Aganaster jagiellonicus, Thuy & Kutscher & Płachno, 2015

Thuy, Ben, Kutscher, Manfred & Płachno, Bartosz J., 2015, A new brittle star from the early Carboniferous of Poland and its implications on Paleozoic modern-type ophiuroid systematics, Acta Palaeontologica Polonica 60 (4), pp. 923-929 : 924-927

Aganaster jagiellonicus sp. nov.

Figs. 2 , 3 .

Etymology: In honour of Jagiellonian University in Kraków, the oldest university in Poland and one of the oldest in Europe, and the Jagiellonian dynasty of Polish kings.

Holotype: Articulated skeleton with all five arms preserved, MZUJ T/0282 (only known specimen).

Type locality: Czatkowice quarry, Dębnik Massif, southern Poland. Type horizon: Mazurowe Doły Formation, Rudawa Group, late Tournaisian (Ivorian, MFZ 4 to 5 foraminiferal zones or RC3α to β rugose coral zones), early Carboniferous .

Diagnosis.—Species of Aganaster with lateral arm plates bearing at least three minute, conical arm spines on the distal edge of the lateral arm plates, shorter than one fifth of an arm segment; ventral arm plates hexagonal and both basal and succeeding proximal arm segments.

Description.—Disc circular, 9 mm in diameter, exposing ventral face, dorsal disc plating unknown; ventral interradii completely covered by few (no more than ten per interradius) plates, no granules or spines discernible, plates rounded, largest near disc edge; genital slits very slender, extending from beside the second arm segment to the edge of the disc; oral shield slightly shorter than one fourth of the disc radius, arrow-head shaped with pointed to almost right proximal angle, deeply incised latero-distal edges and rectangular distal end; madreporite indiscernible; adoral shields long and slender, extending around the lateral edge of the oral shield and separating it from the lateral arm plates of the first arm segment, proximal tips of adoral shields meeting in front of oral shield; proximal portions of oral plates of rather stout aspect; jaws approximately as long as wide (distance between the tip of the jaws and the boundary between the first ventral arm plate and the adoral shield nearly equal to the distance between the first ventral arm plates on both sides of the jaws); first ventral arm plate smaller than following ventral arm plates, nearly hexagonal; second oral tentacle pore opening into the mouth slit, not superficial, covered by single leaf-like lateral papilla, slightly longer than wide, sitting on the adradial edge of the adoral shield; at least four additional, slightly larger lateral oral papillae discernible, in continuous row and contiguous, leaf-like, nearly as wide as long, sitting on adradial edge of oral shield; pointed, conical papilla, two to three times smaller than above-described papillae, discernible on lateral edge of one jaw tip, either representing the proximalmost lateral papilla or a slightly displaced apical papilla; no teeth discernible.

All five arms preserved intact until median to distal arm segments; longest arm approximately 45 mm long (measured from the proximal edge of the first arm segment) and preserving at least 46 arm segments including those incorporated into the disc; arms very slightly increasing in width at the basalmost freestanding segments, then very slowly tapering over the next 10 segments, more quickly tapering over the following few segments and again very gently tapering towards tip of the arms; arm segments not bulging; lateral arm plates of thick aspect, devoid of conspicuous outer surface ornamentation or constriction, non-bulging, meeting ventrally from the first arm segments not incorporated into the disc; distal edge of lateral arm plates with three or more very small spine articulations, either sunken into depressions of distal lateral arm plate edge or integrated into outer surface stereom, structural details of spine articulations not discernible; very small, conical arm spines, parallel to the arm axis, shorter than one fifth of an arm segment.

Lateral arm plates of proximalmost 15 arm segments including those incorporated into the disc with deep, conspicuous between-plate tentacle pores (3 arms scorable); ventro-distal edge of tentacle notch in lateral arm plates lined by a thin, thickened and sharply defined ridge; lateral arm plates of following arm segments with a small, hardly discernible within-plate tentacle pore near ventro-distal tip; two large, leaf-like tentacle scales on most (1st to 14th) of arm segments with between-plate tentacle pores, probably only one tentacle scale on distalmost (15th) segment with between-plate tentacle pore, no scales discernible on (16th and more distal) arm segments with within-plate tentacle pores; tentacle scales slightly oblique to arm axis, most probably pointing proximalwards or towards the arm midline and borne by the edge of the tentacle notch of the lateral arm plate; ventral arm plates of the proximalmost 14 to 15 arm segments (including those incorporated into the disc) approximately hexagonal, with slightly obtuse distal angle, concave and conspicuously thickened and bulging lateral edges, with longitudinal central groove as a result of the bulging lateral edges; proximal angle of ventral arm plates nearly right, truncated in proximalmost segments; ventral arm plates approximately 1.5 times longer than wide in proximal segments, slightly longer in median to distal ones; ventral arm plates separating lateral arm plates in segments incorporated into the disc; successive ventral arm plates separated by lateral arm plates.

Remarks.—The described specimen is characterised by ventral disc plates lacking granules or spines, a continuous series of block-like lateral oral papillae, second oral tentacle pores opening into the mouth slit, thick and non-bulging lateral arm plates, very small appressed arm spines, and within-plate tentacle pores from median arm segments onwards. This combination of characters typically occurs in the extant genus Ophiomusium Lyman, 1869 , formerly assigned to the family Ophiolepididae but recently found to be sister to the Ophiuridae instead ( O’Hara et al. 2014), and the morphologically very similar extant Ophiosphalma Clark, 1941 . In these two genera, however, the within-plate tentacle pores replace between-plate ones in almost all arm segments, whereas this new species displays between-plate tentacle pores until median arm segments. In this respect, the described specimen has greater similarities with Jurassic and Cretaceous forms commonly regarded as closely related to extant Ophiomusium , in particular the Lower Jurassic Ophiomusium? murravii ( Forbes, 1843) (Thuy et al. 2011) and the Late Jurassic Ophiomusium gagnebini Thurman, 1851 ( Hess 1960), which both display within-plate tentacle pores from median arm segments onwards. The genus Mesophiomusium Kutscher and Jagt, 2000 was established on the basis of dissociated lateral arm plate fossils from the Late Cretaceous of Denmark to accommodate fossil Ophiomusium -like forms with within-plate tentacle pores in median to distal arm segments. The lateral arm plates of the type species M. moenense Kutscher and Jagt, 2000 , however, are more reminiscent of those found in another extant ophiolepidid genus Ophiomastus Lyman, 1878 , rather than Ophiomusium or Ophiosphalma (BT unpublished material). Clearly, most of the Jurassic and Cretaceous Ophiomusium - like forms, including the above-mentioned Ophiomusium and Mesophiomusium species, need thorough reappraisal as to their generic assignment and relation with extant relatives. However, they almost certainly belong to different genera than the Carboniferous specimen described herein on account of its non-bulging distal portion of the lateral arm plates, the grooved, nearly hexagonal proximal ventral arm plates and the very large basal tentacle pores.

Among the modern-type Paleozoic ophiuroids known to date, the described specimen shares closest similarities with species of the genera Aganaster and Archaeophiomusium Hattin, 1967 , both assigned to the Ophiolepididae ( Hattin 1967; Hotchkiss and Haude 2004). In Stephanoura belgica , in contrast, proximal arm segments display spine articulations on a strongly elevated ridge, resulting in an undulating ventro-lateral arm profile, and bear needle-like spines which exceed the length of an arm segment ( Haude and Thomas 1983). Furthermore, the lateral arm plates of S. belgica are composed of coarsely meshed stereom. The type species Aganaster gregarius from the early Carboniferous of North America differs from the described specimen in bearing the arm spine articulations on a slightly elevated ridge at some distance from the distal edge of the lateral arm plates. It furthermore has smaller basal tentacle pores and nearly hexagonal ventral arm plates only in the basalmost segments, followed by bell-shaped ones in succeeding segments. The other species assigned to Aganaster ( Aganaster cingulatus Easton, 1943 , from the Carboniferous of Arkansas, USA; A.? fujiaensis Liao and Wang, 2002 and A. huaanensis Wu, 1982 , from the Permian of China), although poorly known, also do not display the combination of characters found in the described specimen. Archaeophiomusium burrisi ( Miller, 1958) from the Permian of Kansas, USA, the type species of the genus, differs in having bulging lateral arm plates, smaller tentacle pores on the basal arm segments, and non-grooved ventral arm plates which are bell-shaped rather than hexagonal. These differences hold true for Archaeophiomusium bispinosum Mayou, 1969 , from the Permian of Nevada, USA, which, in addition, displays superficial second oral tentacle pores and minute papillae on the distal tip of a plate pair that seem to correspond to the genital plates. Archaeophiomusium andinum Sanchez, 1983 from the Permian of Venezuela, and Ophiomusium calathospongum Berry, 1939 from the Late Devonian of Pennsylvania, USA which may possibly belong to Archaeophiomusium , are both poorly known. The descriptions and illustrations available fail to convincingly diagnose the respective species. Due to the poor preservation of the type material, it is likely that the species in question are, in fact, nomina dubia. The scarce morphological details at hand, in particular with respect to the shape of the lateral arm plates, however, preclude confusion with the here-described specimen. Since no currently known species is compatible with the specimen in question, we describe it as a new species.

Geographic and stratigraphic range.— Type locality and horizon only.

Figures

Fig. 2. Ophiurin brittle star Aganaster jagiellonicus sp. nov. from the upper Tournaisian to lower Visean (lower Carboniferous) Mazurowe Doły Formation, Rudawa Group of Czatkowice quarry, Dębnik Massif, southern Poland; MZUJ T/0282, holotype. A. General view of the specimen, exposing the ventral side (also illustrated by O’Hara et al. 2014). B. Oral skeleton. C. Detail of the oral skeleton; photograph (C1), interpretative drawing (C2). D. Detail of basal arm segments; photograph (D1), interpretative drawing (D2).

Fig. 3. Ophiurin brittle star Aganaster jagiellonicus sp. nov. from the upper Tournaisian to lower Visean (lower Carboniferous) Mazurowe Doły Formation, Rudawa Group of Czatkowice quarry, Dębnik Massif, southern Poland; MZUJ T/0282, holotype. Detail of ventral side (A); median arm segments, showing the transition from between-plate tentacle pores to within-plate tentacle pores (B); median arm segments in ventro-lateral view (C); proximal arm segments in dorso-lateral view (D).