Himerometridae A.H. Clark, 1908

Family Himerometridae A.H. Clark, 1908 View in CoL

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 View Fig ). Coelomic groove patterns are very similar in Himerometra ( Fig. 3A View Fig ) and Discometra ( Fig. 4 View Fig ) 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 View Fig ). 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 View Fig ). 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 View Fig ).

Stratigraphical range

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