Asterolepis cf. radiata Eichwald, 1840 sensu Agassiz 1844

Asterolepis cf. radiata Rohon, 1899 Asterolepis cf. radiata Rohon, 1899: 8. Asterolepis sp. – Daeschler et al. 2006: 759. — Downs et al. 2011a: 994. “Asterolepid placoderm” – Downs et al. 2013: 48. “Asterolepid antiarch” – Downs et al. 2018: 1. LOCALITY AND HORIZON. — NV2K17 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). DIAGNOSIS. — Antiarch referred to Asterolepis by the presence of an anterior median dorsal plate that broadly overlaps the mixilateral plates (Moloshnikov 2008). Referred to Asterolepis cf. radiata by the presence of an external skeletal ornament that is composed entirely of rows of tubercles and/or ridges that radiate from the center of ossification to the margins of the plate (Rohon 1899). REFERRED MATERIAL. — NUFV 795, 797-800, 804, anterior median dorsal plate; NUFV 809, 825, posterior median dorsal plate; NUFV 846, 863, mixilateral plate; NUFV 883, partial anterior dorsolateral and mixilateral plates. DESCRIPTION OF THORACIC AND PECTORAL SKELETONS Anterior median dorsal plate (Fig. 6A, B) The five complete anterior median dorsal plates of Asterolepis cf. radiata (NUFV 795, 797-800) exhibit a size and shape range consistent with known material of A. radiata. Among these five specimens, the midline length of the plate is between 79.75 mm (NUFV 798) and 94.94 mm (NUFV 800) and width across the lateral corners (Fig. 6A [lc]) is between 54.41 mm (NUFV 798) and 68.09 mm (NUFV 800). The width/length index of the plate (width across the lateral corners × 100 / midline length), then, is between 65 and 73 for the five specimens. The lateral corners of these plates are positioned close to the midpoint of the plate’s length. This presents a differently shaped anterior median dorsal plate than that of those Asterolepis species with anterior median dorsal plate lateral corners near the plate’s caudal end (Asterolepis essica Lyarskaja, 1981, Asterolepis estonica Gross, 1940, Asterolepis säve-söderberghi Stensïo & Säve-Söderbergh, 1938, and Asterolepis syasiensis Lyarskaja, 1981). The tergal angle (Fig. 6A [ta]) is cranial to the center of the anterior median dorsal plate in this new sample of Asterolepis cf. radiata, and as previously reported for the species, a low dorsal midline crest extends from the tergal angle to the caudal end of the plate. The cranial end of the plate is blunt (width of cranial margin × 100 / midline length = 13.58 in largest specimen, NUFV 800) and therefore unlike the narrow pointed cranial tip of the anterior median dorsal plate in Asterolepis chadwicki (same index reported as 6.00 in C.U. No. 42344, Wells 1964). The anterior median dorsal plate of Asterolepis cf. radiata is vaulted in the transverse plane and vaulted but less so in the sagittal plane. A steeply depressed zone at the caudal end of the anterior median dorsal plate’s external surface accommodates the overlap of the posterior median dorsal plate (Fig. 6A [cd. PMD]). This overlap zone is crested like the ornamented surface but the crest here is accordingly depressed below the ornamented one. The visceral surface of the anterior median dorsal plate shows the steeply depressed areas of anterior dorsolateral plate (Fig. 6 B’[cv.ADL]) and mixilateral plate (Fig. 6 B’[cv.Mxl]) underlap. These are connected to one another across the lateral corners. Low, wide, oblique ridges (Fig. 6 B’[op.r]) of the visceral surface extend from the area opposite to the tergal angle to the caudolateral corners of the plate. The cranial end of the plate’s visceral surface exhibits a rough area (Fig. 6 B’[r.a]) on the midline that has been interpreted as an attachment site for cranial levator muscles in Asterolepis (Moloshnikov 2008). The external ornament of all five anterior median dorsal plates attributed here to Asterolepis cf. radiata consists of rows of isolated or conjoined tubercles that radiate from the tergal angle out to the margins of the ornamented surface. This ornament is consistent with Rohon’s (translated from 1899: 23) original description of A. radiata (“tubercles arranged in radial lines or of radiating or parallel bars”), but is unlike the dramatically ridged/grooved ornamented surface of PIN No. 3725/1191, an anterior median dorsal plate attributed to A. radiata by Moloshnikov (2008). Posterior median dorsal plate (Fig. 6C, D) The posterior median dorsal plate of Asterolepis cf. radiata (NUFV 809, 811, 825; Fig. 6C, D) is known from two incomplete specimens that permit a limited description. The plate is strongly vaulted in the transverse plane with a sharp midline angle that extends along the entire preserved length of the external surface (Fig. 6C); this angle does not form a raised crest like the caudal end of the anterior median dorsal plate. As is expected for Asterolepis, the posterior median dorsal plate overlaps onto both anterior median dorsal (Fig. 6A [cd.PMD]) and mixilateral (Fig. 6 D’[cv.Mxl]) plates. The area overlapping the mixilateral plate is widest at a position in the plate’s caudal half and tapers both cranially and caudally along the posterior median dorsal plate’s lateral margin. Oblique caudolateral margins form a pointed caudal terminus to the plate but the posterior median dorsal plate does not exhibit either a caudal median process as in Asterolepis maxima (Traquair 1914) and juvenile Asterolepis ornata (Upeniece & Upenieks 1992; Upeniece 2011) nor a caudal median notch as in Asterolepis essica (Lyarskaja 1981) and large adult individuals of A. ornata (Upeniece 2011). The visceral surface exhibits two parasagittal depressions (Fig. 6 D’[ps.d]) that align nearly with the widest portion of the area overlapping the mixilateral plate. Just caudal to these, on the midline, is a large circular pit (Fig. 6 D’[v.p]). The ornament is formed of low, rounded tubercles that are organized into linear rows; these radiate from a midline position near the caudal margin of the plate. Mixilateral plate (Fig. 6E) The mixilateral plate in this sample of Asterolepis cf. radiata is represented by two nearly complete specimens (NUFV 846, 863; Fig. 6E). Wide overlap zones on the external surface of these mixilaterals accommodate the anterior dorsolateral, anterior median dorsal, and posterior median dorsal plates (Fig. 6E [cd.ADL, cd.AMD, cd.PMD]). The external ornament of the dorsal lamina comprises rows of conjoined tubercles radiating from a caudolateral position. The lateral lamina is very short (maximum height is 0.25 times maximum width of the dorsal lamina) and carries a unique slotted, interlocking contact with the posterior ventrolateral plate not seen along the other margins of thoracic plates. The ornament of the lateral lamina is conjoined tubercles that appear to radiate from the same point as those of the dorsal lamina. The dorsolateral ridge of the mixilateral plate forms a low, sharp crest. DESCRIPTION OF OTHER ASTEROLEPIDOID SPECIMENS FROM NV2K17 With one noted exception (NUFV 831, articulated cephalic skeleton; Fig. 7A, B), the specimens described here were collected as isolated plates at the NV2K17 field locality (Fram Formation, early Frasnian) on Ellesmere Island. We assign NUFV 852 (mixilateral plate; Fig. 7N) to Asterolepis sp. indet., NUFV 808 (lateral plate; Fig. 7M) to cf. Asterolepidoidei, and the other described specimens to Asterolepidoidei sp. indet. for the reasons explained below. NV2K17 is a locality with at least two species of Asterolepis so much or all of the material described in this section is likely to belong to one or the other of the species presented above. In addition to the described material, there are antiarch plates from NV2K17, including anterior dorsolateral, anterior ventrolateral, posterior ventrolateral, median ventral, and pectoral appendage plates, that do not present the anatomical details necessary to support a meaningful description or a more specific taxonomic assignment than Antiarchi indet. The discovery of a Bothriolepis sp. indet. anterior median dorsal plate (NUFV 847) at NV2K17, and bothriolepidid material from stratigraphically analogous localities, urges taxonomic caution when considering indeterminate antiarch material from the site. Although Asterolepis is, overwhelmingly, the best represented antiarch group at NV2K17, the presence of Bothriolepis at the locality convinces us to not simply attribute all of this locality’s antiarch material to Asterolepis or even Asterolepidoidei. In the Discussion section of this paper, we report on antiarch material from throughout the Fram Formation, including NUFV 847 and asterolepidoid material from other localities. NUFV 831: Partial cephalic skeleton (Fig. 7A, B) NUFV 831 (Fig. 7A, B) includes nuchal, postpineal, partial left paranuchal, partial left lateral, right paranuchal, and partial right lateral plates. The specimen is attributed to Asterolepidoidei sp. indet. according to the presence of a wide postpineal that sutures with the lateral plates and separates the nuchal plate from the orbital opening (Moloshnikov 2008). This partial cephalic skeleton is strongly vaulted in the transverse (visceral surface of nuchal appears as a tight semi-circular arch in caudal view) and sagittal planes. The otic-occipital depression is deeply developed on the visceral surface of nuchal, paranuchal, lateral, and postpineal plates. The narrow unornamented dorsal-facing obstantic margin of the nuchal wraps around and deepens to form the caudalfacing margin of the paranuchals. The central pit-line groove (Fig. 7 A’[cp1]) is separated into left and right parts; each part weakly arches rostrally and crosses the other at the midline. The grooves meet the nuchal’s margin in a position well caudal to the lateral corners. The ornament is almost entirely anastomosing or linear ridges that radiate from a nearly central position of ea

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