The skull and jaw musculature as guides to the ancestry of salamanders

Abstract

The fossil record provides no evidence supporting a unique common ancestry for frogs, salamanders and apodans. The ancestors of the modern orders may have diverged from one another as recently as 250 million years ago, or as long ago as 400 million years according to current theories of various authors. In order to evaluate the evolutionary patterns of the modern orders it is necessary to determine whether their last common ancestor was a rhipidistian fish, a very primitive amphibian, a labyrimhodom or a ‘lissamphibian’. The broad cranial similarities of frogs and salamanders, especially the dominance of the braincase as a supporting element, can be associated with the small size of the skull in their immediate ancestors. Hynobiids show the most primitive cranial pattern known among the living salamander families and “provide a model for determining the nature of the ancestors of the entire order. Features expected in ancestral salamanders include: (1) Emargination of the cheek; (2) Movable suspensorium formed by the quadrate, squamosal and pterygoid; (3) Occipital condyle posterior to jaw articulation; (4) Distinct prootic and opisthotic; (5) Absence ol otic notch; (6) Stapes forming a structural link between braincase and cheek. In the otic region, cheek and jaw suspension, the primitive salamander pattern (resembles most closely the microsaurs among known Paleozoic amphibians, and shows no significant features in common with either ancestral frogs or the majority of labyrinth odonts. The basic pattern of the adductor jaw musculature is consistent within both frogs and salamanders, but major differences are evident between the two groups. The dominance of the adductor mandibulae externus in salamanders can be associated with the open cheek in all members of that order, and the small size of this muscle in frogs can be associated with the large otic notch. The spread of different muscles over the otic capsule, the longus head ol the adductor mandibulae posterior in frogs and the superficial head of the adductor mandibulae internus in salamanders, indicates that fenestration of the skull posterodorsal to the orbit occurred separately in the ancestors of the two groups. Reconstruction of the probable pattern of the jaw musculature in Paleozoic amphibians indicates that frogs and salamanders might have evolved from a condition hypothesized for primitive labyrinthodonts, but the presence of a large otic notch in dissorophids suggests specialization toward the anuran, not the urodele condition. The presence of either an einarginated cheek or an embayment of the lateral surface of the dentary and the absence of an otic notch in microsaurs indicate a salamander‐like distribution of die adductor jaw muscles. The ancestors of frogs and salamanders probably diverged from one another in the early Carboniferous, Frogs later evolved from small labyrinthodonts and salamanders from microsaurs. Features considered typical of lissamphibians evolved separately in the two groups in the late Permian andTriassic.

Cranial muscles of the anuransleiopelma hochstetteriandascaphus trueiand the homologies of the mandibular adductors in lissamphibia and other gnathostomes

“…The branches of the trigeminal nerve traverse the muscle group but do not define its components. This hypothesis is advanced for anurans with the data presented here and from Luther (1914), Starrett (1968), and Haas (2001), for salamanders from Luther (1914), Carroll and Holmes (1980), Haas (2001), and Lubosch (1938), and in caecilians from Luther (1914) and Kleinteich and Haas (2007). The formulation of this plan for caecilians accepts Haas' (2001) interpretation that the caecilian m. levator mandibulae ''externus'' of Edgeworth (1935) and ''medius'' of Luther (1914) represent the posterior (articularis) component as seen in the other Lissamphibia.…”

Molecular Evidence for the Early History of Living Amphibians

“…The conventional hypothesis, based on morphological characters of living and fossil species, is that lissamphibians arose from a single lineage of late Paleozoic (300-250 myr) amphibians and that frogs and salamanders are closest relatives (Duellman and Trueb, 1986;Duellman, 1988;Gardiner, 1983;Trueb and Cloutier, 1991;Milner, 1988Milner, , 1993a. However, some fossil evidence has suggested a multiple origin for lissamphibians (Carroll and Curie, 1975;Carroll and Holmes, 1980;Smithson, 1985), and a salamandercaecilian relationship has appeared in several molecular studies of nuclear genes (Larson and Wilson, 1989;Hedges et al, 1990;Hay et al, 1995). A key to understanding the early evolutionary history of living amphibians and their biogeography is the relationships of the three orders.…”

Tuataras and salamanders show that walking and running mechanics are ancient features of tetrapod locomotion

“…This 'living fossil' has long been considered the best living model of the basic amniote morphotype (Romer 1956;Carroll 1988). The tiger salamander, A. tigrinum, represents a good living model for a generalized tetrapod: Ambystoma retain many plesiomorphic features of basal tetrapods (Carroll & Holmes 1980;Jarvik 1980) with a body plan that has remained essentially unchanged for at least 150 million years (Gao & Shubin 2001).…”