Silurian ostracoderms from Washington Land (North Greenland), with comments on cyathaspid structure, systematics and phyletic position

Abstract

The few detached dermal elements recorded here document a new ostracoderm fauna comprising undeterminable species of one anaspid, two cyathaspids and a heterostracan which might have its closest relatives among the pteraspids. This fauna is derived from marine deposits of ultimate Wenlock or possibly Early Ludlow age at the top of the Lafayette Bugt Formation in its type section, in Washington Land, western North Greenland. It is probably equivalent to one of the undescribed faunas known from the Monograptus testis - M. nilssoni sequence of the Cape Phillips Formation in the Canadian Arctic Archipelago. Comparative material from Norway and Spitsbergen is considered in this study which prompted general comments on cyathaspid squamation, vestigial fin structure, cyathaspid systematics, their phyletic position relative to the pteraspids, system of stability control in swimming, their habitats and diets.

The three-dimensionally articulated oral apparatus of a Devonian heterostracan sheds light on feeding in Palaeozoic jawless fishes

“…12] reconstructed a generalized pteraspid opening the oral plates, with the postoral plate rotating 40° aborally and the oral plates an additional 50°. Bendix-Almgreen reconstructed two cyathaspids with pteraspid-like oral plates in which only the oral plates themselves moved: Allocryptaspis [ 30 , fig. 4A], where the oral plates rotate adorally by 55° and Anglaspis [ 30 , fig.…”

“…Bendix-Almgreen reconstructed two cyathaspids with pteraspid-like oral plates in which only the oral plates themselves moved: Allocryptaspis [ 30 , fig. 4A], where the oral plates rotate adorally by 55° and Anglaspis [ 30 , fig. 4E] where the oral plates rotate adorally by 55°.…”

“…The heterostracan feeding apparatus is best known in pteraspids, where the oral region is characterized by distinctive macromeric dermal plates [25][26][27][28][29]. The function of these plates has been much debated, variously interpreted as biting [26] or slicing [30] 'jaws', a cyclostome-like feeding apparatus [31][32][33][34], a sediment scoop [29] or a filtering structure [35][36][37][38][39]. Equally varied are the inferred ecologies, with heterostracans interpreted as active predators [26], macrophagous selective predators [14,40], hagfish-like scavengers [31][32][33][34], herbivores [30], detritivores [29,41] or suspension feeders (including filter feeding) [12,27,[42][43][44][45].…”

“…The function of these plates has been much debated, variously interpreted as biting [26] or slicing [30] 'jaws', a cyclostome-like feeding apparatus [31][32][33][34], a sediment scoop [29] or a filtering structure [35][36][37][38][39]. Equally varied are the inferred ecologies, with heterostracans interpreted as active predators [26], macrophagous selective predators [14,40], hagfish-like scavengers [31][32][33][34], herbivores [30], detritivores [29,41] or suspension feeders (including filter feeding) [12,27,[42][43][44][45]. The most recent investigation suggests that heterostracans were suspension feeders because the analysed oral plates exhibited no evidence of the wear anticipated of a 'tooth-like' function [27].…”

Testing hypotheses of pteraspid heterostracan feeding using computational fluid dynamics

“…Most of these feeding hypotheses are based on morphological comparisons with extant jawed vertebrates or with extant jawless fish, which have specialized feeding modes. They propose a mechanical function either related to active biting or crushing (Kiaer, 1928), scavenging (Janvier, 1974; Jarvik, 1980; Stensiö, 1932), herbivorous macrophagy (Bendix-Almgreen, 1986), or detritus feeding (White, 1935).…”