Some developmental aspects of the head skeleton of the 35–37 mm Squalus acanthias foetus

Abstract

Two specimens of a group of five foetuses, 35-37 mm in total length, of Squalus acanthias were serially sectioned and stained. These specimens were studied for the purpose of testing statements made in the literature, especially by Holmgren ('40), regarding cell sources and skeletal development. Many of Holmgren's results were confirmed with some important differences in detail. Limited blastemas of dermal bones were not evident although a diffuse subepidermal blastema was present in certain areas. There is evidence of delamination addition to the jaws and many parts of the endocranium. Although the trabecula is in part of visceral origin, the visceral material does not represent an infrapharyngohyal since it always has been an integral part of the endocranium, never a formed element of the arch. The same applies to the pharyngo region of the hyoid arch. The pharyngohyal tissue forms the area of articulation of the hyomandibula and the lamina hypotica. Thus a typical lateral commissure is not formed and the resulting otic capsule-basicranial association is unique to the elasmobranchs. The hyomandibula is the epihyal and there is no evidence of a symplectic in this group. The jaws appear to incorporate delamination tissue, associated in teleostomes with dermal bone, in addition to the neural crest proper.The development of the internal chondral head skeleton of elasmobranch fishes has been described by many individuals including Sewertzoff (1899), van Wijhe ('22), deBeer ('37), Holmgren ('40), and ElToubi ('47, '49, '52). From these studies one gets the general impression of a rather orderly developmental pattern of parts such as are observable in other vertebrates (summarized by deBeer, '37).Unlike the other studies, Holmgren (and Klaatsch, 1894) suggested that one should be concerned with the appearance and source of the earliest skeletal blastemas as well as end structures. He proposed a more extensive ectomesodermal contribution to the endoskeleton than previously allowed, and an ectomesodermal, delamination origin for the dermal system of bones, blastemic vestiges of which occur in sharks.The purpose of this investigation is not to reexamine in detail the development of the head skeleton in this group, but to determine the usefulness of the existing re-J. MORPH., 133: 17-40. ports in understanding ontogenetic events which might have phylogenetic significance. Such an evaluation is particularly necessary since concern exists as to the reality of some of Holmgren's observations, particularly the occurrence of delaminated dermal bone blastemas and the contribution of mesectodermal cells derived from the subepidermal layer to the endocranium. This study is based on a group of five foetal individuals of Squalus acanthias, 35-37 mm in total length (snout to tip of tail). The specimens were obtained from a commercial preparation used in the Comparative Anatomy Laboratory. The foetuses were in good condition, but shrunken (figs. 1 , 2 ) . Two were serially sectioned at 10 p and stained with Mallory's Triple. ...

Braincase of the Upper Devonian Shark Cladodoides Wildungensis (Chondrichthyes, Elasmobranchii), With Observations on the Braincase in Early Chondrichthyans

“…Several investigators considered that the lateral commissure is of neurocranial origin (Swinnerton, 1902;De Beer, 1937;Hammar-berg, 1937;Hubendick, 1943;Daget and d'Aubenton, 1957;Bertmar, 1959, Schaeffer, 1981, but others have suggested a visceral arch origin (e.g., in actinopterygians; Holmgren, 1943, and in sharks;Jollie, 1971; see discussion in Gardiner, 1984a). There has been little modern developmental work on this part of the braincase, but according to Bertmar (1959) the lateral commissure in Neoceratodus is derived from the hyoid arch (which suggests it may have formed in the hyoid/preotic stream of neural crest tissue).…”

“…However, its development is completed fairly late in ontogeny (De Beer, 1931;Holmgren, 1940;Jollie, 1971). The medial capsular wall is also chondrified in Mesozoic hybodonts (e.g., Hybodus, Tribodus) although its extent in earlier hybodonts such as Hamiltonichthys is unknown.…”

“…The hypotic lamina (ϭ ''lateral pharyngohyal plate'' of Jollie, 1971) in neoselachians is a lateral extension of the basal (parachordal) plate that secondarily fuses with the otic capsule at the posterior basicapsular commissure. In osteichthyans and chimaeroids, a hypotic lamina is absent and the glossopharyngeal nerve typically leaves the braincase ventrally rather than ventrolaterally.…”

Remarks on the inner ear of elasmobranchs and its interpretation from skeletal labyrinth morphology

“…This fact, coupled with the view that elasmobranchs represent a primitive level of vertebrate evolution, has led many investigators to regard the elasmobranch inner ear (either implicitly or explicitly) as a model for gnathostomes in general (e.g., Grassé, 1957;Torrey, 1962;Hildebrand, 1974). The ontogeny of the elasmobranch inner ear (along with the rest of the braincase) has been investigated by several workers, especially de Beer (1931), Holmgren (1940), and Jollie (1971), and has also been regarded as generally representative of gnathostomes. Despite such assertions, however, there is an overwhelming body of functional, ecological, and even morphological data suggesting that the inner ear of elasmobranchs is highly specialized (see references in Corwin, 1981;Bleckmann and Hofmann, 1999).…”

Holocephalan Embryo Provides New Information on the Evolution of the Glossopharyngeal Nerve, Metotic Fissure and Parachordal Plate in Gnathostomes

“…By contrast, in modern elasmobranchs, the medial capsular wall is chondrified, the glossopharyngeal nerve leaves the braincase via the remnant of the metotic fissure, and the nerve is floored by the hypotic lamina, so that it has a more horizontal course than in osteichthyans after leaving the endocranial cavity. The course of the glossopharyngeal nerve relative to the otic capsule in elasmobranchs has been extensively discussed: De Beer [4] and Jollie [6] described the glossopharyngeal nerve as passing external to the otic capsule, whereas Holmgren [7], [8] argued that the nerve actually entered the auditory capsule. Unfortunately, no subsequent detailed studies have been presented, although the interpretation of De Beer [4] and Jollie [6] is commonly accepted.…”