ISCHNACANTHIFORMES BERG, 1940

Burrow, Carole Jan & Elliott, David Kenneth, 2023, Acanthodian fauna from the Early Devonian (Emsian) of Death Valley, California, PaleoBios 40 (2), pp. 1-7 : 3

ORDER ISCHNACANTHIFORMES BERG, 1940 FAMILY, GENUS, SPECIES INDET.

( Fig. 2 A-D)

Referred material —Fin spines FMNH-PF 14560, FMNH-PF 14561, dentigerous jaw bone fragments FMNH-PF 14562, FMNH-PF 14563.

Description —The fin spines are laterally flattened, and ornamented with closely spaced smooth longitudinal ridges that parallel the leading edge. The more complete spine ( FMNH-PF 14560; Fig. 2A ) is c. 27 mm long, maximum depth 2 mm, possibly lacking the proximal end. The leading edge ridge is slightly wider than the five lateral ridges on the exposed side of the spine. The other specimen ( FMNH-PF 14561; Fig.2B ) is a 9 mm long fragment of the proximal half of a spine, maximum depth 1 mm, also with five ridges on the exposed side. On both specimens the ornament ridges extend the whole length.

The more complete dentigerous jaw bone specimen ( FMNH-PF 14562; Fig. 2C ) is 30 mm long and is the posterior part of the bone, lacking the posterior end, with the smooth labial surface exposed. Eight teeth are preserved on the occlusal surface. The teeth increase in height anteriorly, curve slightly lingually, have a smooth labial surface, and are slightly recurved antero-posteriorly. They appear to be monocuspid ( Fig. 2D ), lacking denticles or ancilliary cusps. The other specimen ( FMNH-PF 14563; not figured) is a small jaw fragment with one smooth cusp intact.

Comparison —Laterally flattened spines with equal width longitudinal ridges paralleling the leading edge, and a lack of a differentiated insertion, characterise the Ischnacanthiformes ( Burrow 2021). The spines of Poracanthodidae and Acritolepidae ( Valiukevičius 1992, 2003, Burrow 2011) typically have more lateral ridges than the Death Valley specimens, and the Ischnacanthidae , where known,have fewer ridges ( Burrow 2007). The spines most likely belong to an ischnacanthid. However, most ischnacanthiform taxa are based on isolated dentigerous jaw bones and scales rather than articulated fish, and their fin spine morphology is thus unknown, so we assign these spines to an undetermined ischnacanthiform.

Dermal dentigerous jaw bones are only found in ischnacanthiform acanthodians ( Burrow 2004a, 2011). The morphological features of the lingual side of the bones and teeth are important characters for identifying genera (e.g., Long 1986, Burrow 2004a, 2004b). The size and exposed morphology of the Death Valley specimens conforms with that of Cacheacanthus utahensis Burrow, 2007 from the Emsian of Utah, but they cannot be definitively assigned to Cacheacanthus as some characters diagnostic for Cacheacanthus are based on the parabasal shape of the teeth and presence or absence of a lingual tooth row, and these features are not visible on the described specimens.

Figures

Figure 2. Ischnacanthiform acanthodian dermal elements. A, fin spine FMNH-PF14560; B, fin spine fragment FMNH-PF14561. C, D, dentigerous jaw bone FMNH-PF14562, labial view; boxed area in C is enlarged in D. Scale bars=0.5 mm in A–C, 0.1 mm in D. Arrow indicates rostral direction.