1991/06/01 by Allen E. Greer
Environmental Science · Earth and Planetary Sciences · Agricultural and Biological Sciences · #Amphibian and Reptile Biology #Paleontology and Evolutionary Biology #Animal Behavior and Reproduction
paper · doi:10.2307/1564644
An attempt is made to identify all squamate lineages having undergone limb reduction in the sense of having lost one or more bones from either the front or rear limb. The limb osteology of the ancestors of these limb reduced lineages is also inferred. On this basis squamates have undergone limb reduction at least 62 times in 53 lineages. Most reductions begin with the loss of a single phalange; however, some may begin with loss of all elements more distal than the mesopodials. Three different patterns of incipient loss occur in the front limb and two in the rear limb. These trends are discussed in the light of the developmental constraint and adaptation hypotheses. l. , . , . 166-173, 1991 ibians and Reptiles : I i icatio of t e s Limb reduction in tetrapods is one of the few cases in which large scale evolutionary change is amenable to study. There are two reasons for this. First, in the absence of a detailed phylogeny for any particular taxon, the polarity of character state change can be inferred from a general knowledge of vertebrate relationships and limb structure. Second, many vertebrate taxa show many intermediate steps in limb reduction, allowing fairly detailed comparisons to be made. The vertebrate taxon with the largest number of lineages having experienced some limb reduction is squamates (lizards and snakes) and consequently this group has been much studied in this regard (e.g., Fiirbringer, 1870; Sewertzoff, 1904,1931; Essex, 1927; Bellairs, 1950; Gans, 1975; Presch, 1975; Renous and Gasc, 1979; Greer, 1987, 1990; Choquenot and Greer, 1989). The traditional approach in limb reduction studies is to bring together a variety of species showing various stages of reduction and then to arrange them in a morphocline based on a parsimonious sequence of primitive to derived. This method is dictated by the fact that actual phylogenies are generally unknown and is based on the assumption that reversals do not occur. Within this conceptual framework the history of limb reduction studies in squamates has been to focus on smaller and smaller taxonomic groups. For example, while early studies focused on species from several families (e.g., Fiirbringer, 1870; Sewertzoff, 1931; Steiner and Anders, 1946), more recent studies focus on species within several closely related genera (Renous and Gasc, 1979) or a single genus (e.g., Presch, 1975; Greer, 1987, 1990; Choquenot and Greer, 1989). This trend has been due largely to the increasing knowledge of the world lizard fauna. An important consequence of the trend is to eliminate much of the phylogenetic noise inherent in comparing species of distant relationship. ar ange them in a morph cline based on a arsimonious sequenc of primitive to derived. is method is dictated by the fact th t ac ual ylogenies are g nerally unknown and is based the assumption that reve sals do not occur. ithin this conceptual framework the his ory limb reduction studies in quamates has been 166 This content downloaded from 207.46.13.11 on Mon, 17 Oct 2016 05:40:48 UTC All use subject to http://about.jstor.org/terms LIMB REDUCTION IN SQUAMATES What is lacking in limb reduction studies to date is a classification of types of limb reduction. As a first step towards such a classification, I present here an attempt to identify all the squamate lineages that have undergone limb reduction through loss of one or more limb bones, and to characterize the hypothetical common ancestor of each of those lineages in terms of its limb bone morphology. It should be emphasized that outright loss of a bone(s) is but one of two often interrelated types of limb reduction, the other being diminution of structure. However, because the latter requires precise knowledge of ancestors and involves detailed statistical comparison of allometric curves it is the more difficult side of limb reduction to study. MATERIALS AND METHODS Squamates were surveyed from the literature, X-rays, cleared and stained specimens, and dried osteological specimens for taxa in which one or more bones had been lost from the front or rear limb. The relationships of these taxa were then inferred from either the literature or personal observation to a level of inclusiveness such that the next closest relative of the group was a form with the primitive squamate limb morphology. These limb-reduced lineages were assessed on an extremely conservative basis, thereby minimizing the number of overall cases of presumed limb reduction. Variations within single species were viewed as having occurred only once, an extremely conservative assumption. Data on phalangeal formulas for lineages (Table 1) are given as modifications from the presumed ancestral squamate condition of 2.3.4.5.3/ 2.3.4.5.4 (outgroup Sphenodon; also Romer, 1956), with each change of state underlined. Where the changes have occurred within the lineage, the primitive condition for the lineage is given first with the variants following. Where reductions appear to have occurred simultaneously in the front and rear limb, the bone characterizations for the two limbs are joined by a + sign. Where no data are given the primitive condition is implied. Cases of fixed hyperphalangy are treated as primitive. Details of the mesoand metapodials have not been recorded due to the difficulty of interpretation from X-rays, and ambiguities as to homologies in the literature. The taxonomic ranks accorded reflect my own personal view of the level of morphological distinctiveness of the taxa involved.