Acanthodian fauna from the Early Devonian (Emsian) of Death Valley, California Author Burrow, Carole Jan Author Elliott, David Kenneth text PaleoBios 2023 2023-04-11 40 2 1 7 http://dx.doi.org/10.5070/p940253335 journal article 298073 10.5070/P940253335 c3872b03-1bb2-4abd-989f-d4aeee1cb8e8 0031-0298 10913403 BRYANTONCHUS PERACUTUS ( BRYANT, 1934 ) ( Fig. 3 A-C) Referred material —Complete fin spine FMNH-PF 14564, spine fragments FMNH-PF 14565–14572. Description —The one complete spine ( FMNH-PF 14564; Fig. 3A ) is 20 mm long with a straight trailing edge and slightly curved leading edge; maximum depth is 1.5 mm near the proximal end of the exserted part. The insertion is less than 2 mm long. The exserted part of the spine is smooth, without any ridges or other ornamentation, and the short insertion shows fine closeset longitudinal ridges ( Fig. 3B ). The insertion is also exposed on specimen FMNH-PF 1458 ( Fig. 3C ), with a medial groove along the trailing edge also visible. The other fragments also exhibit the smooth surface on the exserted part, characteristic of the taxon. The spines appear to have the circular cross-section that typifies B. peracutus , except FMNH-PF 14567 (not figured) that is somewhat laterally flattened. Comparison —The Death Valley specimens fit within the range shown by Bryantonchus peracutus fin spines from the type stratum of the Beartooth Butte Formation at Beartooth Butte, Wyoming, and also in the Sevy Dolomite of the northern Egan Range, Nevada, Holland Quarry Shale, northwestern Ohio ( Denison 1960 ), and lower Grassy Flat Member, Water Canyon Formation of Utah ( Burrow 2007 ). The taxon is not known from any other region. Figure 3. Acanthodian fin spines and scapulocoracoid. A-C, Bryantonchus peracutus : A, B complete spine FMNH-PF14564; C, proximal end of spine FMNH-PF14568. D, E, Machaeracanthus sp. : D, incomplete spine FMNH-PF14573, lateral view; E, abraded spine FMNH-PF14574, dorsoventrally compressed. F-H, acanthodian indet. scapulocoracoid: F, counterpart FMNH-PF14575; G, part FMNH-PF14576; H, outline sketch. Abbreviations: IEB, insertion-exsertion boundary; k/w, worn keel or wing; plac, placoderm plate; le, leading edge; te, trailing edge; teg, trailing edge groove. Scale bars=0.5 mm in A, C–E, 0.1 mm in B, F–H. Order indet. FAMILY MACHAERACANTHIDAE BURROW & YOUNG, 2005 MACHAERACANTHUS NEWBERRY, 1857 MACHAERACANTHUS SP. ( Fig. 3D, E ) Referred material —Fin spines FMNH-PF 14573, possibly FMNH-PF 14574. Description —Specimen FMNH-PF 14573 is 17 mm long, with a marked longitudinal curvature and lacks the proximal and distal ends ( Fig. 3D ). A lateral surface is exposed, showing an abraded keel or wing and one or two longitudinal grooves on the convexly curved upper surface. FMNH-PF 14574 is the distal part of a spine, 16 mm long with maximum width c. 2.5 mm ( Fig. 3E ). Most of the spine has eroded away, but the lateral wing and keel and the central core are partially preserved in the more eroded distal end. Comparison —Despite their poor preservation, these small spines are identifiable as Machaeracanthus based on their distinctive morphology, with lateral wing and keel extending out from a central axial body. The weak grooves on the upper surface of FMNH-PF 14573 indicate it could be a small specimen of M. sulcatus , which has been described from the Coils Creek Member (?Emsian), McColley Canyon Formation, Nevada (Burrow et al. 2010, fig. 4C–J). However, another species M. kayseri Kegel, 1913 also has longitudinal grooves and ridges on the same surface (Burrow et al. 2010, fig. 1G), so we can only assign the spine to M . sp. Bryant (1934) erected a new species Machaeracanthus minor for a spine from the Beartooth Butte Formation, Wyoming , but Denison (1979) considered it to be a junior synonym of O. penetrans . Burrow (2007) suggested that the holotype of the latter species could rather be an eroded specimen of B. peracutus , but given some uncertainty due to the preservation, the taxon Onchus penetrans is retained ( Burrow 2021 ). In the western United States , Machaeracanthus has only otherwise been recorded from the Emsian McColley Canyon Formation in the Early Devonian and the Red Hill Beds, Simpson Park Range, Nevada in the uppermost Middle Devonian ( Reed 1986 ).