New Data on the Internal Anatomy of the Heterostraci (Agnatha), with General Remarks on the Phytogeny of the Craniota

Abstract

Some new impressions have been observed on the visceral surface of a dorsal shield of Ctenaspis sp. from the Lower Devonian of Spitsbergen. They indicate the probable position of the ophthalmicus profundus and o. lateralis nerves, and of two branchial nerves. The impressions left by the branchial arches are also visible on the same specimen. Furthermore, a median dorsal series of circular impressions previously observed in Seretaspis zychi StensiÖ is compared to similar impressions observed in two osteostracans, and it is suggested that vertebral elements were present in the Heterostraci. Finally, the affinities of the Heterostraci are discussed and it is suggested that they may represent the sister group of the Myopterygii (Cephalaspidomorphi + Gnathostomata).

Agnathans recent and fossil, and the origin of jawed vertebrates

“…6) differs from all others previously suggested ( (1899); (B) after Stensi6 (1927); (C) after Halstead (1982); (D) after Janvier and Blieck (1979); (E) after Janvier (1981b).…”

“…A cartilaginous endoskeleton is primitive for craniates and assumed to have been present in all 'ostracoderms'. Impressions of a backbone can be seen to mark the inside of the head shield of some heterostracans (Janvier and Blieck, 1979) and osteostracans (Belles-Isles, 1989). And in at least one thelodont -Lanarkia horrida -traces of an endoskeleton around the notochord and the braincase have recently been described (Turner and Brugghen, 1993).…”

Origin and early evolution of vertebrate skeletonization

“…Thus, the fossil record provides little data on the subsequent evolution of the endoskeleton prior to the origin of jawed vertebrates. Nevertheless, we can infer from the absence of a mineralized splanchnocranium in any stem-gnathostome, bar conodonts, and thelodonts, that the condition expressed by lampreys is representative of all prejawed vertebrates; this is supported by indirect evidence of an unmineralized branchial skeleton in heterostracans (Janvier and Blieck, 1979), and the preservation of carbonized outlines of putatively cartilaginous branchial skeletons in a number of anaspid-like forms (Arsenault and Janvier, 1991;Woodward, 1900;Stensiö, 1939;Ritchie, 1960Ritchie, , 1968 that compare favorably to the branchial skeleton of lampreys. It is likely also that this skeleton was composed of the same or similar noncollagen-based cartilage.…”

“…Indeed, the only evidence of an axial endoskeleton is in heterostracans and osteostracans that exhibit impressions in the base of the dorsal dermal skeleton that indicate the presence of unmineralized neural arcual elements (Janvier and Blieck, 1979). It should be noted that there is no evidence for the absence of hemal arcual elements.…”

The characters of Palaeozoic jawed vertebrates

“…The branchial chamber lies beneath the braincase in osteostracans (Janvier, 1985a, b), galeaspids (Halstead, 1979;Gai et al, 2011), pituriaspids (Young, 1991), heterostracans (Janvier & Blieck, 1979), and at least partially in osteichthyans (Jarvik, 1980;Gardiner, 1984b). We see no alternative but to interpret this character as a primitive gnathostome trait (see also Zangerl, 1981).…”