Lissodus sardiniensis, Fischer & Schneider & Ronchi, 2010

Fischer, Jan, Schneider, Jörg W. & Ronchi, Ausonio, 2010, New hybondontoid shark from the Permocarboniferous (Gzhelian-Asselian) of Guardia Pisano (Sardinia, Italy), Acta Palaeontologica Polonica 55 (2), pp. 241-264 : 245-253

Lissodus sardiniensis sp. nov.

Figs. 4–7.

Etymology: Named after the island of Sardinia (southern Italy), where the fossil site is situated.

Holotype: FG 589/T/027 a complete tooth with root (Fig. 4A).

Type locality: Northern slope of the Guardia Pisano hills, close to Gonnesa (Sulcis area, SW Sardinia, Italy).

Type horizon: Lacustrine limestone horizon at the top of lithofacies B, latest Carboniferous–earliest Permian (Gzhelian–Asselian), based on sporomorphs and radiometric dating.

Referred material.— Paratypes include teeth FG 589/T/028 (Fig. 4D), FG 589/T/031 (Fig. 4B), FG 589/T/059 ( Fig. 5A ), and FG 589/T/060 ( Fig. 5B ). Fin spine FG 589/F/001 ( Fig. 7A ). Dermal denticles FG 589/S/004 ( Fig. 7F ), FG 589/S/002 ( Fig. 7J ), FG 589/S/007 ( Fig. 7M ).

Additional material.—100 complete teeth and> 400 tooth fragments (mostly crowns),> 150 placoid scales and numerous fin spine fragments.

Diagnosis.—The favourable taphonomic situation enables fin spines and dermal denticles in addition to teeth to be included in the diagnosis of the new species of Lissodus .

Teeth minute, weakly heterodont, measuring from 0.34– 1.31 mm in length. Central cusp prominent standing nearly upright in lateral teeth, becoming strongly labially inclined in mesial and posterior teeth; flanked by one pair of smaller lateral cusplets, clearly leaning toward the central cusp. In occlusal view, crown slightly asymmetric curving away from the mid−point to the labial edges in many teeth. Crown faces triangular−shaped and smooth, lacking vertical striations, accessory cusplets or nodes on the crown shoulders. Occlusal crest compressed into a rather sharp ridge with no crenulation. Labial peg (= labial buttress) usually prominent and not supported by a labial root buttress from below, often showing a tiny subterminal cusplet. Crown/root junction clearly incised around the whole tooth. Root lingually directed and less than one−half crown height but mostly longer than the crown. Root hybodontoid, showing three to five large, simple vascular foramina with anaulacorhize organisation. Central longitudinal pulp cavity situated high up at the crown/root junction. Upper labial root face usually with a single row of small foramina. Basal face crescent−shaped and strongly labially concave. Closest to the species are Lissodus cf. zideki ( Soler−Gijón 1993) and Lissodus lopezae Soler−Gijón, 1997 . However, the new material differs significantly from all other published Palaeozoic and Mesozoic species by the exhibition of a single prominent central cusp, which is flanked by one pair of curved, smaller lateral cusplets.

Fin spines gently curved posteriorly, ornamented with four well−developed smooth longitudinal costae on both sides. Anterior edge with a distinctive keel. Posterior side with a single median row of ventrally curved and weakly alternating hook−like denticles of about 0.5 mm in length. Cross section roughly ovoid of typical hybodontiform organisation. The overall appearance of the fin spines mostly resembles material of L. lopezae Soler−Gijón, 1997 , but differs in the number of denticles.

Dermal denticles vary in shape and size forming distinctive scale morphotypes, most of hybodontoid non−growing type. Crown single to multicuspid; in most scales pointed posteriorly and ornamented with longitudinal ridges on the crown surface. Average crown height 0.5 mm; length varies from 0.5–1.34 mm. Sub−crown smooth; in some specimens a mesial ridge can be recognised. No distinct neck between base and crown. Base in central position, and wider than the crown base to all margins. Undersurface of base (= basal plate) slightly curvate with a large central pulp canal opening. Base outline circular with crenulated margin (multipetaloid), 0.5–1.0 mm in diameter. The denticle assemblage is most similar to Palaeozoic hybodont material described by Gebhardt (1986).

Description of the teeth.—Three morphotypes can be distinguished, which are linked by a gradual transition:

Tooth morphotype I (Fig. 4): The shape and size of teeth in morphotype I vary considerably but are united into one morphotype because of the many transitions between them. The length along the occlusal crest ranges from 0.55– 0.92 mm. The central cusp is prominent but often appears low because of a strong labial inclination in most specimens (Fig. 4G, I) and so the crown of most specimens curves away occlusally from the midpoint to the labial edges (Fig. 4A 2, B 2, C 2, D 2). The lateral cusplets are one−third to one−half of the central cusp height with steeply dipping sides (Fig. 4A 1, D 1, E, F, H); they are pointed and tend to lean toward the cen−

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tral cusp (Fig. 4A 1, B 3, E, F, H). The occlusal crest is moderate, but in some specimens strong on the lateral cusplets (Fig. 4D 3 –F). The labial peg is prominently developed (Fig. 4A 2, D 2, J), protruding aborally (Fig. 4A 1, B 1, C 1, I), and in most specimens carries a tiny subterminal cusplet (Fig. 4A 1, G–J). A lingual peg is occasionally developed (Fig. 4C 3, E, F). The crown/root junction is moderately incised. The root is flat, slightly longer than the crown (Fig. 4B 3, E, I) and less than half the crown height. The lingual root face shows three to five simple vascular foramina (Fig. 4B 3, C 1, D 3 –F) and the labial side has a row of up to seven smaller foramina (Fig. 4A 1, D 1). This morphotype represents ~75% of all teeth and is the most common tooth−type.

Tooth morphotype II ( Fig. 5A–H ): The length along the occlusal crest ranges from 0.34–0.54 mm. The sharp central cusp is prominent ( Fig. 5A 3 , C, D, G) and strongly labially inclined in most specimens ( Fig. 5A 1, B 1, E) so that the crown is distinctly asymmetrical occlusally ( Fig. 5A 2, B 2). The lateral cusplets are slightly rounded ( Fig. 5E, F ), most lean toward the central cusp and half the height of the central cusp. The occlusal crest is moderate ( Fig. 5F, G ) and the labial peg is prominently developed ( Fig. 5H ) with a minute subterminal cusplet. In total, the whole shape of the crown resembles a spade, supported by a strongly incised crown/ root junction ( Fig. 5C, G ). The root is normally half the crown height and noticeably shorter than the crown ( Fig. 5B 1). Teeth of this morphotype have the highest coronal profile. On the lingual root face up to five simple vascular foramina are present ( Fig. 5F–H ) and there are up to seven smaller foramina arranged in a row on the upper labial side ( Fig. 5A 1). This morphotype represents ~20 % of all teeth and is the second most common tooth−type.

Tooth morphotype III ( Fig. 5I–N ): The length along the occlusal crest ranges from 1.01–1.31 mm. The crown is elongate and relatively small. The central cusp is prominent and the lateral cusplets are well rounded perhaps as a consequence of abrasion ( Fig. 5K, M, N ). The lateral cusplets lean toward the central cusp ( Fig. 5L ) and are one half of its height. In occlusal view the central cusp lies more or less in a line with the lateral cusplets. The occlusal crest is strong and the labial peg is moderate developed ( Fig. 5I ). The crown/ root junction is incised. The root is flat measuring less than half the crown height but is somewhat longer than the crown. On the lingual root face there are five vascular foramina ( Fig. 5J ) and on the labial side up to 11 small foramina are present in a line ( Fig. 5I, K ). This is the least common morphotype represents ~5% of all teeth. Only a few complete teeth have been found: most specimens are crowns or half teeth.

Tooth histology ( Fig. 6 ): Sectioned tooth crowns and complete teeth from probably mesial or anterolateral positions revealing a layer of single crystallite enameloid (SCE) up to 40 µm thick ( Fig. 6 ), and especially well developed on the labial crown side ( Fig. 6B–D ). Orthodentine is developed beneath the enameloid in the crown and contains long, sub−parallel, sometimes branched tubules. The dentine tubules are evenly distributed over the crown and show a weak fan−shaped radiation from the central cavity ( Fig. 6D ). The central pulp cavity is clearly developed ( Fig. 6B ), now filled with sediment ( Fig. 6A ). All the teeth of L. sardiniens sp. nov., which have been examined for histology show a distinct orthodont internal structure (sensu Reif 1973).

Description of the fin spines.—( Fig. 7A–E ) Fin spines are only preserved as many small fragments, measuring from 0.30–7.0 mm in length, 1.30 mm in width and 0.50–1.50 mm in height. Some larger specimens possess a gently curved posterior face ( Fig. 7A ). The lateral faces of the spines are well ornamented with continuous smooth longitudinal costae ( Fig. 7A–E ). Normally four costae are present laterally ( Fig. 7D ); the more proximal part of the spine is unknown. The intercostal spaces are comparatively wider in the anterior part of the spine ( Fig. 7C ). Irregular foramina lie between the costae ( Fig. 7C ). Neither costal bifurcation nor anastomosis are observed. Toward the spine tip, the number of costae decreases to three. One strong costa forms a keel along the anterior border of the spine ( Fig. 7D, E ).

Hooked denticles are arranged in a single median row along the posterior face of the spine ( Fig. 7A–C ). The denticles are sharp, laterally compressed, longer than high and possess a strong dorsal crest ( Fig. 7C ). The length of each denticle is about 0.50 mm and the width is about 0.25 mm. Many fragments show a tight array of denticles, which suggests a closed denticle row on a complete fin spine. An exception is provided by FG 589/F/005 ( Fig. 7B ), which shows a small space between the single denticles. The denticles are weakly arcuate laterally and slightly displaced alternately to the left and right of the midline. Toward the spine tip the height of the denticles decreases noticeably. There would be approximately 20 denticles per cm in a complete fin spine. The posterior spine face also displays two smooth, small costae and small foramina marginally.

The subovoid cross section shows an outer, cavernous, highly vascularised layer of osteodentine, an inner lamellar layer with few canals and a large central cavity ( Fig. 7D, E ).

Description of the dermal denticles.—( Fig. 7F–O ) Three basic morphotypes can be distinguished amongst the scales:

Scale morphotype 1 ( Fig. 7F–I ): These are non−growing scales, measuring up to 1.34 mm in length and 0.89 mm in height. The crown is centrally placed, upright with a single central cusp, which is usually cone− or dome−shaped, and in

Fig. 4. Teeth of hybondontoid shark Lissodus sardiniensis sp. nov. Morphotype I, Gzhelian– Asselian of Guardia Pisano , Sardinia, Italy. A. Holotype FG H 589/T/027, labial (A 1), occlusal (A 2), and lingual (A 3) views. B. Paratype FG 589/T/031, labial (B 1), occlusal (B 2), and lingual (B 3) views. C. FG 589/T/032, labial (C 1), occlusal (C 2), and lingual (C 3) views. D. Paratype FG 589/T/028, labial (D 1), occlusal (D 2), and lingual (D 3) views. E. FG 589/T/023, lingual view. F. FG 589/T/025, lingual view. G. FG 589/T/010, lateral view. H. FG 589/T/009, labial view. I. FG 589/T/018, labial view. J. FG 589/T/019, oblique labial view. Scale bars 100 µm .

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some cases more thorn−like ( Fig. 7G ) and slightly curved posteriorly. The crown surface is ornamented with numerous strong vertical ridges that meet at the crown apex ( Fig. 7F–I ). In specimen FG 589/S/004 ( Fig. 7F ) the ridges bifurcate twice or more. A distinct neck is missing in the flat dome−shaped scales but developed in more thorn−like specimens. The base is wider than the crown to all margins and the outline is multipetaloid. The undersurface of the base is slightly concave, and the surface carries radial ridges, which are partial continuations of the crown ridges; numerous foramina for basal canals occur on all sides. This scale−type comprises less than 15% of the total dermal denticle assemblage.

Scale morphotype 2 ( Fig. 7J–L ): Like morphotype 1, these are non−growing scales, measuring up to 0.73 mm in length and 0.54 mm in height. The upright crown is lanceolate, strongly compressed laterally, and the posterior cusp is sharply curved backwards so that in lateral view it appears hook−shaped ( Fig. 7J ). A sharp median crest bifurcates the anterior rim resulting in a crown ornament with three strong vertical ridges ( Fig. 7K 2). Laterally, on the crown surface of some specimens further moderate ridges are developed. The crown is situated centrally on a large base ( Fig. 7K ). The sub−crown is smooth and restricted laterally by a ridge on each side. In some specimens a mesial ridge can be recognised on the sub−crown. The neck is not very well developed. The base is wider than the crown to all margins ( Fig. 7K 2), and the undersurface is slightly concave from below. The outline of the base is multipetaloid and its surface carries radial ridges, which are partial continuations of the crown ridges. Numerous foramina occur on all sides of the scale ( Fig. 7K ). A small number of specimens (~5% of all scales) present the basal fusion of two unicuspid scales of this morphotype forming a multicuspid scale ( Fig. 7L ). This hook−like morphotype represents ~70% of all scales and is the most common scale−type in the microfossil sample.

Scale morphotype 3 ( Fig. 7M–O ): These are growing scales, measuring up to 0.60 mm in length and 0.74 mm in height. The crown stands upright, is very elongate laterally but very thin antero−posteriorly ( Fig. 7M 1, N), exhibiting several strong to moderate ridges on the convex anterior side ( Fig. 7M–O ) whereas the concave posterior side is completely smooth. Up to six posterior sharp cusps are developed in this complex scale−type. The neck is moderate and vascular canals are visible near the crown/base junction ( Fig. 7N ). The base is poorly preserved in most specimens, and wider than the crown. The basal surface outline is multipetaloid to cycloid, and radial ridges on the surface are weakly developed. The undersurface of the base is strongly concave with a central pulp cavity. This scale−type forms ~15% of the total of scales in the sample.

Discussion of the teeth.—The teeth from Guardia Pisano show the presence of some diagnostic features of the genus Lissodus ( Duffin 1985; Rees and Underwood 2002): a crown with a triangular contour, a well developed central cusp, flanked by smaller lateral cusplets, a moderate to strong occlusal crest, a strong labial peg, a lingually inclined root that is narrower than the crown, and a single row of small foramina near the crown/root junction.

In addition, the teeth also share some diagnostic features of Lonchidion as determined by Rees and Underwood (2002): the teeth are extremely gracile, only 0.34 mm long in some specimens, the root is generally wider than the lowermost part of the crown with a strongly concave labial side.

Altogether, the character combination found in the teeth from Guardia Pisano most clearly resembles in certain respects that of the Palaeozoic teeth belonging to L. cf. zideki ( Soler−Gijón 1993) , L. lopezae Soler−Gijón, 1997 , L. lacustris Gebhardt, 1988 , L. sp. (subtype no. 107 of Tway and Zidek 1983), L. sp. (NM) ( Hampe 1996), and L. zideki ( Johnson, 1981) because of the symmetrical, mostly non−ornamented crown with a distinctive occlusal crest, triangular outline in occlusal view and pointed but prominently labially inclined central cusp. All these taxa are from the Late Palaeozoic and assigned to Lissodus after Duffin (1985) but classified as “Palaeozoic genus 1” in open nomenclature by Rees and Underwood (2002). This arrangement into a separate group besides Lissodus was justified by the specific character combination of the teeth (“… the labially inclined cusps and the lack of lateral cusplets, in combination with the heterodonty pattern ...”), which would be atypical for the morphological range of Lissodus according to Rees and Underwood (2002: 477). However, an inclined prominent cusp is not restricted to these forms, as it is also known from Mesozoic species such as Lonchidion selachos . Moreover, not all of the teeth of these Palaeozoic species lack lateral cusplets—see for instance, L. cf. zideki , L. zideki , and L. sp. (subtype no. 107 of Tway and Zidek 1983). Altogether, the separation of Palaeozoic species as “Palaeozoic genus 1” is not convincing. Therefore, the former assignment of these species to Lissodus by Duffin (1985) and subsequent authors is retained and the Sardinian specimens are attributed to Lissodus .

The Palaeozoic species closest to L. sardiniensis sp. nov. are L. cf. zideki ( Soler−Gijón 1993) and L. lopezae Soler−Gijón, 1997 from the Late Carboniferous (Stephanian doi:10.4202/app.2009.0019

C = Gzhelian–Asselian) of Puertollano in central Spain. Teeth of these three species are tiny, overlapping in size from L. cf. zideki (0.31–0.62 mm) over L. sardiniensis sp. nov. (0.34–1.31 mm) to L. lopezae (0.95–1.19 mm). They also share a tiny subterminal cusplet on the labial peg, although the Spanish species lack well−defined lateral cusplets. Furthermore, nodes and a longitudinal ridge along the labial crown shoulder are present in both Spanish species but are absent in L. sardiniensis sp. nov.

Lissodus lacustris Gebhardt, 1988 from the Late Carboniferous (Stephanian C) of Germany differs from the Sardinian specimens in the presence of nodes on a clearly crenulated crown shoulder, weak or absent lateral cusplets and a labial root buttress.

Lissodus zideki ( Johnson, 1981) from the late Early Permian (late Artinskian−Kungurian) of Texas, USA. differs from L. sardiniensis sp. nov. with incipient or absent lateral cusplets, occasionally labial nodes and size (1.5–2.0 mm).

Lissodus sp. (subtype no. 107 of Tway and Zidek 1983) from the Late Carboniferous (Stephanian C) of Kansas differs in possessing incipient cusplets and lacks a subterminal cusplet on the labial peg.

Lissodus sp. (NM) ( Hampe 1996) from the Early Permian (Asselian) of Germany differs in possessing a poorly developed labial peg, a vertical ridge from the central cusp to the labial peg, the absence of lateral cusplets and a mesiodistal length of 2.0–4.0 mm.

The Mesozoic species closest in morphology to L. sardiniensis sp. nov. is Lonchidion selachos Estes, 1964 from the Late Cretaceous (Campanian–Maastrichtian) of Wyoming, USA. The two species share a non−ornamented crown, a tiny subterminal cusplet on the labial peg, and a labially inclined central cusp. In addition, some symphyseal teeth show the same triangular shape of the crown with a prominent cusp and one pair of curved lateral cusplets ( Estes 1964: fig. 2b). However, only some symphyseal teeth of Lonchidion selachos develop this distinctive shape, whereas it is a universal feature in L. sardiniensis sp. nov.

Histologically, teeth with one layer of SCE belong to the “ α tooth type ” of Reif (1973). The fan−shaped radiation of dentinal tubules from the central cavity is similar to structures described in teeth of Lonchidion by Estes (1964: fig. 2d); see also Patterson (1966: pl. 5: 1), and Heckert et al. (2007). The absence of an osteodentine core in any of the available teeth distinguish the Sardinian teeth from those of Lissodis zideki ( Johnson, 1981) , and L. angulatus Stensiö, 1921 from the Lower Triassic of Spitsbergen. The latter show two types of histology (osteodentine− and orthodentine type) within one taxon ( Błażejowski 2004) whereas L. sardiniens sp. nov. is exclusively orthodont.

In spite of all the similarities with Palaeozoic and also Mesozoic species of Lissodus and Lonchidion , the teeth from Guardia Pisano differ significantly from all other published species especially in one characteristic feature. Exclusively L. sardiniensis sp. nov. alone possesses a single prominent central cusp, which is flanked by one pair of curved, smaller lateral cusplets in all of its teeth. On the basis of a hypothetical reconstruction of a dentition of L. nodosus ( Seilacher, 1943) by Duffin (1985: fig. 12) the Sardinian morphotype I probably represents a mesial to anterolateral position whereas morphotype II being derived from a symphyseal position. The small root in the latter suggests that they were most likely not posteriors because of the crushing forces involved at the posterior end of the jaw in durophagous sharks. Morphotype III most likely occupied a lateral position because of its size and the more elongate shape.

Discussion of the fin spines.—The histology of the fin spine fragments from Guardia Pisano corresponds exactly to that described for hybodontiform fin spines by Maisey (1978). Unfortunately, isolated dorsal fin spines of hybodont sharks can only be assigned to the generic level. Applying the argument of Milner and Kirkland (2006) fin spines of Lissodus are ornamented laterally by costae, whereas those of Lonchidion are characterised by smooth lateral sides, with the exception of Lonchidion humblei Murry, 1981 , which has costae. For that reason Milner and Kirkland (2006) suggested that Lonchidion humblei should be assigned to a taxon other than Lonchidion ; in our opinion this should be referred to Lissodus . The Sardinian spine material can only be compared with material of Palaeozoic and Mesozoic hybodontoid species showing laterally ornamentation.

Spines of L. africanus ( Broom, 1909) from the Early Anisian of South Africa ( Brough 1935), and L. cassangensis ( Teixeira, 1956) from the Scythian of Angola ( Antunes et al. 1990) bear a double row of denticles along each posterolateral margin, in contrast to the Sardinian spine fragments. Furthermore, the African species are ornamented with six to seven costae whereas in the Sardinian fragments no more than four costae are present.

Comparison with Lissodus (Lonchidion) humblei Murry, 1981 from the Late Triassic (Carnian–Rhaetian) of the southern USA reveals differences in the number of costae with up to 12 at the proximal end of the spine and the development of two parallel denticle rows proximally in the American fin spines.

Dorsal fin spine material of Lonchidion sp. from the British Wealden (Tithonian–Berriasian) ( Patterson 1966) shows many similarities with the Sardinian fragments. Up to five costae are present, showing no bifurcation or anastomosis and a single median row of hook−like denticles is also developed. Differences include the wider array of single denticles in the median row and a length of about 70 mm, which most likely was not reached by the Sardinian specimens.

Isolated fin spines from the Late Carboniferous (Stephanian C) of the Saale Basin, Germany, which were first questionably assigned to Limnoselache vincinalis by Schnei− der (1986: figs. 2a–c, pl. 1: 6–8) and subsequently attributed to Lissodus lacustris by Soler−Gijón (1997: 162), show an anterior keel along the entire spine length, and six smooth longitudinal costae laterally, of which only two to three reach the distal end of the spine. Bifurcation or anastomosis is ab−

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sent, a median denticle row is present and the cross section is similar to that described for L. sardiniensis sp. nov. Differences include a higher number of lateral costae, an average number of six denticles per centimetre, and the wider distance between the single denticles.

Soler−Gijón (1997: fig. 6, pl. 2: 9) described spines from the Late Carboniferous (Stephanian C) of the Puertollano Basin, Spain, which he assigned to L. lopezae . These correspond to the Sardinian remains in nearly all morphological criteria except the number of denticles per centimetre (six in L. lopezae ), which in turn correlates with the spine material described by Schneider (1986) probably belonging to L. lacustris based on associated teeth from the same horizon.

Because of the co−occurrence in Sardinia hybodontiform spine fragments and teeth from the same horizon, and the absence of any other hybodontiform shark remains in these beds, the most parsimonious explanation is that both belong to the same species. Based on the size of the single fragments the original fin length can only be estimated at 40–50 mm, corresponding roughly to the size of the spine material of the other Palaeozoic Lissodus species described by Schneider (1986) and Soler−Gijón (1997). Neither is it easy to determine how many fin spines are represented in the collection, although the huge number of fragments indicates the presence of more than one original spine. Differences in size of the fragments or in the distance between denticles possibly represent individual variation. Furthermore, it is possible that some spines are from juveniles and others from adults. However, this question cannot be answered from the available isolated material. Discussion of the dermal denticles.—Morphotype 1 is of strong hybodontoid affinity. It agrees well morphologically with some cone−shaped scales from the lower jaw and the roof of the mouth cavity of Hybodus delabechei Charlesworth, 1839 from the Early Jurassic (Sinemurian) of England ( Reif 1978: fig. 2a) as well as with unidentified scales of “hybodontiform morphotype 1” from the Late Jurassic (Kimmeridgian) of northern Germany ( Thies 1995: fig. 4a–d). Maisey (1983: fig. 23c, d) found such scales in the head region of Hybodus basanus from the Lower Cretaceous of England. Furthermore, Delsate et al. (2002: figs. 17−1b, pl. 10a) described similar scales of a undetermined “hybodontiform type 2, group a” from the Early Jurassic (Middle Hettangian) of South Belgium. Therefore, the record from Guardia Pisano extends the record of scales of this morphotype from the genus Hybodus as questioned by Thies (1995), to Lissodus . Moreover, Schnei− der (1986: pl. 3: 6, 8) assigned scales from the Late Carboniferous (Stephanian C) of the Saale Basin, Germany to Limnoselache vicinalis (= Sphenacanthus Soler−Gijón 1997 ), which show a similar crown shape but a convex basal plate in lateral view. The same scale type from the same locality was also described by Gebhardt (1986: pl. 1: 3) as “ type H d2”, there with a more hook−like shape in lateral view. From the Early Permian of the middle and southern Urals Ivanov (2005: fig. 5J) described a “ Petrodus ” type denticle that shares this morphology. Duffin (1985) reported entirely simple, stud−like scales with upright crowns from the squamation of L. africanus , which are similar to morphotype 1. Duffin (1993: fig. 14d, e) described simple, stud−like scales with bifurcate vertical ridges of an undetermined “ type 2”. Rees (2002: fig. 9.1–3) described similar hybodontoid scales from the earliest Cretaceous Vitabäck Clays of southern Sweden as “morphotype 1”. This simple hybodontoid scale morphotype ( Reif 1978) is also known in all articulated hybodont specimens of the Jurassic and Cretaceous ( Duffin 1993).

Morphotype 2 is also considered to be of hybodontoid affinity. It is morphologically similar to thorn−shaped scales of Hybodus delabechei Charlesworth, 1839 from the Early Jurassic (Sinemurian) of England ( Reif 1978: fig. 2d) as well as with some unidentified scales of the “hybodontiform morphotypes 2 and 3” from the Late Jurassic (Kimmeridgian) of northern Germany ( Thies 1995: fig. 4f–i). It also resembles specimens described from the Early Jurassic (Middle Hettangian) of southern Belgium from undetermined “hybodontiform scale−type 2, group b” by Delsate et al. (2002: fig. 18, pl. 10c). Hampe (1996: figs. 7a–c) described as “morphotype 2A” similar lanceolate, posteriorly recurved and keeled scales of L. sp. (NM) from the Early Permian (Lower Rotliegend) of Germany. Other unicuspid denticles with lanceolate cusps curved posteriorly are known from the Late Carboniferous (Stephanian C) of the Saale Basin in Germany called “ type F d3” and “d6” by Gebhardt (1986: pl. 3: 1,4); these undetermined dermal denticles are from the same horizon as L. lacustris Gebhardt, 1988 and are very similar to morphotype 2 material from Sardinia in showing a smooth crown surface with strong anterior ridges and a median posterior crest. The only difference is the narrow basal plate in the German material. Rees (2002: fig. 9.4) documented a similar scale as “morphotype 3” from the Cretaceous of southern Sweden. In Recent sharks, Squalus acanthias possesses similar scales with a single lanceolate and backwards−curved crown and a polygonal base in the posterior part of the oral cavity ( Reif 1985: pl. 8, M2). Multicuspid scales similar to the fused specimens of morphotype 2 are described by Reif (1978: fig. 8d, e) for Hybodus delabechei and Reif (1985: pl. 15) for placoid scales of the Recent shark Echinorhinus brucus . These primary unicuspid scales become fused at their bases in the case of irregular spacing during formation−time. Such scales cannot be regarded as growing scales ( Reif 1978). The frequency of scales of morphotype 2 in the microfossil sample (~70% of all scales) probably indicates that this scale−type was the principal squamation morphotype of L. sardiniensis sp. nov. covering the bulk of the shark’s body.

Morphotype 3 strongly resembles a scale referred to Ctenacanthus from the Late Permian of Greenland ( Reif 1978: fig. 1e). However, growth rings on the lower side on the base are not recognisable in our morphotype 3. Mader and Schultze (1987: fig. 4a, b) described two different undetermined scales from the Early Carboniferous (Viséan) of western Germany showing a serrated crown of several separated ridges. Gebhardt (1986: pl. 1: 2) described similar scales from the Stephanian Wettin Subformation of Germany as “ type H d1”, which possess at least two lanceolate ridges forming a multicuspid shape but with a more cylindrical crown. Moreover, Soler−Gijón (1997: pl. 2: 1) showed a multicuspid scale from the Late Carboniferous (Stephanian C) of Spain, which he assigned to the?ctenacanthid Sphenacanthus carbonarius , and which resembles the rake−like shape, but with a convex basal undersurface and a round crown base. Masson and Rust (1983: fig. 7) described an undetermined elasmobranch denticle from the Late Pennsylvanian Morian Group of the Sydney Basin, Nova Scotia, Canada, which resembles the multicuspid rake−shape of morphotype 3 in lateral view. Ginter and Sun (2007: fig. 13E 1, E 2) displayed such scales from the Early Carboniferous (Tournaisian) of Muhua, southern China, identifying them as ctenacanth scales. A scale assemblage from the Early Permian of the Middle and Southern Urals also contains a similar scale, described as “ Listracanthus ” denticles by Ivanov (2005: fig. 5L). Finally, Johns (1996: pl. 2: 7) created a key to Triassic elasmobranch scales from north−eastern British Columbia, Canada, which contains a similar scale−type with lanceolate and inclined crown with multiple paired ridges. Interestingly Johns (1996) assigned this scale−type to the hybodontoid scale morphotype after Reif (1978). The same assignment was done by Rees (2002) with a similar shaped “morphotype 6” from the Cretaceous of southern Sweden.

The assignment of morphotype 3 is difficult. Although these scales are most similar to the ctenacanthid morphotype of Reif (1978), no other remains of ctenacanthid sharks were found in the Sardinian samples. Furthermore, the scales described by Gebhardt (1986) are from the same stratigraphic level as remains of L. lacustris Gebhardt, 1988 , and the scales described by Soler−Gijón (1997) are from the same stratigraphic level as remains of L. lopezae Soler−Gijón, 1997 . It seems to be a strong possibility that the scales from Germany and Spain in fact belong to Lissodus . This characteristic scale−morphotype probably represents a primitive complex scale form that occurred since the Devonian in ctenacanthid ( Basden et al. 2006: fig. 11) as well as in hybodont sharks but because of the disarticulated hybodontoid remains especially from the Palaeozoic this cannot be verified.

Assignment to generic or even species level based on disarticulated scales is extremely difficult because most fossil and also Recent sharks show heterosquamation ( Reif 1985; Johns 1996). The scale morphology varies greatly from one elasmobranch family to another, from one genus to another within the same family and also within one species according to ontogenetic stage, region of the body, between specimens of different size and even between different gen− der ( Reif 1974; Cappetta 1987; Kemp 1999). So far placoid scales possess low taxonomic significance because of this wide variability ( Reif 1985; Thies 1995; Duffin 1999). Because of this and the poor record of scales from articulated squamations from a single elasmobranch species fossil shark scales can often only be assigned to the familial level. However, the co−occurrence of undoubtedly hybodontiform scales and teeth from the same stratigraphical horizon of Guardia Pisano supports the assignment to the same taxon as above for the spines.

After comparison with other described material, the scale assemblage from Guardia Pisano shows greatest affinity with specimens described by Gebhardt (1986) from the Late Carboniferous (Stephanian C) of the Saale Basin, Germany, which is also the type locality of L. lacustris Gebhardt, 1988 . Because the scales are disarticulated, the position on the shark’s body is only generally determinable.

Based on the above discussion the material from the Gzhelian–Asselian of the Guardia Pisano Basin of Sardinia is referred to the new species Lissodus sardiniensis sp. nov., encompassing teeth, fin spines, and dermal denticles.

Figures

Fig. 5. Teeth of hybondontoid shark Lissodus sardiniensis sp. nov. Morphotype II (A–H) and morphotype III (I–N), Gzhelian–Asselian of Guardia Pisano, Sardinia, Italy. A. Paratype FG 589/T/059, labial (A1), occlusal (A2), and lingual (A3) views. B. Paratype FG 589/T/060, labial (B1), occlusal (B2), and oblique lingual (B3) views. C. FG 589/T/051, labial view. D. FG 589/T/052, labial view. E. FG 589/T/053, labial view. F. FG 589/T/058, lingual view. G. FG 589/T/055, lingual view. H. FG 589/T/056, lingu−lateral view. I. FG 589/T/062, labial view. J. FG 589/T/066, lingual view. K. FG 589/T/061, labial view. L. Crown FG 589/T/065, lingual view. M. Crown FG 589/T/064, labial view. N. Crown FG 589/T/063, labial view. Scale bars 100 µm.

Fig. 7. Spine fragments (A–E) and dermal denticles: morphotype 1 (F–I), morphotype 2 (J–L), and morphotype 3 (M–O) of hybondontoid shark Lissodus sardiniensi sp. nov., Gzehlian–Asselian of Guardia Pisano, Sardinia, Italy. A. FG 589/F/001, lateral view. B. FG 589/F/005, lateral view. C. FG 589/F/002, lateral view. D. FG 589/F/003, cross−section with anterior side above. E. FG 589/F/004, cross−section of the anterior side with the keel. F. FG 589/S/004, oblique lateral view. G. FG 589/S/009, oblique lateral view. H. FG 589/S/010, oblique lateral view. I. FG 589/S/011, oblique lateral view. J. FG 589/S/002, lateral view. K. FG 589/S/001, oblique lateral (K1), dorsal (K2) views. L. coalesced denticle FG 589/S/003, oblique dorsal view. M. FG 589/S/007, lateral (M1), anterior (M2) views. N. FG 589/S/006, oblique lateral view. O. FG 589/S/005, anterior view. Scale bars: A, 1 mm, B–G, 300 µm, H–O, 100 µm.

Fig. 3. Diagrammatic illustrations of teeth, fin spines and dermal denticles of Lissodus sardiniensis sp. nov. A. Tooth (following Duffin 1985), labial (A1), lingual (A2), occlusal (A3), and lateral (A4) views. B. Fin spine (modified from Schneider 1986), posterior (B1), lateral (B2) views, cross−section (B3). C. Denticles (following Thies 1995), oblique anterior (C1), and lateral (C2) views.

Fig. 6. Thin sections of teeth of hybondontoid shark Lissodus sardiniensis sp. nov. shown under ordinary light, Gzhelian–Asselian of Guardia Pisano, Sardinia, Italy. A. FG 589/T/033 complete tooth. B. FG 589/T/034 complete tooth. C. FG 589/T/035 tooth crown. D. FG 589/T/037 tooth crown. Scale bars 100 µm.