Asterolepis alticristata Downs & Daeschler & Lo & Carey & Shubin 2019, n. sp.

Asterolepis alticristata n. sp. Asterolepis sp. – Daeschler et al. 2006: 759. “Large asterolepidid” – Downs et al. 2011a: 994. “Asterolepid placoderm” – Downs et al. 2013: 48. “Asterolepid antiarch” – Downs et al. 2018: 1. HOLOTYPE. — NUFV 897, anterior median dorsal plate of thoracic skeleton (Fig. 2). ETYMOLOGY. — From the Latin ‘altus,’ high, and ‘crista,’ crest, in reference to the tall midline crest of the anterior median dorsal plate that helps to diagnose the species. TYPE LOCALITY AND HORIZON. — NV 2K17 locality (77˚09’59.1”N, 86˚16’9.42”W), Fram Formation near the eastern arm of Bird Fiord on southern Ellesmere Island, Nunavut Territory, Canada. Palynological data indicate an early Frasnian age (medius and maclarenii zones of Chi & Hills 1976). REFERRED MATERIAL. — NUFV 1265, nuchal plate; NUFV 812, 813, premedian plates; NUFV 822, partial anterior median dorsal plate; NUFV 814, 815, 823, 828, 834, 835, 842, 845, 849-851, 854, 856, 860-861, 864, 867, 870-873, 876-878, 884-886, 889, 890, 896, 1270-1277, crests of anterior median dorsal plates; NUFV 796, 806, 874, 879, 898, posterior median dorsal plates; NUFV 802.1, 802.2, 821, 848, posterior median dorsal fragments; NUFV 892, articulated partial thoracic and pectoral skeleton including median ventral, right and left anterior dorsolateral plates, anterior ventrolateral plates, dorsal central 1 and ventral central 1 plates; NUFV 895, right mixilateral plate. DIAGNOSIS. — Antiarch referred to Asterolepis by the presence of an anterior median dorsal plate that broadly overlaps the mixilateral plates (Moloshnikov 2008). Distinguished from previously described species by the following combination of anatomical features: 1) a tall, rounded midline crest that rises from a position near the tergal angle of the anterior median dorsal plate and loses most of its height before reaching the posterior median dorsal plate and (2) a midline crest of the posterior median dorsal plate that decreases in height cranial to caudal. REMARKS Because the new species is diagnosed by a crest of the anterior and posterior median dorsal plates, we use spatial association with diagnostic plates or plate fragments to refer additional material to the species (including isolated premedian, mixilateral and nuchal plates and an articulated partial skeleton [NUFV 892] that lacks the median dorsal plates). All of these referred materials also share a size category and dermal ornament with the Asterolepis alticristata n. sp. type specimen. DESCRIPTION OF CEPHALIC SKELETON Nuchal plate (Fig. 3 A-C) The nuchal plate (NUFV 1265: Fig. 3 A-C) is short and wide; the width/length index (maximum transverse width × 100 divided by maximum sagittal length) is 195. The maximum reported index for Asterolepis is 210 in Asterolepis scabra (Young 1987). Several smaller, unassignable nuchals from NV2K17 (NUFV 1247, 1248, 1252, 1254, 1263) have an index that is comparable with, though none higher than, this reported maximum (See Description of Other Asterolepidoid Specimens from NV2K17). As in Asterolepis ornata, the width across the lateral corners is slightly greater than the width across the caudolateral corners; this is despite the presence of pronounced caudolateral processes. As is true of all known species of Asterolepis, the notch for the postpineal plate is shallow, reaching no further caudal than the lateral corners of the nuchal plate. The central pit-line groove (Fig. 3 A’[cp1]) extends across the width of the nuchal; it exhibits a pronounced median dip and meets the lateral margins of the plate well caudal to the lateral corners. Tiny, parasagittal foramina just caudal to the pit-line are weakly visible among the tubercles of the ornament. Stensiö (1931) presumed these to be the external openings of the endolympatic ducts (Fig. 3 A’[d.end]). The nuchal is a thick plate with broad and deep zones of overlap onto lateral (Fig. 3 A’[cv.L]) and paranuchal (Fig.3 A’[cv. Pn]) plates. There is a very short obtected area (Fig. 3 A’[nm]) along the nuchal’s caudal margin that is unornamented though not depressed. The caudal margin of the plate is notched in parasagittal positions close to the plate’s midline. These notches border a short caudal midline process (Fig. 3 A’[ml.p]) of the plate similar to the one described in A. ornata (Upeniece 2011). The visceral surface of the nuchal exhibits a supraotic thickening (Fig. 3 B’[sot]) that surrounds the large supraoccipital pits (containing the internal openings of the endolymphatic ducts; Stensiö 1931; Fig. 3 B’[sop]). The caudal surface of the nuchal plate exhibits a median occipital crista (Fig. 3C [mo.c]) and, on either side of it, parasagittal fossae that were interpreted by Stensiö (1931) to be attachment areas for cranial levator muscles. In caudal view, the visceral surface of the plate has a tight, nearly semicircular arch to it. The areas lateral to the visceral arch are depressed relative to the area external to it such that a pronounced horizontal ledge runs across the plate at the most external reach of the visceral arch. The caudal surface of the nuchal is the thickest part of the plate and there is a dramatic decrease in nuchal thickness rostrally from the depressed visceral extensions of the plate’s caudal surface. The ornament of the nuchal plate is coarsely tuberculate. Tubercles are fused into ridges close to the rostral and lateral margins of the plate; these ridges radiate from the center. Premedian plate (Fig. 3 D-I) The two premedian plate specimens referred to Asterolepis alticristata n. sp. (NUFV 812 [maximum sagittal length of 29.85 mm] and 813 [same length = 22.02 mm]: Fig. 3 D-I) share a size category and an ornament type but differ from one another in some specific details. The premedian plate has been recognized as a source of intraspecific variation throughout Antiarchi (Young 1984; Downs et al. 2011b). Both specimens exhibit the generalized premedian plate shape that has been well described in Asterolepis ornata (Lukševičs 2001). Both are thick plates, especially at the caudal end, and are arched across the midline. Both have unornamented zones in lateral positions along the depressed rostral margin of the plate; the rostral margin exhibits a midline notch. In both, the tall caudal surface is depressed rostrally (analogue of the bothriolepidid preorbital recess; Lukševičs 2001) and this surface has a prerostral process (Fig. 3F [pr.p]) across the midline with a small depression on the midline (the nasal notch of Lukševičs 2001). The prerostral process separates the orbital cavities. Both premedian specimens are widest at the lateral corners and these corners are closer to the rostral margin than they are to the caudal one. The two premedian specimens referred to A. alticristata n. sp. have a width/length index (as calculated for the nuchal above) of 134 (NUFV 812) and 157 (NUFV 813). These values are within the range of variation that Upeniece (2011) reported for adult Asterolepis ornata (120-233). This broad range of values for adult A. ornata again highlights the variability of this plate within a single antiarch species. The principal section of the infraorbital pit-line groove (Fig. 3D [ifc1], G[ifc1]) extends across the width of the premedian but follows a different path on each of the two specimens. In NUFV 812 (Fig. 3G), the groove forms a caudally directed point at the midline and strongly arches rostrally on either side of the midline. It is positioned at about mid-length along the plate, such that the midline dip of the groove reaches into the caudal half of the plate. In contrast, the groove on NUFV 813 (Fig. 3D) has a shallow, rounded dip at the midline and is much straighter across its entire width. The groove is situated close to the rostral margin of the premedian such that no part of it reaches into the plate’s caudal half. Tiny foramina (Fig. 3E [ps.f], H[ps.f]) penetrate the visceral surface of the premedian; the two most prominent are parasagittally located in the rostral half of the plate. The external ornament of the premedian is similar to that of adult Asterolepis ornata in that rows of tubercles radiate out from the center of ossification. Close to the lateral margins of the plate, the tubercles within a row fuse together to form ridges. DESCRIPTION OF THORACIC AND PECTORAL SKELETONS Anterior median dorsal plate (Fig. 2) The anterior median dorsal plate of Asterolepis alticristata n. sp. features a tall rounded crest on the midline. The remarkable height (c. 60 mm in the largest specimen, NUFV 897, maximum anterior median dorsal plate width = 112.55 mm) and thickness (c. 18 mm in NUFV 897) of the crest is unlike any previously described for Asterolepis (Fig. 2). The crest rises sharply from a position just cranial to the tergal angle and reaches its maximum height just caudal to the center of the anterior median dorsal plate, nearly in line with the plate’s greatest width. The long dorsal margin of the crest caudal to the maximum height is slightly convex to deeply concave. The closest point of comparison among antiarchs, for a crest in this position and of this height, is in Bothriolepis cristata Traquair, 1895 (Scotland). A tall dorsal crest also appears in Bothriolepis gippslandiensis Hills, 1929 (Victoria, Australia); Bothriolepis cullodenensis Long, 1983 (Victoria Australia); and Bothriolepis zadonica Moloshnikov, 2004 (Central European Russia). In each of these three examples, however, the crest is formed nearly equally by anterior median dorsal and posterior median dorsal plates; the crest rises from near the anterior median dorsal plate’s tergal angle and reaches its maximum height only at the contact between anterior median dorsal and posterior median dorsal plates (Moloshnikov 2004). In addition to the three partial anterior median dorsal plates with crests (NUFV 822, 896, 897 [holotype]), the A. alticristata n. sp. referred material includes thirty-seven isolated crests from anterior median

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