Abstract:BackgroundSqualiform sharks represent approximately 27 % of extant shark diversity, comprising more than 130 species with a predominantly deep-dwelling lifestyle. Many Squaliform species are highly specialized, including some that are bioluminescent, a character that is reported exclusively from Squaliform sharks within Chondrichthyes. The interfamiliar relationships within the order are still not satisfactorily resolved. Herein we estimate the phylogenetic interrelationships of a generic level sampling of “sq… Show more
“…Hexanchids and echinorhinids are often considered primitive, given their plesiomorphic characteristics and early origin in the fossil record (Barnett et al ., ; Crofts, ; da Cunha et al ., ; Daly‐Engel et al ., ). Echinorhinus brucus and the prickly shark Echinorhinus cookei Pietschmann 1928 are basal elasmobranchs with an estimated family radiation from other squaliform sharks roughly 148 M BP (Straube et al ., ). Hexanchiformes, the oldest extant order of sharks, are estimated to have diverged from the rest of Squalimorphii about 203 M BP (Straube et al ., 2015).…”
Discovery of an unusual rectal gland in the Atlantic sixgill shark Hexanchus vitulus led us to examine the rectal glands of 31 species of sharks to study diversity in rectal‐gland morphology. Twenty‐four of 31 species of sharks had digitiform glands (mean width–length ratio ± SD = 0.17 ± 0.04) previously assumed to be characteristic of all elasmobranchs regardless of habitat depth or phylogenetic age. Rectal glands from the family Somniosidae were kidney bean‐shaped (mean width: length ± SD = 0.46 ± 0.05); whereas those from families Echinorhinidae and Hexanchidae were lobulate (mean width: length ± SD = 0.55 ± 0.06). Rectal gland width: length were different among species with digitiform morphology and lobulate morphology (ANOVA; R2 = 0.9; df = 15, 386; 401, F = 219.24; P < 0.001). Histological and morphological characteristics of the digitiform morphology from deep‐sea sharks were similar to those from shallow‐water sharks. Histology of lobulate rectal glands from hexanchids were characterised by tubule bundles separated by smooth muscle around a central lumen. Additionally, we examined plasma chemistry of four species of sharks with digitiform rectal glands and two species with lobulate rectal‐gland morphology to see if there were differences between morphologies. Plasma chemistry analysis showed that urea and trimethylamine N‐oxide (TMAO) followed the piezolyte hypothesis, with TMAO being highest and urea being lowest in deep‐sea sharks. Among electrolytes, Na+ was highest in species with lobulate rectal glands. Hexanchids and echinorhinids both have lobulate rectal glands similar to those of holocephalans, despite the more than 400 million years separating these two groups. The morphological similarities between the lobulate rectal‐gland anatomy of primitive sharks and the secretory morphology of holocephalans may represent an intermediate state between Holocephali and derived shark species.
“…Hexanchids and echinorhinids are often considered primitive, given their plesiomorphic characteristics and early origin in the fossil record (Barnett et al ., ; Crofts, ; da Cunha et al ., ; Daly‐Engel et al ., ). Echinorhinus brucus and the prickly shark Echinorhinus cookei Pietschmann 1928 are basal elasmobranchs with an estimated family radiation from other squaliform sharks roughly 148 M BP (Straube et al ., ). Hexanchiformes, the oldest extant order of sharks, are estimated to have diverged from the rest of Squalimorphii about 203 M BP (Straube et al ., 2015).…”
Discovery of an unusual rectal gland in the Atlantic sixgill shark Hexanchus vitulus led us to examine the rectal glands of 31 species of sharks to study diversity in rectal‐gland morphology. Twenty‐four of 31 species of sharks had digitiform glands (mean width–length ratio ± SD = 0.17 ± 0.04) previously assumed to be characteristic of all elasmobranchs regardless of habitat depth or phylogenetic age. Rectal glands from the family Somniosidae were kidney bean‐shaped (mean width: length ± SD = 0.46 ± 0.05); whereas those from families Echinorhinidae and Hexanchidae were lobulate (mean width: length ± SD = 0.55 ± 0.06). Rectal gland width: length were different among species with digitiform morphology and lobulate morphology (ANOVA; R2 = 0.9; df = 15, 386; 401, F = 219.24; P < 0.001). Histological and morphological characteristics of the digitiform morphology from deep‐sea sharks were similar to those from shallow‐water sharks. Histology of lobulate rectal glands from hexanchids were characterised by tubule bundles separated by smooth muscle around a central lumen. Additionally, we examined plasma chemistry of four species of sharks with digitiform rectal glands and two species with lobulate rectal‐gland morphology to see if there were differences between morphologies. Plasma chemistry analysis showed that urea and trimethylamine N‐oxide (TMAO) followed the piezolyte hypothesis, with TMAO being highest and urea being lowest in deep‐sea sharks. Among electrolytes, Na+ was highest in species with lobulate rectal glands. Hexanchids and echinorhinids both have lobulate rectal glands similar to those of holocephalans, despite the more than 400 million years separating these two groups. The morphological similarities between the lobulate rectal‐gland anatomy of primitive sharks and the secretory morphology of holocephalans may represent an intermediate state between Holocephali and derived shark species.
“…Figure 9.Tooth development patterns of adult and embryo squaliform taxa plotted against the phylogenetic models of figure 1. ( a – d ) The phylogeny of Straube et al [5] showing dental arrangements of lower ( a ) and upper ( b ) teeth of adults and lower ( c ) and upper ( d ) teeth of embryos. ( e – h ) The phylogeny of Naylor et al [3] showing dental arrangements of lower ( e ) and upper ( f ) teeth of adults and lower ( g ) and upper ( h ) teeth of embryos.…”
Section: Discussionmentioning
confidence: 99%
“…The major radiation within the clade was probably only possible after the appearance of the strongly heterodont forms (e.g. [5,6]) in the Late Cretaceous. It is possible that the change in tooth development from that of Squalus to that of forms exemplified by Deania , Centroscymnus and Etmopterus contributed to this radiation.…”
Section: Discussionmentioning
confidence: 99%
“…Embryos of the pristiophorids Pliotrema and Pristiophorus were examined (see [13] for details of the specimens), representing a sister group to the Squaliformes within the Squalea (figure 1 b ) [3,5], with dentitions clearly showing an alternate pattern in the adult. The Pristiophoridae therefore possess the same dental patterning present in all galean sharks and batoids yet studied, and therefore preserve the probable plesiomorphic dental condition to which the Squaliformes can be compared.…”
Section: Methodsmentioning
confidence: 99%
“…All taxa mentioned in the text are included to give an overview of the position of both squaliforms and non-squaliform squalean sharks. Note the discrepancy in the relative positions of Squalus in Straube et al [5] and Naylor et al [3]. Images not to scale; slightly modified from FAO publications under a Creative Commons Attribution-Noncommercial 3.0 license; Trigonognathus redrawn from various sources.Figure 2.Dentitions of squaliform sharks.…”
The squaliform sharks represent one of the most speciose shark clades. Many adult squaliforms have blade-like teeth, either on both jaws or restricted to the lower jaw, forming a continuous, serrated blade along the jaw margin. These teeth are replaced as a single unit and successor teeth lack the alternate arrangement present in other elasmobranchs. Micro-CT scans of embryos of squaliforms and a related outgroup (Pristiophoridae) revealed that the squaliform dentition pattern represents a highly modified version of tooth replacement seen in other clades. Teeth of Squalus embryos are arranged in an alternate pattern, with successive tooth rows containing additional teeth added proximally. Asynchronous timing of tooth production along the jaw and tooth loss prior to birth cause teeth to align in oblique sets containing teeth from subsequent rows; these become parallel to the jaw margin during ontogeny, so that adult Squalus has functional tooth rows comprising obliquely stacked teeth of consecutive developmental rows. In more strongly heterodont squaliforms, initial embryonic lower teeth develop into the oblique functional sets seen in adult Squalus, with no requirement to form, and subsequently lose, teeth arranged in an initial alternate pattern.
The morphology of the articular region of the pectoral girdle and associated basals in Etmopteridae is revised in light of new evidence provided by taxa unavailable for previous studies. Such studies considered that etmopterids plesiomorphically had a single pectoral articular condyle, and only Etmopterus had two separate ones. Our reanalysis indicates that the possession of two separate condyles, one for the articulation of the propterygium and the second for the meso‐ and metapterygium, is the most widespread condition in this group. However, the presence of two separate articular condyles is not recovered as a synapomorphy for Etmopteridae. Previous studies also proposed that etmopterids lack a hook‐like process on the anteroproximal margin of the anteriormost pectoral basal. We document that the hook‐like process is plesiomorphically present in Etmopteridae, thus corroborating the hypothesis of closer relationships between this family and the other squaliforms that also share this process, namely Centrophoridae, Dalatiidae, Oxynotidae, and Somniosidae.
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