Himerometridae A.H. Clark, 1908

Eléaume, Marc, Roux, Michel & Philippe, Michel, 2020, Discometra luberonensis sp. nov. (Crinoidea, Himerometridae), a new feather star from the Late Burdigalian, European Journal of Taxonomy 729, pp. 121-137 : 126

Family Himerometridae A.H. Clark, 1908

Remarks

Hess & Messing (2011) placed Himerometridae within the superfamily Mariametroidea A.H. Clark, 1909 (later changed to Himerometroidea by Taylor et al. 2017) in which rod-shaped basals are absent.All genera in Himerometridae ( Heterometra A.H. Clark, 1909 , Himerometra , Craspedometra A.H. Clark, 1909 , Amphimetra A.H. Clark, 1909 ), except Discometra , show coelomic grooves on the adoral side of centrodorsal. Amphimetra has multiple radiate coelomic grooves while Heterometra , Himerometra and Craspedometra show Y-shaped grooves with more or less wide branches. In Discometra , such Y-shaped features are present as canals included in the stereom and located just beneath the surface of the proximal facet of radials ( Fontannes 1880; Sieverts-Doreck 1961). As a consequence, coelomic grooves are absent from the adoral surface of the centrodorsal. In Himerometra , the aboral facet of each radial displays a pair of wide grooves running in parallel in the inner part and diverging in a Y shape in the outer part; this feature is also visible on the oral side of the centrodorsal ( Fig. 3A ). Coelomic groove patterns are very similar in Himerometra ( Fig. 3A ) and Discometra ( Fig. 4 ) but differ markedly in each of the other genera ( Rasmussen 1978: figs 593–594). The numerous radiating coelomic grooves (A.H. Clark 1915) distinguish Amphimetra the most from the other genera, and Hemery (2011), Summers & Rouse (2014) and Taylor et al. (2017) questioned its attribution to Himerometridae . In Himerometra , the general shape of the centrodorsal varies from hemispherical to almost discoidal. The insertions of cirri on the centrodorsal have a very variable aspect on the same individual, with or without a slight rectangular transverse relief ( Fig. 3B ). The radial circlet delimits a more or less wide adoral cavity whose flanks corresponding to the inner face of the radials are sculpted by strong vermiculate figures ( Fig. 3C ). The distal articular facet of the radials is sub-trapezoidal, the internal (adoral) ligamentary areas, with no distinguishable limit adorally, are separated by a large depression, and the muscular areas are reduced and nearly inconspicuous ( Fig. 3D ).

Stratigraphical range

Miocene–Recent (Western Pacific), possibly since the Eocene.

Figures

Fig. 3. Centrodorsal and radials in Himerometridae A.H. Clark, 1908. A–D. Himerometra robustipinna (Carpenter, 1881) (MNHN-IE-2012-862). A. Adoral side of centrodorsal. B. Cirrus sockets on lateral face of centrodorsal.C–D. Radials.C. Adoral view showing vermiculate grooves on inner face.D. Distal articular facet. E. Discometra rhodanica (Fontannes, 1877) from Picabrier (MHNL 20.062687), cirrus sockets on centrodorsal. Scale bars: A–D = 1 mm; E = 0.5 mm.

Fig. 4. Cross sections near aboral surface of radial circlet showing Y-shaped coelomic canals in the genus Discometra Gislèn, 1924. A. Discometra rhodanica (Fontannes, 1877). B. Discometra eggenburgensis (Schaffer, 1912). Modified from Sieverts-Doreck (1961) in which A was modified from Fontannes (1880).