Skeletal anatomical description of Isogomphodon oxyrhynchus (Müller & Henle, 1839) and discussion of its phylogenetic position within Carcharhinidae (Chondrichthyes, Carcharhiniformes)

Abstract

The Daggernose shark Isogomphodon oxyrhynchus (Müller & Henle, 1839), sole species in its genus, is endemic to northern South America, from Southern Maranhão State in Brazil to Trinidad and Tobago. Although the species is traditionally classified as estuarine, it occurs also in entirely marine environments. Such habitat shift is seasonal, related to the rain regimen. I. oxyrhynchus is easily recognized by the elongated snout, enlarged pectoral fins and reduced eyes, characteristics which account for its unique general aspect in the family Carcharhinidae.The anatomy of I. oxyrhynchus remains poorly known and its phylogenetic position is still controversial. In this study, we propose a contribution to the knowledge of I. oxyrhynchus, offering: 1. A description of the entire skeleton and dermal denticles, based on dissections, computerized tomography and Scanning Electron Microscopy; 2. A comparative anatomical study of I. oxyrhynchus with other representatives of Carcharhinidae, which is compared with results from a novel molecular analysis; 3. Review of fossil taxa assigned to Isogomphodon.The skeletal description revealed much new information for I. oxyrhynchus, including new information on complexes which were previously entirely unknown in the taxon, such as the appendicular skeleton, mandibular arch and clasper. The comparative and phylogenetic study is organized into 46 morphological characters, which qualitatively supports Carcharhinus porosus as sister-group of I. oxyrhynchus. Results also support the hypothesis of Isogomphodon as a modified member of the genus Carcharhinus. Lastly, a critical review of the fossil taxa assigned to Isogomphodon demonstrated that from the five species previously assigned to Isogomphodon, only two can be positively attributed to the genus.

Evolutionary History and Taxonomic Reclassification of the Critically Endangered Daggernose Shark, a Species Endemic to the Western Atlantic

“…Sorenson et al [39] also found that C. porosus and T. obesus are the carcharhinid species that are most similar to C. oxyrhynchus. Although no exclusive similarities were found between C. oxyrhynchus and C. porosus, 26 (58%) of the 45 traits analyzed were highly similar, including the morphology of the lower teeth, which have an elongated crown, with narrow roots in the anterior rows [71]. The distinctive morphology of C. oxyrhynchus does not exclude this species from the genus Carcharhinus, however, given that the tree topology and genetic distances found in the present study emphatically support the inclusion of this species in Carcharhinus.…”

“…The head and fins are also sexually dimorphic [69], which may be associated with mating behavior patterns [70]. Silva [71] presented an anatomical description of C. oxyrhynchus based on an analysis of skeletal traits, including the dental series and morphology of the neurocranium, mandibular and brachial arches, claspers, fins, and the dermal denticles, which was used for systematic comparisons with other members of the family Carcharhinidae. Silva [71] initially compared C. oxyrhynchus with the fossil species attributed to the genus Isogomphodon, including Isogomphodon acuarius (Probst, 1879), Isogomphodon lerichei (Darterville and Casier, 1943), Isogomphodon gracilis (Jonet, 1966), Isogomphodon caunellensis ( [65]), and Isogomphodon aikenensis (Cicimurri and Knight, 2019), affirming that only the fossil taxa C. aikenensis and C. acuarius should be maintained in Isogomphondon while other taxa were assigned to Carcharhinus.…”