Drepanoistodus Lindström, 1971

Rasmussen, Jan Audun, Eriksson, Mats E. & Lindskog, Anders, 2021, Middle Ordovician Drepanoistodus (Vertebrata, Conodonta) from Baltica, with description of three new species, European Journal of Taxonomy 774, pp. 106-134 : 117-118

Genus Drepanoistodus Lindström, 1971

Type species

Oistodus forceps Lindström, 1955 , subsequently designated by Lindström (1971).

Remarks

Drepanoistodus is here interpreted as quinquemembrate and comprises four nongeniculate coniform elements and one geniculate coniform element that collectively make a curvature-transition series from erect to recurved element types (e.g., Stouge & Bagnoli 1990; Rasmussen 1991). The nongeniculate elements comprise a suberectiform element associated with drepanodontiform type-l, type-2 and type-3 elements. In general, Middle Ordovician nongeniculate Drepanoistodus elements from Baltica can be described as follows: the suberectiform element is characterised by a straight, erect cusp. The drepanodontiform type-1 element has a strongly recurved cusp, which is keeled both anteriorly and posteriorly. The anterior keel is twisted strongly inwards. An extension, sometimes triangular in outline, may occur at the anterobasal corner. The drepanodontiform type-2 element has a recurved cusp which is keeled. It is separated from the drepanodontiform type-l element by the straight or only weakly twisted anterior margin, and by the consistent presence of an anterobasal flare, commonly with a triangular outline. The drepanodontiform type-3 element is typified by a slightly recurved cusp, which is anteriorly and posteriorly keeled and not twisted. As opposed to the drepanodontiform type-1 and type-2 elements, it lacks the anterior triangular flare. For a more comprehensive description, see Rasmussen (1991).

Many coniform conodont apparatuses are not easily placed in the locational PMS notation scheme favoured by Sweet (1981, 1988), or the more biologically correct terminology advocated by Purnell et al. (2000), because it is very difficult or even impossible to identify locational homologues with the ozarkodinid notation ( Smith et al. 2005). This is primarily a consequence of the lack of natural assemblages in many conodont genera and families, including Drepanoistodus .

In most cases, it is extremely difficult to distinguish between individual Middle Ordovician Drepanoistodus species based on the largely homeomorphic nongeniculate elements ( van Wamel 1974; Dzik 1983; Stouge 1984; Rasmussen 2001), and this is indeed also the case with respect to the species studied herein. Because our material includes nothing but isolated conodont elements (as opposed to articulated clusters or natural multi-element assemblages), the identification of Drepanoistodus at the species level is solely based on the geniculate element, which are described below. The stratigraphical distribution of the studied specimens of Drepanoistodus is shown in Table 3 .