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Metamorphosis and Neoteny: Alternative Pathways in an Extinct Amphibian Clade

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Metamorphosis and Neoteny: Alternative Pathways in an Extinct Amphibian Clade Evolution, 60(7), 2006, pp. 1467–1475 METAMORPHOSIS AND NEOTENY: ALTERNATIVE PATHWAYS IN AN EXTINCT AMPHIBIAN CLADE RAINER R. SCHOCH1,2 AND NADIA B. FRO¨ BISCH3,4 1Staatliches Museum fu¨r Naturkunde Stuttgart, Rosenstein 1-D-70191 Stuttgart, Germany 2E-mail: [email protected] 3Redpath Museum, McGill University, 859 Sherbrooke Street West, Montre´al, Quebec H3A 2K6, Canada 4E-mail: [email protected] Abstract. The Branchiosauridae was a clade of small amphibians from the Permo-Carboniferous with an overall salamander-like appearance. The clade is distinguished by an extraordinary fossil record that comprises hundreds of well-preserved specimens, representing a wide range of ontogenetic stages. Branchiosaurids had external gills and weakly ossified skeletons, and due to this larval appearance their status as neotenic (perennibranchiate) forms has long been accepted. Despite their extensive fossil record large specimens with an adult morphology appeared to be lacking altogether, but recently two adult specimens were identified in a rich sample of Apateon gracilis collected in the 19th century from a locality near Dresden, Saxony. These specimens are unique among branchiosaurids in showing a high level of ossification, including bones that have never been reported in a branchiosaur. These highlight the successive formation of features believed to indicate terrestrial locomotion, as well as feeding on larger prey items. Moreover, these transformations occurred in a small time window (whereas the degree of size increase is used as a proxy of time) and the degree of concentration of developmental events in branchiosaurids is unique among tetrapods outside the lissamphibians. These specimens are compared with large adults of the neotenic branchiosaurid Apateon caducus from the Saar-Nahe Basin, which despite their larger body size lack the features found in the adult A. gracilis specimens. These specimens give new insight into patterns of metamorphosis (morphological transformation) in branchiosaurids that are believed to be correlated to a change of habitat, and clearly show that different life-history pathways comparable to those of modern salamanders were already established in this Paleozoic clade. Key words. Amphibians, Branchiosauridae, development, developmental pathways, evolution, metamorphosis, ne- oteny, Temnospondyli. Received November 10, 2005. Accepted April 7, 2006. The evolutionary origin of metamorphosis in amphibians aquatic animals, which underwent neither a change of habitat is still very poorly understood. The three extant clades, sal- nor a significant morphological transformation during on- amanders, frogs, and caecilians, reveal widely divergent pat- togeny. Notably, the most characteristic and feature-inde- terns and processes usually referred to as metamorphosis, and pendent criterion for metamorphosis is not observed, namely the homology of metamorphosis-related features across these the concentration of developmental events into a short phase clades is far from clear. During metamorphosis, the remod- of development (Alberch 1989). eling of the skeleton as well as changes in the crucial organ Recently, one of us reviewed the evidence for metamor- systems have been found to be quite different in the three phosis in temnospondyls, the largest clade of Paleozoic am- extant clades (Fox 1984; Duellman and Trueb 1986; Fritzsch phibian-like tetrapods (Schoch 2002a). The emerging picture 1990; Rose and Reiss 1993; Hall 2003; Rose 2003). However, is that major morphological transformations occurred only in the underlying system of endocrine control appears to be three small clades, which all had more or less terrestrial rather conservative (Gundernatsch 1912; Norris 1997; Shi adults. However, in only one of these clades there was a 2000), and common features of morphological transformation substantial concentration of developmental events, the Per- have been mentioned among the three extant clades (Reiss mo-Carboniferous family Dissorophoidea. The family in- 1996, 2002). It is therefore unclear which aspects of am- cludes mostly small species with adults having well-ossified phibian metamorphosis were actually inherited from a com- and long limbs suggesting a terrestrial existence. However, mon ancestor and which evolved in parallel. Most generally, most species of dissorophoids are known by few size classes, the origin of metamorphosis in the evolutionary history of which leaves major gaps in their ontogenetic trajectories. The amphibians is puzzling. group with the best-known fossil ontogenies, the Branchio- Here, we approach the problem from the perspective of the sauridae, was until recently known exclusively from larval fossil record. In Paleozoic amphibians, or more precisely specimens. early anamniote-grade tetrapods, ontogeny has been studied In the present study, we report on the discovery of meta- in various clades. These studies were mostly based on dif- morphosed adults in a particular branchiosaurid species and ferent size classes of specimens from the same taxon and analyze the transformations that characterize this previously horizon, revealing particularly detailed information on the unknown type of metamorphosis. Unlike all other temno- ontogeny of the skull (Boy 1974; Schoch 1992, 2003, 2004; spondyls, these changes were dramatic and rapid, that is, the Boy and Sues 2000). Regarding the problem of metamor- crucial developmental events occurred in a small time win- phosis, these analyses arrived at a rather surprising result: a dow, whereas the degree of size increase is taken as a proxy morphological transformation that would indicate some sort for time. This recalls the situation in modern amphibians, in of metamorphosis is rarely found among these extinct groups. which metamorphosis is a short phase with drastic morpho- Instead, many clades appear to have included predominantly logical changes. This is contrasted by other branchiosaurid 1467 ᭧ 2006 The Society for the Study of Evolution. All rights reserved. 1468 R. R. SCHOCH AND N. B. FRO¨ BISCH species, which merely extended the larval trajectory to form Recently, Werneburg (1991) reexamined the branchiosaurid neotenic adults. The latter are throughout similar to larvae, material of this collection and correctly identified the adults although having a higher level of ossification. of Apateon gracilis (referred to as Melanerpeton gracile in Based on these characteristics, branchiosaurids are the first his study). The collection consists of an extensive body of extinct lower tetrapod group to document the complete ne- material collected at just one locality and horizon, the otenic and metamorphosing trajectories in closely related spe- Plauenscher Grund at Niederha¨slich near Dresden. The ma- cies. They are indeed the first group for which a lissamphi- terial comprises an extensive growth series of several taxa, bian-like metamorphosis can be proven. among them Onchiodon labyrinthicus, Acanthostomatops vor- Branchiosaurids were first mentioned by Meyer (1844) and ax, Branchierpeton amblystomum, and the species of interest later recognized as a distinct group by Fritsch (1879), who here, A. gracilis (Boy 1987; Werneburg 1991). The main also coined the family name (meaning ‘‘gilled lizards’’). objectives of the present study are therefore to describe the Originally the group was used as a repository for immature unique features of the large specimens and to assess which specimens with poorly defined features. Despite their ex- changes characterized the latest events in the ontogeny of A. quisite preservation, which includes skin remains and exter- gracilis. These data will be compared with that from large nal gills, branchiosaurids have posed problems, that have not adults of the neotenic branchiosaurid Apateon caducus from yet been surmounted. Since Fritsch, they have been referred the Saar-Nahe Basin, based on material not described before. to as a stereotype of larval amphibians in the Paleozoic. Romer (1939) hypothesized that branchiosaurids were MATERIAL nothing but larval morphotypes of well-known, 2 to 3 m long temnospondyl amphibians such as Onchiodon or Scleroce- The material referred to in this paper is housed in the phalus. This was indeed an attractive explanation, because following institutional collections: Institut fu¨r Geowissen- larval specimens of these groups during that time were oth- schaften, University of Mainz (Germany): GPIM-N 1230– erwise unknown and by analogy with extant amphibians, 1840; Landesamt fu¨r Denkmalpflege, Mainz (Germany): metamorphosis should have involved major transformations LDM-N-9u2; Landesamt fu¨r Umwelt und Geologie, Freiberg in Paleozoic amphibians as well. (Germany): LFUG-SS 13958, 13962, 13964, 13965, 13976, Boy (1972) was able to reject Romer’s hypothesis by iden- 13986, 14011, 14015, 14032, 14037, 14064, 14045, 14072, tifying true larval specimens of Sclerocephalus and Onchio- 14073, 14048; LFUG-RS 14770, 14775, 14782, 14783 (A. don and by recognizing autapomorphic larval specializations gracilis); Museum fu¨r Naturkunde, Humboldt Universita¨t zu of branchiosaurids, which distinguished them from both lar- Berlin (Germany): MB.Am.55, 363, 364, 370, 374, 376, 398, vae and adults of all other temnospondyls. Boy (1972) con- 399, 402–406, 412, 413, 945 (A. gracilis); Staatliches Mu- cluded that members of the Branchiosauridae formed a clade seum fu¨r Mineralogie und Geologie, Dresden, (Saxony, Ger- of their own, being closely related to dissorophoids such as many): SGD 2144, 2149
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