The early elasmobranch Phoebodus : phylogenetic relationships, ecomorphology and a new time-scale for shark evolution

Abstract

Anatomical knowledge of early chondrichthyans and estimates of their phylogeny are improving, but many taxa are still known only from microremains. The nearly cosmopolitan and regionally abundant Devonian genus Phoebodus has long been known solely from isolated teeth and fin spines. Here, we report the first skeletal remains of Phoebodus from the Famennian (Late Devonian) of the Maïder region of Morocco, revealing an anguilliform body, specialized braincase, hyoid arch, elongate jaws and rostrum, complementing its characteristic dentition and ctenacanth fin spines preceding both dorsal fins. Several of these features corroborate a likely close relationship with the Carboniferous species Thrinacodus gracia , and phylogenetic analysis places both taxa securely as members of the elasmobranch stem lineage. Identified as such, phoebodont teeth provide a plausible marker for range extension of the elasmobranchs into the Middle Devonian, thus providing a new minimum date for the origin of the chondrichthyan crown-group. Among pre-Carboniferous jawed vertebrates, the anguilliform body shape of Phoebodus is unprecedented, and its specialized anatomy is, in several respects, most easily compared with the modern frilled shark Chlamydoselachus . These results add greatly to the morphological, and by implication ecological, disparity of the earliest elasmobranchs.

A symmoriiform from the Late Devonian of Morocco demonstrates a derived jaw function in ancient chondrichthyans

“…Phylogenetic significance. Results of phylogenetic analysis place the symmoriiform sharks, including Ferromirum, as a clade branching from the holocephalan stem, consistent with recent and related results 3,13,14,35 (Fig. 5).…”

“…Consistent with this treatment, both traditional and some of the more current phylogenetic hypotheses have placed this group (whether grade or clade) on the chondrichthyan stem 11,12 . However, an alternative series of analyses resolve symmoriiforms as stem holocephalans 2,3,10,13,14 . Notably, the most recent of these hypotheses employ the abundance of new data from X-ray tomography, especially concerning the neurocranium 3,11,15,16 .…”

First African thylacocephalans from the Famennian of Morocco and their role in Late Devonian food webs

“…Late Devonian strata of the eastern Anti-Atlas are known to be highly fossiliferous, yielding both vertebrates and invertebrates [13][14][15][16] . The Middle Famennian outcrops contain a layer attributed to the Maeneceras genozone 13,[17][18][19] , erroneously called 'Phyllocarid Layer' in an earlier paper (Ref.…”

Mineralisation of theCallorhinchusvertebral column (Holocephali; Chondrichthyes)

“…Following phylogenetic review Coates et al (2017Coates et al ( , 2018Dearden et al, 2019;Frey et al, 2019), several taxa that were previously resolved as stem group chondrichthyans (basal to the clade Elasmobranchii + Holocephali; Pradel et al, 2011), are now resolved as stem holocephalans, joining more crownward stem holocephalans including the Iniopterygiformes (Zangerl and Case, 1973), Helodus (Moy-Thomas, 1936), Kawichthys, Debeerius and Chondrenchelys, the latter being the sister taxon to the crown group Holocephali (chimaeroids) (Figure 8). Tessellated calcified cartilage has been variously identified among these stem-group Holocephali: this includes taxa assigned to the Symmorida, such as Dwykaselachus (Coates et al, 2017: extended data figure 1d), Ozarcus (Pradel et al, 2014), Cladoselache ("minute granular calcifications"; Dean, 1894; Figure 9A), Akmonistion ("prismatic calcified cartilage" Coates and Sequiera, 2001), Damocles and Falcatus (Lund and Grogan, 1997), also present in Cobelodus ( Figure 9B).…”

“…We report the presence of a layer of mineralisation in the Callorhinchus embryo, comprising small, irregularly-shaped units, maintained in adults, and lacking many of the characteristics of tesserae in the elasmobranchs. To provide further phylogenetic context we also examined mineralised tissues in fossil members of the Callorhinchidae (Edaphodon; Nelson et al 2006), as well as stem-group holocephalan taxa (e.g., Cladoselache, Cobelodus, Helodus, Iniopterygiformes;Coates et al, 2017Coates et al, , 2018Frey et al, 2019). The tesserae in these stem-group holocephalans are larger than in Callorhinchus, and more similar in shape to polygonal elasmobranch tesserae.…”