Asterolepis cf. radiata Eichwald, 1840 sensu Agassiz 1844

Autor: Downs, Jason P., Daeschler, Edward B., Lo, Nathanael, Carey, Emily N., Shubin, Neil H.
Rok vydání: 2019
Předmět:
ISSN: 0272-4634
DOI: 10.5281/zenodo.3703484
Popis: 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 s��ve-s��derberghi Stens��o & S��ve-S��derbergh, 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 each plate present. The only isolated tubercles present are caudal to the central pit-line groove on nuchal and paranuchal plates and immediately lateral to the orbital opening on the lateral plate. Premedian plates (Fig. 7 C-F) Five small premedian plates from NV2K17 (NUFV 817, 833, 857: Fig. 7C, D; NUFV 1266, 1267: Fig. 7E, F) are impossible to refer to either of the two species described above but all may be referred to Asterolepidoidei sp. indet. according to the presence of a medial notch in the rostral margin (Moloshnikov 2008). Among these five is morphological variation that does not scale against size (plate width). This may result from multiple species represented in the sample or from the high level of variability in the antiarch premedian (Young 1984; Downs et al. 2011b). Of the four more completely preserved specimens, the smaller two (NUFV 1266 and 1267) are relatively short and wide (width/length index = 154 in NUFV 1267) relative to the larger two (NUFV 833 [same index = 111] and 857 [= 116]). Two types of external ornament are present in the sample, one predominantly ridged with ridges radiating from the center of the plate and tubercles, when present, clustered centrally along the rostral and caudal margins (NUFV 817, 857; Fig. 7C, D; 1266), and the other predominantly tuberculate with short radiating ridges along the lateral margins (NUFV 833, 1267; Fig. 7E, F). One of these ornament types is no more likely than the other at a given premedian size. In these unreferred specimens, the principal section of the infraorbital pit-line groove follows the two different paths described above for Asterolepis alticristata n. sp. One path is close to the rostral margin and follows the contour of that margin but for a median caudal dip. The other exhibits the median dip but is straighter across its length and is located in the middle of the premedian plate. As with the ornament types, a premedian of a given size is no more likely to show one path than the other. Nuchal plates (Fig. 7 G-L) We refer 24 nuchals from NV2K17 (NUFV 819, 829, 880 [Fig. 7 G-I]; 1120, 1121, 1247-1251, 1252 [Fig. 7 J-L]; 1253- 1264, 1269) to Asterolepidoidei sp. indet. according to the lack of an orbital margin. These nuchals (with widths between 24.32 mm in NUFV 1260 and 39.26 mm in NUFV 1258) belong to a different size category than that of Asterolepis alticristata n. sp. (nuchal width of 62.42 in the one referred specimen, NUFV 1265). There is no consistent relationship between width/length index of the nuchal (whether length is measured as midline length or maximum sagittal length) and nuchal size (width) thus suggesting against the entire sample representing a single growth series. However, the top eight values of the index using maximum sagittal length (from 176 in NUFV 819 to 202 in NUFV 1248) are among the top ten widest nuchals in the sample. In other words, the widest nuchals also tend to be relatively short. The variation in this index for these twenty-four nuchals includes the value calculated for A. alticristata n. sp. (195 in NUFV 1265). Vaulting of the nuchal in the transverse plane is differently developed across the sample; some are tightly arching, others are flatter. This too is inconsistent relative to size of the nuchal plate. In this sample of nuchals, the middle pit-line groove exhibits variation in its path. In all cases, it extends across the width of the nuchal and arches rostrally on left and right sides. In some specimens, the groove is separated into left and right components that cross one another at the midline (e.g., NUFV 880; Fig. 7 G-I). In others, there is one unbroken middle pit-line groove that dips caudally at the midline (e.g., NUFV 1252; Fig. 7 J-L). There is also variation in where the groove meets the lateral margin of the plate, from a rostral position close to the nuchal���s lateral corners (e.g., NUFV 880; Fig. 7 G-I) to a caudal position just rostral to the caudolateral corners (e.g., NUFV 1252; Fig. 7 J-L). These sources of variation do not trend with width of the nuchal plate. Ornament is variable in this sample of nuchals. Whenever ornament is observable, there are linear ridges close to the rostral and lateral margins of the plate; these ridges present a loosely radial pattern that originates from a caudocentral position. In larger nuchals, centrally, the ornament is tuberculate and these tubercles are either arranged into rows that extend from a caudocentral position to the aforementioned ridges (e.g., NUFV 1252; Fig. 7 J-L) or the tubercles do not exhibit any discernible organization (e.g., NUFV 1249). In smaller nuchals, more of the ornamented surface is ridged, often having the appearance of fused tubercles, and these ridges radiate from a caudocentral position to the rostral and lateral margins (e.g., NUFV 880; Fig. 7 G-I). When tubercles are present in the smaller nuchals, they are in line with these ridges. NUFV 808: Lateral plate (Fig. 7M) A single, incomplete lateral plate (NUFV 808; Fig. 7M) lacks the caudal end but preserves the thick orbital margin and contact surface for the premedian plate. The depth of the medial margin and the dermal ornament of the plate are consistent with the lateral plate of NUFV 831, the partial cephalic skeleton (described above) that is diagnosable as asterolepidoid. We refer NUFV 808 to cf. Asterolepidoidei sp. indet. due to a lack of diagnostic features on this incomplete, isolated plate. The preserved length of NUFV 808 is 46.86 mm; the plate is thickest (14.81 mm) at the rostral end of the orbital margin. The rostral margin of the plate is unornamented and depressed below the ornamented surface. The path of the infraorbital pitline (Fig. 7M [ifc1]) interrupts the ornament of the plate���s rostral end. The visceral surface of NUFV 808 is smooth and does not preserve the lateral���s contribution to the otic-occipital depression. The external ornament is different than that on the lateral plate included in NUFV 831. In NUFV 808, the ornament is entirely tuberculate. Fine tubercles are tightly clustered lateral to the orbital opening and larger tubercles, some conjoined, are organized into linear or curving rows between orbital margin and pit-line. Rostral to the pit-line, similarly large tubercles, some conjoined, do not exhibit any specific organization. NUFV 852: Mixilateral plate (Fig. 7N) NUFV 852 (Fig. 7N) is a mixilateral plate in a much smaller size category than that of Asterolepis alticristata n. sp. Its assignment to cf. Asterolepis sp. indet. is supported by a wide area of overlap by the anterior median dorsal plate (Moloshnikov 2008). The anterior median dorsal plate of Grossolepis also overlaps the mixilateral plate; we assign NUFV 852 to cf. Asterolepis sp. indet. according to the pronounced width of the overlap zone. The dorsal lamina ornament is densely tuberculate with no apparent organization to the tubercles; this is a close match to the mixilateral ornament in A. alticristata n. sp. The lateral lamina is only partially preserved so height is impossible to measure. Ornament of the preserved portion of the lateral lamina appears similar to that of the dorsal lamina.
Published as part of Downs, Jason P., Daeschler, Edward B., Lo, Nathanael, Carey, Emily N. & Shubin, Neil H., 2019, Asterolepis alticristata n. sp. (Antiarchi) from the Upper Devonian (Frasnian) of Nunavut, Canada, and a report on the antiarch diversity of the Fram Formation, pp. 679-698 in Geodiversitas 41 (19) on pages 688-691, DOI: 10.5252/geodiversitas2019v41a19, http://zenodo.org/record/3695594
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