characteristics. However, the structure of the head musculature clearly supports a somewhat intermediate position of Lanthanotus between the Helodermatidae and the Varanidae.

Olivier Rieppel

Summary: A Hypothesis to test

The head musculature as described and discussed above leads to the formulation of a possible phylogeny of anguinomorph lizards which differs to some extent from ideas put forward by other authors (McDowell and Bogert, 1954; Russell, 1967; Meszoely, 1970). Most of this earlier work is based on skull structure. In view of discrepancies of interpretation it becomes now necessary to test the phylogeny of anguinomorph lizards based on the structure of their head musculature by an investigation of the skull.

Olivier Rieppel

The Skull of Anguinomorph lizards

The second section of the present study deals with the skull anatomy of the anguinomorph lizards in order to get a new set of data which will be used to test the phylogenetic hypotheses based on muscular structures and, where possible, to refine them.

Olivier Rieppel

The Skull of the Anguidae

When discussing the bearing of myological evidence on the reconstruction of anguid phytogeny, a discrepancy arouse between the current interpretation of fossil evidence (Meszoely, 1970) and the interpretation of the structure of the head musculature. Meszoely (1970) considers the Anguinae as the most primitive group, based mainly on the presence of extensive palatal dentition in Ophisaurus apodus. Therefrom, the diploglossines are believed to have diverged on the one hand, the gerrhonotines on the other. The latter subfamily is regarded as a specialised group mainly because of the fused frontal bones and because of osteoderm structure. The head musculature on the contrary seems to indicate that the gerrhonotines are to be interpreted as the most primitive anguid subfamily. Apart from primitive features such as the vestiges of the second epibranchials and the primitive phalangeal count found in gerrhonotines (McDowell and Bogert, 1954), a more detailed discussion of the skull appears to be necessary to support the supposed primitive status of the Gerrhonotinae among anguid lizards.

Olivier Rieppel

The Skull of the Xenosauridae

Barrows and Smith (1947) considered the family Anguidae to be most closely related to Xenosaurus grandis. Their assumption was later specified by Estes (1963) who postulated the derivation of Xenosaurus from a gerrhonotid ancestor.

Olivier Rieppel

The Skull of the Anniellidae

The assumption of a close relationship of Anniella with the genus Anguis is generally accepted (see discussion above and Bezy, Gorman, Kim and Wright, 1977). While McDowell and Bogert (1954) derive Anniella from an Anguis-like ancestor, the analysis of the head musculature favours the assumption of a common ancestry of the Anniellidae and the anguine lineage within the Anguidae. The anatomy of the skull will be used to discriminate between the two hypotheses.

Olivier Rieppel

The Skull of the Helodermatidae

Apart from Russell (1967), the Helodermatidae are generally accepted as primitive platynotan lizards, an assumption corroborated by the analysis of their head musculature (see above). The osteology of the two species, H. suspectum (Fig. 23) and H.horridum, has first been analysed by Boulenger (1891). As it could be confirmed by personal investigation, the two species are very close in cranial anatomy. Almost no clearcut differences could be recorded in all the specimens of H. suspectum and H. horridum investigated. The number of premaxillary teeth varies randomly in both species from 3-1-3 to 4-1-4, maxillary teeth range from 8 to 9 in H. suspectum and from 6 to 7 in H. horridum, dentary teeth range from 8 to 10 in both species. H. suspectum may bear teeth on both palatines and pterygoids, or teeth may be present on either of these two bones only, or the palatal dentition may be lacking altogether. The same variability is observed in H. horridum. Contrary to Boulenger’s statement (1891), the nasal process of the premaxilla was found to be of the same width in H. suspectum (MBS. 81594) as in one specimen of H. horridum (BM[NH]11.1A).

Olivier Rieppel

The Skull of the Lanthanotidae

As it has been discussed above, the phylogenetic position of Lanthanotus is still a matter of considerable controversy.

Olivier Rieppel

The Skull of the Varanidae

The skull of the Varanidae has received very extensive attention already by Bahl (1937), Mertens (1942b), Bellairs (1949), McDowell and Bogert (1954) and Frazzetta (1962). Little will be added in the present context (Fig. 25).

Olivier Rieppel

The Atlas-Axis Complex

In lizards, the atlas consists of the first intercentrum (h1) which is fused with or sutured to the two half neural arches. It thus forms a ring which supports the ventral and lateral edges of the occipital condyle. The odontoid forms an apex dentis which comes to lie within a trough on the dorsal surface of the occipital condyle. The second intercentrum (h2) is fixed below the centrum of the axis, and constitutes the anterior hypapophysis which is generally directed posteriorly. In the Anguinomorpha, the third intercentrum (h3) is fused with or sutured to the posterior part of the proceeding centrum, i. e. the axis, which consequently bears two hypapophyses. These may sometimes fuse into a single crest (Hoffstetter and Gasc, 1969). The discussion of the anguinomorph atlas-axis complex will concentrate on the most conspicuous features only. To take the details of the structures into account seems not safe enough without previous analysis of possible individual variation which may be very extensive. For example, the h2 and h3 on the axis were found to have fused into a continuous crest in one specimen of Ophisaurus apodus (Fig. 26, c), but they remained separate in a second specimen.

Olivier Rieppel

The Phylogenetic Relationships of Lanthanotus: An Inquiry into the Fossil Record

Based on the skull characters as analysed above, Lanthanotus has to be derived from a stage of platynotan evolution where the following characters must have been retained: 1)palatine and pterygoid teeth still present2)vomerine processes of premaxilla not elongate and pointed3)posterior wall of prefrontal ventrally in contact with jugal4)descensus parietalis projects at least slightly in front of the alar process of the prootic5)vomers retaining extensive palatal shelves6)palatines retaining deep and elongate choanal grooves7)contact of prefrontals with nasals retained8)Meckelian groove closed through the contact of the splenial with the dentary in front of the anterior inferior alveolar foramen.

Olivier Rieppel

The Phylogeny and Classification of the Anguinomorpha

The Anguinomorpha are a natural group of lizards characterised by the tongue which is subdivided into a retractile fore portion and a broad, elastic hind portion through a zone of invagination (McDowell and Bogert, 1954, McDowell, 1972).

Olivier Rieppel

Outlook: The Origin of Snakes

How did snakes evolve and where did they come from? Two different questions relate to the origin of snakes: who were the ancestors of snakes and what was the biology of the transition from the lacertilian to the ophidian status? Work on both of these questions is in continuing progress. Relevant to the preceeding analysis of anguinomorph anatomy is the question of snake ancestry.

Olivier Rieppel

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