The systematic position of Lanthanotus and the affinities of the anguinomorphan lizards. Bulletin of the AMNH ; v. 105, article 1

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New York : [American Museum of Natural History]
"The external features of the Bornean lizard, Lanthanotus borneensis Steindachner, are described, together with the skull, the hyoid, the tongue, and the roentgenogram of the postcranial osteology. The various theories concerning the relationships of Lanthanotus are evaluated. The belief that Lanthanotus is related to the Chinese lizard Shinisaurus crocodilurus Ahl is shown to be unsupported by either the internal or external morphology. Nor is there evidence of close relationship between Lanthanotus and Heloderma, although these two genera are most frequently placed together in the family Helodermatidae. On the other hand, structural resemblances between Lanthanotus and Varanus are found to be both numerous and important. However, it is here shown that there is still greater affinity between Lanthanotus and three Cretaceous families believed to be closely related to Varanus, the Aigialosauridae, Mosasauridae, and Dolichosauridae. Lanthanotus represents a monotypic family closely related to aigialosaurs and dolichosaurs. Lanthanotus shows many of the features to be expected in an ancestor of the snakes, and the close similarities between Lanthanotus and the boid-like, colubrid-like, and particularly the leptotyphlopid snakes, materially strengthen the argument that the snakes are derived from platynotan (Varanus-like) lizards. In contrast to other limbless reptiles commonly referred to the Serpentes, the Typhlopidae show no particular resemblance in cranial structure to either the platynotan lizards or the snakes (including Leptotyphlops). Reasons are advanced for believing that the Typhlopidae are wrongly placed in the Serpentes (or Ophidia) and represent a line of anguinid-like lizards that has become convergent to the Serpentes. Incidental to the work upon which we base the conclusions summarized above, several additional observations have been made on the anatomy and relationships of the saurian infraorder Anguinomorpha. The most important conclusions reached are as follows: 1. The family Pygopodidae is not related to the Anguinomorpha but represents an aberrant line of geckos, modified by a reduction of the limbs but retaining many features of the gekkonids. 2. The family Cordylidae (long known as Zonuridae) is not related to the Anguinomorpha but instead is closely related to the Gerrhosauridae, a family that differs in relatively minor ways from the Lacertidae. 3.The fossil family Euposauridae, frequently placed in or near the Anguinomorpha, shows no clear evidence of affinity to the Anguinomorpha and is not even definitely referable to the Squamata. 4. After the removal of these three families, the Anguinomorpha become a natural group, the members of which are characterized by their peculiar tongue structure, their method of tooth replacement, and a unique combination of osteological characters. The group, here considered an infraorder, contains the families Anguinidae (Anguidae auct.), Xenosauridae, Anniellidae, Helodermatidae, Varanidae, Aigialosauridae, Mosasauridae, Dolichosauridae, and Lanthanotidae. 5. This group, the Anguinomorpha, appears to have undergone an early separation into two main lines of evolution, here considered taxonomically as two superfamilies: Diploglossa (or Anguinoidea) and Platynota (or Varanoidea). 6. The line here called Diploglossa (Anguinidae, Xenosauridae, and Anniellidae) includes the more primitive families, primarily adapted to a crushing action of the jaws, with rigid mandibles. None of the known forms can be considered directly ancestral to the platynotan line. 7. The remainder of the Anguinomorpha represent a line (Platynota or Varanoidea) the jaws of which are primarily adapted for a grasping action, with a progressive development through the families of a hinge in the middle of the ramus of the lower jaw. This line includes forms distinctly more predatory in habits than is usual in the members of the first line. 8. The beaded lizards (Heloderma) have most recently been classified among the members of the first (diploglossan) line. But careful study of the morphology of Heloderma shows it to be close to Varanus, and a member of the second (platynotan) series, as was believed by most of the earlier workers. Although specialized in many respects, in other features of its structure Heloderma appears to be near the ancestral form that gave rise to the Platynota. 9. Upon examination of the skull, the Chinese genus Shinisaurus of previously unknown affinities proves to be a close relative of the Mexican genus Xenosaurus. The fossil genera Exostinus, Melanosaurus, and Necrosaurus (also known as Palaeovaranus) also appear to be members of the Xenosauridae. A revised classification of the Anguinomorpha is presented"--P. 134-135.
142 p., 16 p. of plates : ill. ; 27 cm.
Includes bibliographical references (p. 138-141).