Rhomphaiodon sp.

Rhomphaiodon sp.

( Figure 3 View FIGURE 3 )

Specimens referred. NHMD–1811668 – tooth; NHMD–1811669 – tooth; NHMD–1811670 – tooth; and 69 additional teeth stored under the collections number NHMD–1811711.

Locality. From site 62/91/G, western side of Tait Bjerg , Jameson Land, East Greenland ( 71°28´34´´ North 22°40´43´´ West) GoogleMaps .

Horizon and age. Thin bone bed in Carlsberg Fjord Member of the Ørsted Dal Formation in the Fleming Fjord Group. Late Triassic (Norian).

Description. The teeth of this species are the third most abundant remains from site 62/91/G, with 72 specimens. The size is around 1.5--2 mm, but it can reach 3 mm apicobasally and 2 mm mesiodistally. Teeth are symmetrical tricuspids ( Figure 3A View FIGURE 3 , D-E), pentacuspid ( Figure 3B View FIGURE 3 ), or heptacuspid ( Figure 3C View FIGURE 3 ), but they can be asymmetrical. They display one principal cusp and up to three pairs of lateral cusplets. In rare cases, the number of cusplets is unequal on each side. The connection between the cusp and cusplets has the shape of a V, with an acute angle, sometimes even a right angle in some specimens. Principal cusp is triangular in labial and lingual views. The edges of the triangles can be straight or slightly curved. The principal cusp can be conical or shaped as a grain of rice in occlusal view. It can be curved lingually and occasionally laterally curved, as for the lateral cusplets. The principal cusp is one to 3 mm, and the lateral cusplets decrease in size towards the distal end. On the teeth with a 1 mm high principal cusp, a pattern can be observed in the decrease in size of the lateral cusplets. The first pair of lateral cusplets are inferior to or equal to half of the principal cusp height. When there is a second pair of lateral cusplets, they are half the height of the first pair of lateral cusplets. If there is one, this pattern continues for the third pair of lateral cusplets. Vertical striations run from the apices of the cusp and cusplets to the base of the crown on the lingual and labial sides. The root is usually present. It is lower but wider than the crown, and its height is around one-fifth of the tooth total height or even less than half of the crown in the highest teeth in lingual view. The root is more elevated in labial view, around one-third of the tooth. The base can be curved or completely flat. The root is more extended in its lingual part and gives the shape of a wall to the front of the tooth, as the cusps are close to the labial part. The lingual part of the root is extended lingually forming a broad, bulbous structure called the lingual torus ( Figure 3D View FIGURE 3 ). The root possesses multiple foramina aligned on both lingual and labial sides. The foramina are close to the base on the labial, and sometimes the pits are not closed. On the lingual side, some foramina are more elongated vertically.

Remarks. The identification is supported by the presence of a crown profile with very high principal cusp and lateral cusplets and the presence of lingual torus on the root. Two species belong to this genus, Rhomphaiodon minor (Agassiz, 1837) and Rhomphaiodon nicolensis Duffin, 1993b . The species R. minor was first described as Hybodus minor by Agassiz (1837) based on a dorsal fin spine. Similarities were noted between R. nicolensis and ‘ Hybodus’ minor by Duffin (1993b). Then, the isolated teeth of R. minor were assigned to the genus Rhomphaiodon by Cuny and Risnes (2005), based on the enameloid microstructure and the morphology, and identified as a member of the Synechodontiformes after a suggestion that the species shares vascularised root typical of Synechodontiformes (Cuny et al., 2000). The genus is common in Europe during the Late Triassic and Early Jurassic (Duffin, 1993b; Duffin and Delsate, 1993; Delsate and Godefroit, 1995; Cuny et al., 2000; Cuny and Risnes, 2005; Norden et al., 2015).

Superclass OSTEICHTHYES Huxley, 1880

Class ACTINOPTERYGII Cope, 1887

Order SAURICHTHYIFORMES Aldinger, 1937

Family SAURICHTHYIDAE Owen, 1860 (sensu Stensiö, 1925)

Genus SAURICHTHYS Agassiz, 1834

Saurichthys sp.

( Figure 4 View FIGURE 4 A-C)

Specimens referred. NHMD–1811674 – tooth; NHMD–1811675 – tooth; NHMD–1811676 – tooth; and 32 additional teeth stored under the collections number NHMD–1811712.

Locality. From site 62/91/G and “No field” # 92/G, western side of Tait Bjerg , Jameson Land, East Greenland ( 71°28´34´´ North 22°40´43´´ West) GoogleMaps .

Horizon and age. Thin bone bed in Carlsberg Fjord Member of the Ørsted Dal Formation in the Fleming Fjord Group. Late Triassic (Norian).

Description. The 35 teeth identified as Saurichthys sp. are elongated and conical, with a circular cross-section. The teeth can be slender or stocky, slightly curved, or even sigmoidal in general shape ( Figure 4 View FIGURE 4 A-B). Their size varies considerably; the biggest is around 5.5 mm long ( Figure 4A View FIGURE 4 ), while the smallest measures 1.75 mm ( Figure 4C View FIGURE 4 ). The acrodin cap, the tip of the tooth, is smooth and can be translucent for the smallest specimens. The cap of the largest teeth can be pearl coloured at the base. There are strong vertical ridges all along the lower part of the teeth, named the shaft. The ridges are thicker closer to the base, giving a folded appearance to the flared base. The shaft and the cap are separated by a neck, a horizontal ridge of one to 2 mm in diameter. The height of the cap depends on the neck position, but usually, the cap corresponds to 10--30% of the total height of the tooth.

Remarks. The teeth are assigned to Saurichthys due to the relative height of the cap, of the total height of the tooth, and the vertically ridged shaft. The dentition of Saurichthys is composed of two distinct sizes: the largest, around 3 mm and the smallest, 1 mm long (see Kogan and Romano, 2016). The largest and longest caps are usually assigned to Saurichthys sp. , while the smallest specimens remain doubtful. In that case, they are counted as actinopterygian indeterminate teeth. The teeth have been compared to the teeth of Saurichthys longidens Agassiz, 1844 , described in a dozen articles on the microvertebrate fauna of the Rhaetian of England. The morphology of the teeth of Birgeria acuminata differs from that of Saurichthys longidens by being larger with ridges on the lingual side of the cap and both taxa have been described as two morphotypes of Severnichthys acuminatus (Agassiz, 1844) in a dozen of studies on the fauna of the English Rhaetian (Allard et al., 2015; Korneisel et al., 2015; Norden et al., 2015; Lakin et al., 2016; Mears et al., 2016; Slater et al., 2016; Landon et al., 2017; Cavicchini et al., 2018; Cross et al., 2018; Moreau et al., 2021). However, Diependaal and Reumer (2021) considered the genus Severnichthys as a nomen dubium by arguing that Birgeria and Saurichthys are two different animals. Considering the hypothesis of Diependaal and Reumer (2021), and the absence of any teeth sharing a similar morphology with Birgeria among our material, it seems that our material tends to support that said hypothesis. Regarding the material from Greenland, it was not possible to identify to the species-level.

The genus has a global distribution, with records in the Lower Triassic of Alberta, British Columbia, China, France, Great Britain, Greenland, Spitsbergen, Madagascar, Nepal, Tasmania, Australia, Russia, and South Africa (Rieppel, 1992; Bender and Hancox, 2004). The oldest Saurichthys ever found is from the Early Triassic Wordie Creek Formation of East Greenland (Kogan, 2011). Its presence was mainly restricted to the north-western Tethys in the Late Triassic, especially in Europe (Romano et al., 2012). The genus was also observed in the Fleming Fjord Formation, only reported as Saurichthys sp. by Jenkins et al. (1994) but first described here.