Type species

Pechoralepis zinaidae (Valiukevičius, Reference Valiukevičius2003a)

Included species

Pechoralepis zinaidae, P. adzvensis (Valiukevičius, Reference Valiukevičius2003a), P. valentinae (Valiukevičius, Reference Valiukevičius2003a), P. juozasi sp. nov.

Etymology

For Lithuanian scientist Dr Juozas Valiukevičius, the world's premier worker on acanthodian microremains over the last thirty years.

Holotype

Scale NMV P228897 (Fig. 2a & b); paratypes are the other material figured here.

Fig. 2. Scales and possible fin spine fragments of acritolepid acanthodian Pechoralepis juozasi sp. nov. from the Aztec Siltstone, locality MC7, Mount Crean, Antarctica. a. & b. Holotype NMV P228897 in crown and lateral views, c. & d. NMV P228898 in lateral and basal views, e. & f. NMV P228899 in crown and lateral views, g. & h. NMV P228900 in crown and lateral views, i. & j. NMV P228901 in crown and lateral views, k. & l. vertical transverse section NMV P228941, showing whole section and closeup of central crown, m. horizontal section through apex of base and lower crown NMV P228942, showing area at lateral corner with bone cell lacunae of upper base and odontocytes and Stranggewebe of lower crown. n.–r. Fin spine fragments possibly from P. juozasi. n. & o. fragment NMV P228920. p. Transverse thin section of spine tip fragment NMV P228955 (leading edge to left). q. & r. Longitudinal and transverse thin sections through one side of spine fragment NMV P228954. Scale bar is 0.1 mm in a–q, 0.01 mm in r; anterior of scales is to the right.

Other material

Twenty scales including NMV P228898 (Fig. 2c & d), NMV P228899 (Fig. 2e & f), NMV P228900 (Fig. 2g & h), NMV P228901 (Fig. 2i & j), and thin sections NMV P228941 (Fig. 2k & l) and NMV P228942 (Fig. 2m).

Diagnosis

Pechoralepis with diamond- or square-shaped scales in which the base is wider and longer than the crown; posterolateral ridges along edges of high central crown plate are curved, converging to form a blunt posterior corner; 1–3 subradial ridges on anterior central crown plate fade out by midcrown; the scale neck rarely has vascular canal openings, as radial canals are only rarely developed in the crown; ascending vascular canals are lacking and circular canals are developed at two levels, just above the base and in the angle between the upper and lateral crown areas; the crown has 4–7 superposed growth zones; odontocytes fill the lowest level of the central part in each growth zone; Stranggewebe is only developed low in the posterior crown.

Description

Scales are medium-sized, c. 0.5–1.0 mm long and wide. Scale crowns are usually diamond- or square-shaped with a raised medial area bordered by curved sharp edges from lower lateral areas; the outermost crown perimeter is sharp all round. The crown is relatively flat, and usually pinched in and sloping down slightly at the anterior corner, widening gradually towards the centre and then narrowing to form the posterior point (Fig. 2a, c & i). Rare scales have a straight anterior edge (Fig. 2e). The medial crown area has 1–3 short, evenly-spaced ridges running back from the anterior edge(s) towards the side edges, but fading out before mid-crown; lateral crown areas have one or two (Fig. 2i) ledges with (Fig. 2a) or without (Fig. 2e) shorter ridges. The neck is short or absent. Rarely, large pores are present along the neck-base junction (Fig. 2b & j), and more rarely canals pierce the base (Fig. 2j). Scales bases have a square or rhombic outline, and are usually slightly wider and longer than the crowns; the base is rounded convex and deepest slightly forward of centre.

Histological investigations show that the scale crowns have 4–7 superposed growth zones, which are relatively thin centrally (Fig. 2k & l). Growth zones are filled with odontocytic mesodentine (sensu Valiukevičius & Burrow Reference Valiukevičius and Burrow2005: lacunae often present where dentine tubules branch) plus a small area of Stranggewebe (i.e. mesodentine comprising parallel elongated lacunae and processes) low in the posterior crown, but not in the primordium (i.e. the first formed, central growth zone). The elongated lacunae are very fine and densely packed (Fig. 2m). Odontocytes interconnected by short processes are abundant in the lowest level of all growth zones, across the whole width of the central plate (Fig. 2l). The scales appear to have circular vascular canals at two levels, just above the base and in the angle between the upper and lateral crown areas; other vascular canals are rare. Polygonal or rounded osteocytes are abundant throughout the base, which is formed of very thin bone lamellae. The osteocytes are interconnected by winding processes, forming a particularly dense network in the apex of the base, which is the primordial growth zone (Fig. 2m).

Comparison

The scales are assigned to Pechoralepis because of their distinctive histological structure (fourth structural group sensu Valiukevičius & Burrow Reference Valiukevičius and Burrow2005); morphologically they most closely resemble scales of P. valentinae (Valiukevičius Reference Valiukevičius2003a) from the Lochkovian Toravey Formation of Timan-Pechora, Russia. Histologically, scales of P. juozasi sp. nov. mainly differ from those of P. valentinae (Valiukevičius Reference Valiukevičius2003a, fig. 21) in having more crown growth zones (4–7 vs. 2–4) and only rare vascular canals in the crown. Pechoralepis juozasi also shows some similarites to Iranolepis ginteri Hairapetian, Valiukevičius & Burrow, Reference Hairapetian, Valiukevičius and Burrow2006 from early Frasnian shallow water carbonates in the Chahriseh profile, Kaftari Mountain, central Iran. The main differences between the two taxa are that Iranolepis scales have a pronounced anteromedian sulcus on the crown, lack the sharp perimeter along the anterior crown edge, and have sparser odontocytes in the crown. Both taxa appear to have circular vascular canals at two levels in the crown (Hairapetian et al. Reference Hairapetian, Valiukevičius and Burrow2006, fig. 4D). The overall scale shape resembles that of some of the new undetermined shark scales described below, but scales of P. juozasi are distinguished by having lateral crown ledges rather than lateral odontodes, a symmetrical base rim outline, relatively smooth bases, superpositional crown growth zones, and lacking an anteromedian sulcus on the crown. The distinctive shape of the crown in P. juozasi, with a medial crown area bearing short ridges edged by lower lateral ledges and short oblique neck ridges, is mirrored in many other taxa, including the Early Devonian acanthodians Nobilesquama minilonga (Valiukevičius, Reference Valiukevičius2003a), Nostolepis kozhymica Valiukevičius, Reference Valiukevičius2003a, Nostolepis terraborea Valiukevičius, Reference Valiukevičius2003a, Nostovicina laticristata (Valiukevičius, Reference Valiukevičius, Cherkesova, Karatajute-Talimaa and Matukhin1994), Pechoralepis valentinae and P. adzvensis, as well as the modern, deep water Gulper shark, Centrophorus granulosus (Reif Reference Reif1985, pl. 5, text-fig. 21G).

Material

Ten short mid-distal fin spine fragments including NMV P228920 (Fig. 2n & o), transverse section near spine tip NMV P228955 (Fig. 2p), and vertical longitudinal and transverse sections of a mid-distal spine fragment NMV P228954 (Fig. 2q & r).

Description

Fragments have up to three longitudinal ribs per side and a wider leading edge rib, with a wide central cavity. Lateral ridge crests are slightly convex in transverse section, with an angular lower edge (Fig. 2r). Shallow grooves separating ribs have irregularly-spaced pore openings. The largest fragment is the slender tip of a spine, 3 mm long, with a relatively broad flat trailing face. NMV P228955, the transverse section of this spine tip (Fig. 2p), shows a wide central cavity and numerous longitudinal canals in the upper half (leading edge) of the spine, and a finely lamellar acellular bone forming a thick layer surrounding the central cavity (Fig. 2q). Transverse sections show denteons formed around the longitudinal canals. All sections show mesodentine without lacunal widenings forming a network in the ridges, and an outer durodentine or enameloid layer on the ridges (Fig. 2r).

Comparison

Unfortunately no fragments with an insertion area were found in the residues: a group of Gondwanan diplacanthid genera, including Milesacanthus, are distinguished from other acanthodians with longitudinal ridges on fin spines by having very thin parallel ridges forming the surface of the insertion area. The sections described here show no evidence of the radiating canals and deep inter-ridge grooves with a subcircular cross-section which characterize the Antarctic diplacanthid acanthodians in which spine histology is known (e.g. Young & Burrow Reference Young and Burrow2004, fig. 6H, J, M, O, P). Acritolepids are one of the non-diplacanthiform acanthodians with spines having a morphology similar to the Mount Crean fragments: unpaired and pectoral fin spines of Lochkovian taxa Pechoralepis zinaidae (Valiukevičius, Reference Valiukevičius2003a) and Acritolepis spp. (Valiukevičius Reference Valiukevičius2003b, figs 1, 2, 6B, 13A, 13B, 15A) have equal-width lateral longitudinal ridges with a sharp-edged ‘corner’ in transverse section, and a histological structure (Valiukevičius Reference Valiukevičius2003b, figs 8, 14) similar to the Mount Crean fragments, differing mainly in cellularity and lack of an outer enameloid layer. The Antarctic spines are much younger (Givetian) than the Arctic acritolepids (Lochkovian). Loss of bone cell lacunae in dentinous tissues from older to younger species of the same genus has been noted relatively commonly in Devonian acanthodians and actinopterygians (Valiukevičius & Burrow Reference Valiukevičius and Burrow2005), supporting the possibility that the spines could be from Pechoralepis juozasi n. sp.

Diagnosis

See Valiukevičius & Burrow (Reference Valiukevičius and Burrow2005) for a revised diagnosis.

Type species

Nostolepis striata Pander, Reference Pander1856

Fig. 3. Scales of climatiid acanthodian Nostolepis sp. cf. N. gaujensis Valiukevičius, Reference Valiukevičius1998 from the Aztec Siltstone, locality MC7, Mount Crean, Antarctica. a. & b. NMV P228902 in laterocrown and lateral views, c. & d. NMV P228903 in laterocrown and lateral views, e. & f. NMV P228904 in anterocrown and lateral views, g. & h. NMV P228905 in laterocrown and posterocrown views, i. vertical transverse section NMV P228943, j. horizontal section of crown NMV P228944. Scale bar is 0.1 mm; anterior of scales is to the right unless indicated by arrows.

Material

Thirty-seven scales including figured specimens NMV P228902 (Fig. 3a & b), NMV P228903 (Fig. 3c & d), NMV P228904 (Fig. 3e & f), NMV P228905 (Fig. 3g & h), and thin sections NMV P228943 (Fig. 3i) and NMV P228944 (Fig. 3j).

Description

Scales are relatively large, up to 1.0 mm wide and 1.5 mm long. The horizontal crown bears 4–6 sharp and robust parallel ridges running back from a sharply defined anterior edge; vertical ridges extend down the anterior face of the crown on better-preserved scales. Crown ridges decrease in height posteriorly, extending two-thirds or more of the length of the crown and fading out in the posterior third. Some pores are visible at the crown/neck junction laterally and anteriorly. The base is moderately convex, usually subrhombic in outline, and protrudes in front of the crown; the posterior part of the crown extends beyond the base. A fine honeycomb-like network of polygonal depressions, each c. 15 µm diameter, forms ultrasculpture on well-preserved scale crowns (e.g. Fig. 3h).

Histological study shows that the crown has up to six growth zones, penetrated by circular and interconnecting radial canals (Fig. 3j), with Stranggewebe low in the posterior part of the crown including the primordium, and mesodentine filling the rest of the crown (Fig. 3i). This tissue is odontocytic in the lowest level of each growth zone and syncitial (non-lacunal: sensu Valiukevičius & Burrow Reference Valiukevičius and Burrow2005) in the outer level, where it is orthodentine-like with fine tubules oriented perpendicular to the outer surface; a very thin, birefringent durodentine layer overlies the mesodentine in the central part of the crown. The base is formed of thin bone lamellae, with osteocytes abundant throughout, and a dense network of interconnected processes and large polygonal osteocytes in the primordial growth zone.

Comparison

The type scales of N. gaujensis are from the Frasnian of the Baltic Upper Old Red Sandstone (Valiukevičius Reference Valiukevičius1998). Similar scales, assigned to Nostolepis sp. cf. N. gaujensis, have also been described from ?Frasnian siltstones of the Cuche Formation, Colombia (Burrow et al. Reference Burrow, Janvier and Villarroel2003), carbonates of the Chahriseh profile (Turner et al. Reference Turner, Burrow, Gholamalian and Yazdi2002, Hairapetian et al. Reference Hairapetian, Valiukevičius and Burrow2006) and perhaps the Bidou 1 locality, central Iran (Janvier Reference Janvier1974, pl. 6, figs 2 & 3: see Turner et al. Reference Turner, Burrow, Gholamalian and Yazdi2002). Scales from the ?late Givetian Campo Chico Formation of Venezuela, assigned to Acanthodii indet. by Young & Moody (2002, fig. 16A–C), are possibly conspecific. The main distinguishing features of the Antarctic scales is the sharpness of the ridges and crown edges and the ‘honeycomb’ ultrasculpture. Undoubtedly these features are correlated, with the microornament almost certainly being the imprint of epidermal cells, by comparison with the identical pattern formed on scales of modern sharks (Reif Reference Reif1985). The lack of abrasion and wear on the scale crowns suggests that the scales had not erupted through the skin. Amongst acanthodians, ultrasculpture has previously only been observed on acanthodiform scales (e.g. Trinajstic Reference Trinajstic2001, Beznosov 2005). The most similar pattern in acanthodians is seen on Upper Carboniferous Acanthodes-type scales from the Moscow district (Beznosov Reference Beznosov, Rosanov, Lopatin and Parkhaev2005, pl. 12.4, 8, 10), which have rounded depressions of similar dimensions to the polygonal depressions in the Nostolepis sp. cf. N. gaujensis scale crowns. Amongst other vertebrates, the ultrasculpture is identical to that on tubercles of Psammosteus sp. 2 from the Givetian Abava Beds of Estonia (Märss Reference Märss2006, fig. 4L). Like Nostolepis sp. cf. N. gaujensis, taxa with comparable imprints have an outer birefringent durodentine or enameloid layer.

Remarks

See Young & Burrow (Reference Young and Burrow2004, p. 26) for a revised diagnosis

Type species

Milesacanthus antarctica Young & Burrow, Reference Young and Moody2004

Fig. 4. Scales of Milesacanthus antarctica Young & Burrow, Reference Young and Burrow2004 from the Aztec Siltstone, collecting site MC7, locality 8 of Young (Reference Young1988), Mount Crean, Antarctica. a. & b. NMV P228906, in crown and posterolateral views, c. NMV P228907, in laterobasal view, d & e. NMV P228908, in crown and laterocrown views, f. NMV P228909, in anterocrown view, g. NMV P228910, in crown view, h. NMV P228911, in laterocrown view, i. & j. NMV P228912, in laterocrown view, k. & l. NMV P228913, in laterocrown and lateral views, m. & n. NMV P228914, in anterocrown and lateral views, o. NMV P228915, in laterocrown view, p. & q. NMV P228916, in crown and laterocrown views, r. NMV P228917, in posterocrown view, s. NMV P228918, in crown view, t. & u. NMV P228919, in crown and posterolateral views. Scale bar is 0.1 mm; anterior of scales is to right unless indicated by arrows.

Fig. 5. Ground thin sections of scales of Milesacanthus antarctica from the Aztec Siltstone, locality MC7, Mount Crean, Antarctica. a. WAM 03.1.2, vertical longitudinal section of scale with unworn ornament ridges, deep neck, shallow base, light polarized under Nomarski optics, b. WAM 03.1.1, horizontal section of crown of scale with unworn ornament ridges, deep neck, shallow base, c. NMV P228945, horizontal section of crown of scale with worn ornament ridges, short neck, medium base, viewed under Nomarski optics, d. & e. NMV P228946, horizontal section of crown of scale with slightly worn crown and smooth anterior edge, f. & g. NMV P228947, vertical transverse section of scale, worn crown with only short grooves behind smooth anterior rim; matrix stuck to broken base of scale. Scale bar is 0.1 mm in a–c, e, g, and 1.0 mm in d and f; anterior of scales is indicated by arrows.

Fig. 6. Casts of impressions of squamation patches which had flaked off Milesacanthus antarctica holotype or associated specimens on block ANU V773 from Portal Mountain, Antarctica. a. scale crowns, b. scale bases, c. laterally exposed scales. Scale bar is 0.1 mm.

Material

More than 500 scales including NMV P228906 (Fig. 4a & b), NMV P228907 (Fig. 4c), NMV P228908 (Fig. 4d & e), NMV P228909 (Fig. 4f), NMV P228910 (Fig. 4g), NMV P228911 (Fig. 4h), NMV P228912 (Fig. 4i & j), NMV P228913 (Fig. 4k & l), NMV P228914 (Fig. 4m & n), NMV P228915 (Fig. 4o), NMV P228916 (Fig. 4p & q), NMV P228917 (Fig. 4r), NMV P228918 (Fig. 4s), NMV P228919 (Fig. 4t & u), and thin sections WAM 03.1.2 (Fig. 5a; Young & Burrow Reference Young and Burrow2004, fig. 3D), WAM 03.1.1 (Fig. 5b; Young & Burrow Reference Young and Burrow2004, fig. 4C), NMV P228945 (Fig. 5c), NMV P228946 (Fig. 5d & e), NMV P228947 (Fig. 5f & g).

Description

Scales are medium to large-sized, from 0.5–1.5 mm wide, 0.3–0.5 mm deep, and 0.5–1.7 mm long. Crown ornament comprises 14 to 24 sub-parallel or slightly fanned ridges and grooves extending to at least midcrown, with denticulated posterolateral crown edges. On scales with unabraded crowns bearing full-length ridges and grooves (Fig. 4a, b & g), the scale neck is deep and strongly concave, and the base is shallow and convex, with a marked base-neck rim. Other scales have worn crowns which are smooth except for 6–12 short grooves on the anterior part (Fig. 4k–n, p–r); these scales have relatively shorter necks and more tumid bases, and a less pronounced base-neck rim. Both scale types sometimes have ‘warts’ low on the posterior neck, between deep vertical grooves (Fig. 4b, c, e & l). Some scales show ornamentation intermediate between these two extremes. Most scale crowns are symmetrical and the anterior edges converge at c. 100°. Rare scales are asymmetrical (Fig. 4r–u).

Histological study shows that the scales have up to 12 crown growth zones. Thin, distally branching dentine tubules and canals (Fig. 5a) rise up through the neck and curve over to converge in the upper central parts of the growth zones of the crown (Fig. 5c–e), with the main canals running up to and under the crown grooves (Fig. 5g). Dentine is mostly syncitial mesodentine, forming a spongelike network in the lower crown (Fig. 5f & g). Scales also have a pore canal system penetrating the anterior part of the crown, with canals rising up from large pores in the lower neck and opening out through pores in the grooves between the ribs on the anterior half of the crown (Fig. 5b–e). Branches of this system probably also extend back through the scale crown to the posterior neck, where they possibly ran up grooves on the external surface of the scales (Fig. 4c, l, u). No bone cell lacunae are present in the crown or base; the latter is penetrated by Sharpey's fibre bundles and short, thin, irregularly branching canaliculi (Fig. 5a & g).

Comparison

Many of the scales on the holotype and associated articulated specimens of M. antarctica from Portal Mountain (Fig. 1, locality 12) have well-preserved ridges and grooves. However, most of the scales on those specimens have been split through the middle and the crown ornamentation is mainly known from casts of loose fragments on which scale patches were cleared with 10% hydrochloric acid (Young & Burrow Reference Young and Burrow2004, fig. 4A; Fig. 6a–c). The isolated scales from Mount Crean show a greater range in preservation of ornamentation, from unabraded to almost smooth crowns (Fig. 4a–l). A similar range is seen in the older species M. ancestralis from the Emsian Jawf Formation of Saudi Arabia (Burrow et al. Reference Burrow, Lelièvre and Janjou2006, figs 7 & 8). The main difference between the species is in the numerical range of crown ridges and grooves, with 14–24 in M. antarctica and 6–18 in M. ancestralis. Burrow et al. (Reference Burrow, Lelièvre and Janjou2006, p. 547) described the crown ornament of the latter as usually formed of grooves rather than ridges: in well-preserved scales with deep long grooves, the round-bottomed grooves cut into the flat surface of the crown (e.g. Burrow et al. Reference Burrow, Lelièvre and Janjou2006, fig. 8A). The distinction between grooves and ridges is not so clear-cut in the Mount Crean examples. Milesacanthus ancestralis scales were also distinguished by having wart-like bumps on the posterior part of the neck, but some M. antarctica scales, both in the microremains (Fig. 4b, c, e, h, l–o, q, r & u) and on the type material (Fig. 6c), also show this feature. Hairapetian et al. (Reference Hairapetian, Valiukevičius and Burrow2006) assigned similar scales from the early Frasnian of the Chahriseh profile in central Iran to Milesacanthus sp. aff. M. antarctica. They noted that M. ancestralis scales “differ from the Iranian ones in having a complicated syncitial mesodentinal network in the lower crown (neck area) filling the growth zones between the main branches of the ascending vascular canals … and also lacunal widenings of dentine canals, particularly dense in the lower neck; and the horizontal canals running under the grooves of the crown plate are the longest and widest of the species compared, and supplied by numerous winding canaliculi…”. Scales of M. antarctica, like those of M. ancestralis, have a complex mesodentinal network in the lower crown (Fig. 5f & g). M. antarctica is distinguished by the density of dentine tubules converging in the centre of the upper scale crown (Fig. 5c–e).

Fig. 7. Acanthodidae gen. et sp. indet. a.–d. Scales from the Aztec Siltstone, locality MC7, Mount Crean, Antarctica. a. & b. NMV P228921 in anterocrown view and closeup of ?abraded grooves on the crown, c. NMV P228922 in posterocrown view, d. NMV P228923, vertical longitudinal section of scale. e.–j. Cast of impression of articulated left upper and lower jaws WAM 92.3.7 from the Aztec Siltstone, Mount Ritchie, Antarctica. e. cast of whole specimen, f. Palatoquadrate, jaw articulation area, and posterior end of lower jaw. g. Anterior ossification of Meckel's cartilage, mandibular ossification, and hyoid bar. h. Robust conical gill raker. i. Mandibular ossification of lower jaw preserved on counterpart. j. Labelled sketch of the whole specimen. Scale bar is 0.01 mm in b, 0.1 mm in a, c, d, 5 mm in i, 1 cm in e–h, j; anterior indicated by arrows.

Material

More than 50 scales including NMV P228921 (Fig. 7a & b), NMV P228922 (Fig. 7c), and thin section NMV P228948 (Fig. 7d) from Mount Crean site MC7; set of articulated jaws WAM 92.3.7 (Fig. 7e–j) from Mount Ritchie.

Description

Most scales are small, c. 0.3 mm wide and long, with some larger scales up to 0.5 mm long. The crown surface is flat, smooth and kite-shaped, with a shallow sulcus towards the anteromedian corner. The posterior crown point extends slightly beyond the base. The neck and base are of equal depth; the base is rounded convex and deepest centrally.

Histological study (Fig. 7d) shows that scale crowns have c. 10 thin growth zones. Scales appear to have two or three circular canals in the neck which are intersected by ascending canals in the lower half of neck. Dentine tubules show minimal branching, and lack the network of fine tubules which characterize Acanthodes Agassiz, Reference Agassiz1833 (Denison Reference Denison1979, fig. 9G). A very thin layer of birefringent durodentine forms the upper part in each crown growth zone. The upper surface of the base is relatively flat; canals of Williamson are distributed throughout the base.

Jaws (Fig. 7e–j). WAM 92.3.7 is a slab showing the impression of a pair of left jaws preserved in lateral view. The general size, shape and structure of the jaws is comparable with that of Acanthodes bronni Agassiz, 1833 (e.g. Miles Reference Miles, Greenwood, Miles and Patterson1973, fig. 12) from the Lower Permian of Europe, with separately ossified palatoquadrate and autopalatine cartilages forming the upper jaw (the metapterygoid is missing), and anterior and posterior ossifications of the lower jaw Meckel's cartilage. Short parallel striations of the superficial bone layer are obliquely oriented on the posterior ossification, which has a markedly thickened occlusal edge opposing the ventral edge of the palatoquadrate. Remnants of the hyomandibular cartilages appear stuck to the outer surface of the palatoquadrate. The mandibular ossification is relatively thick and deep compared with that of A. bronni, with a rugose surface ornament. The elongate, concave surfaced element underlying the mandibular bone is probably the hyoid bar. Several conical gill rakers c. 1 cm long are scattered between the upper and lower jaws. Small curved elements preserved in a circular formation in front of the autopalatine are interpreted as sclerotic ring plates. Short thin closely-spaced elements behind the palatoquadrate are branchiostegal rays, and several small tubular elements behind the jaw articulation are sensory line ossicles.

Comparison

All acanthodids have smooth-crowned scales with 10 or more crown growth zones in which birefringent durodentine forms the central upper layer. They also have perichondrally ossified jaws and branchial cartilages; the upper jaw has three ossification centres, the lower jaw has two ossification centres and is supported by a robust ventral mandibular bone. The rugose surface on this bone on the WAM 92.3.7 jaws resembles that of A. sulcatus Agassiz, 1835 (Denison Reference Denison1979, fig. 2A) from the Lower Carboniferous of Scotland. The Mount Ritchie jaws are distinguished from those of other acanthodids by having the thickened perichondrally ossified occlusal shelf on the posterior Meckel's cartilage.

Type species

Aztecodus harmsenae Long & Young, 1995

Fig. 8. Chondrichthyan teeth and denticles from the Aztec Siltstone, locality MC7, Mount Crean, Antarctica. a–c. Teeth of Aztecodus harmsenae Long & Young, Reference Long and Young1995. a. & b. NMV P228829, in labial and lingual views. c. NMV P228830, in lingual view. d.–l. Teeth of Antarctilamna prisca Young, Reference Young1982. d. & e. NMV P228923, multiridged variant with one of two main cusps broken off, in labial and lateral views, f. & g. NMV P228924, in occlusal and lateral views, h. & i. NMV P228925, in occlusal and laterolingual views, j.–l. NMV P228949, transverse section along pulp canal of main preserved cusp of smooth tooth variant (other cusp has broken off), in whole section, and closeups of cusp tip (Nomarski optics) and labial centre of base. m.–p. Elasmobranchii indet. fam., gen. et sp. A. branchial denticles. m. & n. NMV P228926, in crown and posterior views, o. & p. NMV P228927, in crown and lateral views. Scale bar is 0.1 mm in a–i, k–p, 1.0 mm in j.

Revised diagnosis

Aztecondontid omalodontiform with teeth up to 2 cm wide with a low base that is broader than the height of the cusps. Second principal cusp is one-quarter to three-quarters the height of the largest cusp. Both cusps have smooth lingual and labial surfaces and are strongly compressed labio-lingually; the cusps are broad-based and widely separated by a sharp crenulated ridge. Small accessory cusplets are sometimes present laterally. The tooth base is approximately 2.5 times as long as broad.

Remarks

Based on the similarity of forms, and the minor differences between teeth of the two taxa, we follow the recommendation of Hairapetian et al. (Reference Hairapetian, Ginter and Yazdi2008) and regard Anareodus statei Long & Young, Reference Long and Young1995 as a junior synonym of Aztecodus harmsenae.

Material

Teeth NMV P228829 (Fig. 8a & b) and NMV P228830 (Fig. 8c) from Mount Crean locality MC7.

Description

Tooth NMV P228829 closely resembles the holotype of Anareodus statei, although it is much smaller at c. 3.5 mm wide and lacks a small accessory cusplet on the main cusp. The basal surface of the tooth is narrow and flat, forming a horizontal plane to the vertically-directed base, and lacking labial or lingual extensions, directly comparable to the base of the type material of Aztecodus. The two main crown cusps are sharply pointed and markedly asymmetrical, with the large cusp curving lingually and having crenulated lateral cristae. The labial surface of the cusp is scored by random apically-directed grooves, recalling the pleromin composition of cusps of Aztecodus harmsenae (Hampe & Long Reference Hampe and Long1999, fig. 5B). One large foramen penetrates the base below the main cusp, and smaller foramina open along the labial base-crown junction.

The second tooth (Fig. 8c) is c. 3 mm wide and relatively poorly preserved, with slightly asymmetrical worn lateral cusps and a smooth-crested narrow ridge between them. Although only half the base is present, it shows the laterally concave basal surface which characterizes the type specimen of Aztecodus (Long & Young Reference Long and Young1995, fig. 8A), and labio-lingual canals penetrating the base below the base-crown margin. The labial and lingual faces of the base are vertical, without extensions.

Comparison

These two teeth support the assessment of Hairapetian et al. (Reference Hairapetian, Ginter and Yazdi2008) that Anareodus statei and Aztecodus harmsenae are conspecific, with the ‘Anareodus’-like tooth (Fig. 8a & b) having a large nutrient foramen in the base and lacking an accessory cusplet on the main cusp, and thus fitting in the gap between the two original diagnoses. If, as proposed above, Anareodus teeth are composed of pleromin, then this further supports Anareodus being a junior synonym of Aztecodus.

Revised diagnosis (teeth and scales)

Antarctilamnid teeth with 4–7 cusps aligned in a single plane with main lateral cusps directed lingually, a basal projection on the mid-labial margin, and one or more large foramina between the button and the edge of the base; two tooth morphotypes, one with wavy vertical cristae and the other relatively smooth with weak lateral cristae on the main cusps. Ridged teeth have an osteodentine (= trabecular dentine) core and base, orthodentine extending up to half the radius of the cusps, and an outer layer of pallial dentine; smooth teeth have a central canal and orthodentine forming the lateral cusps, an osteodentine base, and a thin outer enameloid layer. Scales are compound, with a flat crown ornamented with semiconcentric odontodes, and a cup-shaped base.

Type species

Antarctilamna prisca Young, 1982

Material

Seven teeth including NMV P228923 (Fig. 8d & e), NMV P228924 (Fig. 8f & g), NMV P228925 (Fig. 8h & i), and thin section NMV P228949 (Fig. 8j–l).

Description

Both the ridged and smooth diplodont tooth variants which were figured in the original description of type species A. prisca are represented in the Mount Crean sample; teeth are 1–2 mm wide. Only one example of the most distinctive teeth (Fig. 8d & e), which is commonly figured as the typical Antarctilamna form, was found. It shows strongly developed vertical ridges on the labial face, comparable to those figured by Young (Reference Young1982, text-fig. 3A–D, pl. 89.7) on the A. prisca holotype. The ridges meet weaker ridges on the lingual face, appearing as a row of chevrons in lateral view on NMV P228923 (Fig. 8e). One of the main cusps has broken off this tooth (Fig. 8d & e), showing that each of the labial ridges has a large central pulp canal. Unfortunately the base is also broken, with the lingual extension missing. The tooth is possibly a pathological variant, as it has accessory cusplets labial to the main cusps.

The tooth form occurring more commonly in the residues has laterolingually-pointing main cusps separated by two or three smaller medial cusps (Fig. 8f–i); where three medial cusps are developed, the central cusp is larger than the other two. All medial cusps are smooth with a subcircular cross-section; the lateral cusps are labio-lingually flattened with weak carinae running up the lateral and medial edges. The lingual edge of the base is slightly convex, with one large foramen opening out below an oval-shaped lingual button on the central upper surface, and one or more smaller foraminae lateral to the large one. The lower surface of the base is concave, and the base outline is subdiamond/trapezoidal. A vertical ground thin section of one of these smooth tooth variants (Fig. 8j–l) shows a single central pulp canal in the lateral cusp joining a network of canals in the tooth base. Fine branching orthodentine tubules radiate up and out from the central canal in the main cusp, overlain by a very thin enameloid layer which merges with the underlying dentinous tissue. Incremental growth lines in the dentine layer run slightly obliquely to, or parallel with, the outer surface (Fig. 8k). Fine branching dentine tubules are also visible in the upper level of the base (Fig. 8l), at a level which probably represents the base of the medial cusps. A few dark spots in this area of the section could be bone cell lacunae, but are more likely to be crosscut dentine canals.

Comparison

Both Turner (Reference Turner1997) and Ginter (Reference Ginter, Arratia, Wilson and Cloutier2004) noted the close similarity between teeth of Antarctilamna and Wellerodus Turner, Reference Turner1997. Hampe & Long (Reference Hampe and Long1999, fig. 3) studied the histological structure of the strongly ridged tooth form. These differ from the smoother tooth form in having cusps with a trabecular core and purportedly lacking an enameloid layer, although the latter seems visible on the left in their figure 3A & B. Hampe & Long (Reference Hampe and Long1999) excluded Antarctilamna from the xenacanthoids based on the lack of a lingual button and a labial basal tubercle on the teeth, but these structures are present on the two Antarctilamna tooth forms (Long & Young Reference Long and Young1995, fig. 4B; Fig. 8f–i). The antarctilamniforms are, however, differentiated from the xenacanthiforms by their vastly different fin spines (Ginter et al. 2008).

Antarctilamna teeth have been recorded from many levels of the Aztec Siltstone, from the base to the ‘Pambulaspis’ zone (Long & Young Reference Long and Young1995). While previously only known from the Antarctic, south-eastern Australia, South Africa (Anderson et al. Reference Anderson, Long, Evans, Almond, Theron and Bender1999), and possibly Saudi Arabia (Ginter Reference Ginter, Arratia, Wilson and Cloutier2004), and South America (Maisey & Melo Reference Maisey and Melo2005), Antarctilamna, and also Portalodus, have now been recorded from the late Givetian-early Frasnian of western New York (Ginter et al. Reference Ginter, Cloutier, Maisey and Purnell2006; see also Turner Reference Turner, Arratia, Wilson and Cloutier2004).

Material

Two branchial denticles NMV P228926 (Fig. 8m & n) and NMV P228927 (Fig. 8o & p), >150 scales including NMV P228928 (Fig. 9a & b), NMV P228929 (Fig. 9c & d), NMV P228930 (Fig. 9e & f), MC7b-17 (Fig. 9g), NMV P228931 (Fig. 9h), and thin sections NMV P228950 (Fig. 9l & m) and NMV P228951 (Fig. 9n & o), four head tesserae including NMV P228933 (Fig. 9i & j), and one umbellate scale NMV P228934 (Fig. 9k).

Fig. 9. Scales of Elasmobranchii indet. fam. gen. et sp. A from the Aztec Siltstone, locality MC7, Mount Crean, Antarctica. a. & b. NMV P228928 in crown and posterior views, c. & d. NMV P228929 in crown and lateral views, e. & f. NMV P228930 in crown and anterocrown views, g. NMV P228931 in crown view, h. ?transitional scale NMV P228932 in anterior view, i. possible head scale NMV P228933 in lateral view, j. & k. umbellate scale NMV P228934 in crown and ?anterior views, l. & m. NMV P228950, transverse section of flank scale, n. & o. NMV P228951, horizontal section through crown. Scale bar is 0.1 mm in a–k, m–o, and 1.0 mm in l; anterior of scales is to right unless indicated by arrows.

Description

Several elements with irregularly dispersed ‘teeth’ on an oval base are interpreted as branchial denticles, by comparison with similar elements from articulated Devonian–Carboniferous sharks (e.g. Antarctilamna prisca in Young Reference Young1982, text-fig. 2C & D). NMV P228926 has eight denticles, with the two larger ones broken off to show that each had a central pulp canal. NMV P228927 has four denticles, with the three largest broken off to show a single central pulp canal in two and multiple canals in the largest, which has broken right at the base presumably at the level of the vascular canal network below where the central canal would have branched off.

Presumed flank scales are of typical ctenacanthoid structure, with the crown formed of bunched, overlapping finger-like odontodes. Many scales (e.g. Fig. 9a & b) show a regular arrangement of odontodes, while others show a less ordered pattern (Fig. 9e–h). Each odontode has a central pulp canal (Fig. 9b, i & j), and the upper posterior area of the scale neck has a row of canal openings (Fig. 9b). Some scales have a large anteromedian odontode (Fig. 9c & d). The upper surface of each odontode is flat or slightly concave (Fig. 9i & j). Other scale forms include presumed head tesserae with areal-growth odontodes (Fig. 9i) and umbellate scales (Fig. 9j & k). The scales usually have a large pulp cavity piercing the base, from which relatively wide canals radiate just above the base (Fig. 9n & o). Branching, fine-calibre orthodentine-like tubules form the odontodes, although dentine in the innermost parts of each growth zone resembles the syncitial mesodentine in some ‘young’ climatiid acanthodians.

Comparison

Many Middle–Late Devonian chondrichthyans have scale variants and branchial denticles with very similar morphologies, as best shown by Williams (Reference Williams1998) for Tamiobatus vetustus Eastman, Reference Eastman1897. The branchial denticles are of the “double row coxcomb or “Stemmatias bicristatus” type” of Williams (Reference Williams1985, p. 156). The flank scales with regular rows of odontodes resemble A. prisca scales (Young Reference Young1982, text-fig. 4D). Scale crown histology is comparable with that of Ohiolepis newberryi Wells, Reference Wells1944 from the Middle Devonian of the eastern United States of America.

Fig. 10. Scales of Elasmobranchii indet. fam., gen. et sp. B from the Aztec Siltstone, locality MC7, Mount Crean, Antarctica. a. & b. NMV P228935 in crown and laterocrown views, c. & d. NMV P228936 in crown and lateral views, e. & f. NMV P228937 in crown and lateral views, g. & h. NMV P228938 in crown and laterocrown views, i. NMV P228939 in posterobasal view, j. NMV P228940 in crown view, k. possible head scale NMV P228941 in crown view, l. vertical longitudinal section NMV P228952, Nomarski optics polarization showing Sharpey's fibre bundles in base and branching orthodentine-like tissue in crown, m. horizontal section of crown NMV P228953 showing unoriented network of dentine tubules of inner parts of crown growth zones. Scale bar is 0.1 mm; anterior of scales indicated by arrows.

Material

More than 150 scales including scales NMV P228935 (Fig. 10a & b), NMV P228936 (Fig. 10c & d), NMV P228937 (Fig. 10e & f), NMV P228938 (Fig. 10g & h), NMV P228939 (Fig. 10i), NMV P228940 (Fig. 10j), and ground thin sections NMV P228952 (Fig. 10l) and NMV P228953 (Fig. 10m); possible head scale NMV P228940 (Fig. 10k).

Description

The scales show superficial resemblance to those of P. juozasi sp. nov., with a sharply delineated central crown flanked by lower crown areas. However, in the chondrichthyan scales these lateral areas are separate dentinous growth zones with upper surfaces that slope down medially. The central crown area has a deep anteromedian sulcus extending back towards the posterior corner (Fig. 10b & d). Anterolateral crown edges are sharp and straight, posterolateral edges are longer and slightly curved, converging to a single point just posterior to the base. All side faces of the central crown are deep and vertical; anterolateral faces sometimes bear vertical ridges (Fig. 10f). A few small canal openings are visible on some scales along the anterior crown-neck margin (Fig. 10c & f), with larger openings scattered over the upper posterior part of the neck (Fig. 10b, d & f). The scale base is quite distinctive, with one, or occasionally both, lateral corners strongly pinched out. The lower surface of the base is very lumpy and pustulose (Fig. 10i), distinguishing scales of this species from those of the acanthodian taxa in the assemblage. NMV P228940 (Fig. 10k) is possibly a head scale from this species, as it has similar pustulose upper and lower basal surfaces as on the small-crowned variant NMV P228938 (Fig. 10g & h). On normal flank scales, the base protrudes in front of the crown and is deepest below the anterior edge of the crown (Fig. 10k). The crown is formed of semicircular growth zones with syncitial mesodentine close to the pulp canals, becoming more orthodentine-like towards the outer surface of each growth zone (Fig. 10k).

Comparison

The scales resemble those of the older climatiid acanthodian Canadalepis Vieth, Reference Vieth1980, which have a blunt tongue-shaped crown. At least one of the scales from the Emsian Jawf Formation of Saudi Arabia, which Burrow et al. (Reference Burrow, Lelièvre and Janjou2006, figs. 4.1, 2) assigned to Canadalepis? sp. closely resembles the new scale form, but its histological structure is not known. The structure of indet. sp. B scales resembles that of Protacrodus wellsi (Gross Reference Gross1973, figs 25–27) from the Late Devonian of eastern USA. No scales have been associated with Aztecodus and Portalodus, which are known only from isolated teeth, thus it is not possible to assign the scales to any known species, even though it seems highly likely that they derive from one of these taxa.