The morphology of Idiocranium russeli (amphibia: Gymnophiona), with comments on miniaturization through heterochrony

Abstract

Idiocranium russeli is among the smallest species of caecilians. The morphology of the skull (e.g., small, widely spaced dermal elements, retention of cartilaginous remnants) and of the posterior vertebrae (incomplete development, retention of a postvertebral notochord) indicate that heterochrony, or change in timing of development, has resulted in the miniaturization of the species. The pattern is apparently a progenetic one, characterized by early offset of development (see Alberch et al., '79). Other unique features of osteology (e.g., extensive sphenethmoid development) correlate with the burrowing habit of the species and may have arisen to compensate for progenetic effects. Several features of the myology, neurology, and visceral morphology are correlated with miniaturization as well. Reproductive maturity occurs at approximately 75 mm total length; gonads are proportionately small, but other aspects of reproductive morphology are typical of direct-developing caecilians.Zdiocranium russeli is one of the smallest caecilian species. Collected only once near the Cameroon-Nigerian border in 1934-1935 by the British Museum (Natural History) expedition led by Ivan T. Sanderson, the species is characterized by a number of unique characters. Several (reviewed below) were listed by Parker ('36) in the type description. Aspects of the ecology and behavior were recounted by Sanderson ('37). Forty-eight paratypes were reported by Parker ('36) to be in the British Museum (Natural History) and the Museum of Zoology, Cambridge University, U.K. Available for study were the holotype (BMNH 1936.3.4.29) and 16 paratypes from the BMNH, 28 specimens from Cambridge, and three specimens from the Museum of Comparative Zoology, Harvard University. Investigation of Idiocranium provides information relevant to several problems in evolutionary morphology. Little was known of its morphology; thus its relationships to other gymnophione taxa are not well assessed. Miniaturization as a developmental and evolutionary phenomenon is receiving renewed attention (Hanken, '84); heterochrony, or alteration of developmental "program," has been proposed as a mechanism for such phenomena (Gould, '77; Alberch et al., '79). Analysis of heterochrony in a lineage presumably already constrained developmentally and phylogenetically by body elongation and limb loss may yield insight into pattern of evolution. This study analyzes characters of Idiocranium common to the gymnophione lineage, and the nature of variation of these characters in a miniaturized taxon. It considers how small size may be achieved, and how the morphological characteristics of the taxon are correlated with its biology, especially its fossorial habits and its direct-development mode of reproduction. A comparison with other West African caeciliids also may be instructive in the absence of opportunities to recollect the species, if it still exists. MATERIALS AND METHODSForty-eight specimens (27 females, 13 males, and eight juveniles) of Idiocranium russeli were examined, ranging...

A redescription ofCarrolla craddocki(Lepospondyli: Brachystelechidae) based on high‐resolution CT, and the impacts of miniaturization and fossoriality on morphology

“…Relative enlargement of sensory regions such as the nasal capsule, eye, and otic capsule is a common feature of size‐reduced vertebrates (Wake,1986; Carroll,1990; Hanken and Wake,1993). The absolute size of the otic capsule in particular is limited by functional requirements of the semicircular canals, meaning that the region is relatively larger in smaller taxa (Jones and Spells,1963; Carroll,1970; Gans,1974; Rieppel,1984).…”

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“…A great deal of recent work has considerably improved our understanding of the evolutionary origins of modern amphibians (frogs, salamanders, caecilians; collectively Lissamphibia; see Lebedkina,2004; Schoch and Milner,2004; Anderson,2008 for recent reviews). A number of studies have expanded our knowledge of amphibamid temnospondyls (Schoch and Rubidge,2005; Huttenlocker et al,2007; Anderson et al,2008a; Fröbisch and Reisz,2008; Fröbisch and Schoch,2009), which are considered to be the sister group to lissamphibians (Bolt,1969,1991; Trueb and Cloutier,1991; Clack and Milner,1993,2010; Milner,1993), or at least batrachians (frogs and salamanders, exclusive of caecilians; Anderson et al,2008b; but see Laurin and Reisz (1997) for an alternative view). The description of Gerobatrachus hottoni (Anderson et al,2008b), a derived amphibamid sharing features uniquely known only among frogs and salamanders, has refocused the question of lissamphibian origins to that of caecilian origins.…”

Cladistic analysis and osteological descriptions of the frog species in theLeptodactylus fuscusspecies group (Anura, Leptodactylidae)

“…In adults of Bolitoglossa occidentalis (Taylor, 1941), the rate of closure towards the centre of the skull of the frontals and parietals is similar to that observed in subadults from other species of the genus (Alberch and Alberch 1981). Idiocranium russeli (Parker, 1936) (Gymnophiona) has a large nasofrontal fontanelle (Wake 1986). The open frontoparietal fontanelle gives limited dorsal protection to the brain and this lack of protection could be compensated by an extended ossification of the sphenethmoid and septum nasi, a trait which is typical of almost all the species of the L. fuscus group.…”

Consequences of miniaturization in insect morphology

“…Основное влияние миниатюри зации на строение позвоночных животных относит ся к редукциям и структурным упрощениям и сво дится к слиянию элементов скелета у рыб [83][84][85], амфибий [32-35, 86,87], рептилий [88,89], млеко питающих [90] и упрощению органов чувств [83,[91][92][93]. Также у многих мелких позвоночных на блюдается аллометрия ЦНС [93][94][95][96][97].…”