Gyrodus sp.

Kriwet, Jürgen & Schmitz, Lars, 2005, New insight into the distribution and palaeobiology of the pycnodont fish Gyrodus, Acta Palaeontologica Polonica 50 (1), pp. 49-56 : 50-52

Gyrodus sp.

Description.—Specimen NLH 102.971 is an almost complete right prearticular with dentition. Only the tip of the rather massive prearticular bone and parts of the coronoid process are missing. The symphyseal area is rather deep and antero−posteriorly elongated ( Fig. 2A 1 , A 2 ). The base of the lateral coronoid process is broad. The dentition consists of crushing teeth. The tritorial teeth are arranged in four longitudinal rows without intercalating teeth between them. The principal row comprises transversely elongated and obliquely placed teeth ( Fig. 2A 1 ). The main teeth are much broader than any of those positioned in medial or lateral rows. In occlusal view, they expose an oval outline with a transverse coronal indent surrounded by a well−marked and crimped wall. The medial edge of the teeth is tapering forming a low and blunt, displaced apex. A second, marginal wall delimiting a shallow, outer groove and following the outer tooth margin is usually developed. Anteriorly, the groove is deepest. Teeth of the medial tooth row are much smaller and restricted to the anterior half of the dentition ( Fig. 2A 1 ). Although more rounded, they display the same occlusal morphology as the principal teeth. However, a low and medially displaced apex on the wall is only developed in the posteriormost teeth. The main row is flanked laterally by two tooth rows. Teeth of the first lateral row are much smaller and more rounded. They are arranged with their long axes parallel to the labial margin. The central indent is very shallow and inconspicuous; thus, the walls meet and form a very blunt apex accompanied laterally by an indent. Teeth of the second lateral row are almost as large as the principal teeth and display generally the same morphology. The long axes of these teeth are arranged perpendicular to the labial margin. The lateral apex is well developed but blunt. A very small and displaced central tubercle is present in the apical furrow of most teeth. The posterior part of the outer rim is absent in several teeth.

The second specimen (NLH 102.972) is a fragmentary prearticular dentition and consists of three teeth of the outermost and two smaller teeth of the first lateral row ( Fig. 2B ). The teeth exhibit the same sculpture as those of the complete specimen. Differences are found in the broader central furrow and the more reduced lateral apex. A central and displaced small tubercle is present in the apical indent of the first lateral tooth.

Discussion.—The combination of four longitudinal prearticular tooth rows without intercalating teeth with generally two crenulated apical rings, a tubercle in the apical indent in at least some teeth, and a medial or lateral apex on the inner wall are considered characteristic for Gyrodus dentitions ( Thies 1985; Kriwet 2000). The two specimens from the Lower Cretaceous of northern Germany display these characters and are accordingly assigned to Gyrodus . Gyrodus is one of the most commonly cited pycnodont genera in the Late Jurassic, with about 32 described species (e.g., Wagner 1851; Woodward 1895; Lambers 1991). Most of these species, however, may not be valid. Only two species: Gyrodus hexagonus (de Blainville, 1818) and G. circularis Agassiz, 1833 were based on more than isolated remains including articulated skeletons from the Late Jurassic (Kimmeridgian and Tithonian) of the Solnhofen and Nusplingen plattenkalks of southern Germany.

A second genus, Polygyrodus White, 1927 may be related to Gyrodus ( Kriwet, 2001a) . The only species, P. cretaceus (Agassiz, 1844) from the Turonian–Campanian of England, Switzerland, and Germany ( Roemer 1841; Woodward 1895; White 1927), is known from isolated dentitions and its assignment to Gyrodontidae might be disputable, because this assumption is based on similarities in dental structures. Teeth of Polygyrodus differ in the arrangement and occlusal sculpture (compare Woodward 1909: pl. 35: 5, 6). The most notable differences to teeth of Gyrodus are the elevated and obtusely acuminate crown with a coarsely wrinkled and irregular apex, and a single, very narrow groove that is generally restricted to one side of the crown. In addition, the teeth are very irregularly arranged so that distinct tooth rows are difficult to identify.

A comparison with all described species of Gyrodus is difficult, because most of the species are based on isolated and fragmentary dental remains and the prearticular dentition in many specimens is not known. Therefore, we compared the dentitions mainly with those of G. hexagonus ( Fig. 2C–E ) and G. circularis ( Fig. 2F ), which are the only two Gyrodus species established for articulated skeletal remains. Species based on isolated dental remains and assigned to Gyrodus were also considered. The two Early Cretaceous specimens described herein resemble G. hexagonus in tooth morphology and sculpture ( Fig. 2C–E ), especially in the presence of an occlusal indent with a central tubercle in several teeth that is surrounded by a crimped wall with an additional anterior one ( Fig. 2E ). The occlusal prearticular tooth surfaces of G. circularis are, conversely, more strongly sculptured than those of G. hexagonus and the specimens presented here when unworn with a less developed central indent and a more wrinkled wall surrounding it ( Fig. 2F ). In addition, the posterior main teeth lack the central apical indent; instead, they expose a broad and blunt, irregularly wrinkled and folded apex (compare also Kriwet 2000: fig. 6). Continuous wear results in constant loss of the crimped walls and the lateral blunt apex, and may render identifications difficult ( Fig. 2B ). Despite the differences to some Gyrodus species (e.g., G. cricularis ) and similarities to others (e.g., G. hexagonus ), an assignment to any Gyrodus species of the specimens described here is impossible without more complete material. Consequently, we leave both specimens in open nomenclature. Minor morphological differences between both specimens may fall within the intraspecific variation of tooth ornamentation or may be the result of wear. However, because of the different stratigraphic occurrences both specimens may belong to different species.

Figures

Fig. 2. Dental remains of Gyrodus spp. A. Right prearticular dentition of Gyrodus sp. (NLH 102.971) from the lower Barremian of clay pit “Gott”; occlusal (A1), mesial (A2), and lateral (A3) views. B. Fragmentary prearticular dentition of Gyrodus sp. (NLH 102.972) from the Hauterivian of clay pit “Engelbostel”; occlusal view. C. Articulated prearticulars of Gyrodus hexagonus (de Blainville, 1818) (MB.f. 1345) from the Kimmeridgian of Solnhofen, Bavaria; occlusal view. D. Posterior principle prearticular teeth of Gyrodus hexagonus (de Blainville, 1818) (BSP−AS VII 1073) from the Kimmeridgian of Solnhofen, Bavaria; occlusal view. E. Latero−posterior prearticular tooth of Gyrodus hexagonus (de Blainville, 1818) (MB.f. 1367) from the Kimmeridgian of Solnhofen, Bavaria; occlusal view. F. Left prearticular of Gyrodus circularis Agassiz, 1844 (JM SOS 3130) from the Kimmeridgian of Wintershof, Bavaria; occlusal view. G. Unidentified pycnodont vomer (aff. Gyrodus? sp.) (MB.f.7133) from the Hauterivian, Lower Cretaceous, of Langenberg near Oker, northern Germany; occlusal view.H. Left prearticular of an unidentified pycnodont (aff.Gyrodus? sp.) (MB.f. 7233) from an Lower Cretaceous erratic of the Baltic Sea; occlusal view. Scale bars 5 mm.

Fig. 1. General sketch map of Germany (A) with position of the clay pits “Gott” and “Engelbostel” (B) that yielded the dental remains described herein. Abbreviations: E, clay pit “Engelbostel”; G, clay pit “Gott”; H, Hanover.