Redescription of two lungfish (Sarcopterygii: Dipnoi) tooth plates from the Late Cretaceous Bauru Group, Brazil

Abstract

Two lungfish tooth plates from the Adamantina and Marília formations, southeastern Brazil, are redescribed. One represents the youngest record of an asiatoceratodontid for Gondwana. This extends the upper temporal range of this family from Cenomanian to Turonian–Santonian. The other represents a new record of ceratodontids for South America. This discovery corroborates previous descriptions of South American fauna, such as the mixture of relict and ancient Gondwana taxa that disappeared by the end of the Cretaceous or the beginning of the Paleogene. The records add to the extant neotropical ichthyofauna, that survived after the South America–Africa separation.

Introduction

The dipnoans, or lungfishes, form a well-defined clade of sarcopterygian fishes with a fossil record since the Early Devonian. Today, they are represented by just three extant freshwater genera distributed in three continents: Lepidosiren (South America), Protopterus (Africa) and Neoceratodus (Australia) (e.g., Schultze, 1992).

Fossil dipnoans in the Paleozoic are well known from articulated specimens, including three-dimensional skulls and isolated tooth plates, sometimes attached to mandibles and maxillary bones (Denison, 1968). There are a few rare exceptions of articulated Mesozoic specimens from the Jurassic of Kyrgyzstan (Nessov and Kaznyshkin, 1985), the Cretaceous of Thailand (Cavin et al., 2007), and several specimens from Australia (White, 1926; Kemp, 1993, 1994, 1996, 1998); most of the post-Triassic fossil record is composed exclusively of tooth plates (e.g., Kemp, 1997b).

This notable shift in the preservation of fossil dipnoans through time is directly related to the observed evolutionary trend, from heavily ossified skeletons with dermal bones, sometimes covered by cosmine, in the Devonian dipnoans, to the weakly ossified skeletons with thin dermal skull bones embedded in the skin present in Mesozoic and Cenozoic dipnoans (Martin, 1982). In parallel, tooth plates became stronger through increasing mineralization with specialized dentines (Martin, 1982; Kemp, 2001; Cavin et al., 2007).

Despite the relatively frequent occurrence of lungfish tooth plates in South America (e.g., Toledo, 2006; Apesteguía et al., 2007; Cione et al., 2007; Soto and Perea, 2010), only six nominal species were identified in Brazil: Ceratodus africanus Haug, 1905 (see Cunha and Ferreira, 1980; Martin, 1984); Lepidosiren megalos Silva Santos, 1987; Asiatoceratodus cf. tiguidiensis (see Dutra and Malabarba, 2001; Castro et al., 2004); Ceratodus humei Priem, 1914 (see Toledo, 2006) (for a discussion of the taxonomy of this species, see Churcher and De Iuliis, 2001); Ptychoceratodus cf. philipsi (see Richter and Toledo, 2008); and Equinoxiodus alcantarensis Toledo et al. (2011).

The lack of morphological data, apart from tooth plates, makes difficult a precise classification for much of the post-Triassic dipnoan fossil record. The recent discoveries of new material in the Mesozoic, specifically in eastern and western Gondwana (e.g., Churcher and De Iuliis, 2001; Castro et al., 2004; Churcher et al., 2006; Cavin et al., 2007; Cione et al., 2007; Gottfried et al., 2009; Soto and Perea, 2010; Toledo et al., 2011), allowed a more accurate review of some dipnoan fossil groups (e.g., Kemp, 1994; Churcher et al., 2006; Apesteguía et al., 2007; Cavin et al., 2007; Cione et al., 2007; Agnolin, 2010; Soto and Perea, 2010). Based on these reviews, we present here a redescription of two specimens collected during fieldwork in the Adamantina and Marília formations (Fig. 1; the localities of Santo Anastácio and Uberaba towns, respectively), of the Bauru Group. Both specimens were originally identified by Gayet and Brito (1989) as Neoceratodus sp. (also published by Brito et al., 2006). However, a re-examination of the specimens provided new anatomical information that allows them to be assigned to other genera (see Discussion).