Hipponicharion eos Matthew, 1886

Álvarez, María Eugenia Dies, Gozalo, Rodolfo, Cederström, Peter & Ahlberg, Per, 2008, Bradoriid arthropods from the lower-middle Cambrian of Scania, Sweden, Acta Palaeontologica Polonica 53 (4), pp. 647-656 : 652

Hipponicharion eos Matthew, 1886

Fig. 3C, D.

1886 Hipponicharion eos sp. nov.; Matthew 1886: 64, pl. 6: 19, 19a, b.

1984 Hipponicharion cf. eos Matthew ; Bednarczyk 1984: pl. 5: 6.

1997 Hipponicharion eos Matthew, 1886 ; Siveter and Williams 1997: 45–46, pl. 6: 4–8, pl. 7: 1, 2 [see for complete synonymy].

1998? Hipponicharion sp. cf. eos Matthew, 1886 ; Williams and Siveter 1998: 18, pl. 2: 6, 7.

2002 Hipponicharion eos Matthew, 1886 ; Gozalo and Hinz−Schallreuter 2002: figs. 4A, 5C.

2004 Hipponicharion ichnusum sp. nov.; Elicki and Pillola 2004: 392, text−fig. 6, pl. 2: 1, 2.

2007 Hipponicharion eos Matthew ; Williams et al. 2007: fig. 5.9.

Material.—One nearly complete right valve ( LO 9349t; Fig. 3C 1 –C 3) preserved as an internal mould and a fragment of the external mould, one incomplete valve and its external mould ( LO 9350t; Fig. 3D 1, D 2), and one almost complete valve preserved as an external mould with a fragment of the internal mould.

Description.—Carapace sub−semicircular in outline. Dorsal margin straight and slightly shorter than maximum valve length. Lateral outline evenly convex. Free margin evenly developed with narrow admarginal ridge well defined, demarcated from lobal area by deep, narrow groove. Lobation consists of two ridge−like lobes and a smaller sinusoidal central lobe in slightly anterior−median position. Anterior and posterior lobes broadening dorsally, subparallel to adjacent valve outline, separated ventrally by a narrow gap. Anterior lobe shorter than posterior lobe. Anterior sulcus ventrally shallow. Posterior sulcus broad and shallow. Surface distinctly granulose.

Remarks.—The gross morphology of the valves at hand corresponds well to those of H. eos , but differs from this species in having a slightly narrower ventral gap between the anterior and the posterior lobe. Modest variation in the length and width of anterior and posterior lobes, as well as the length of the ventral gap between these lobes, can, however, be observed in H. eos (see Siveter and Williams 1997: pl. 6: 4–8, pl. 7: 2).

The specimen figured by Bednarczyk (1984) is poorly preserved, but it closely resembles H. eos , as described by Siveter and Williams (1997).

Elicki and Pillola (2004) described a new species, H. ichnusum , from the uppermost Bilbilian Stage of Sardinia. The holotype (a right valve) is incomplete but clearly exhibits the anterior and posterior lobes. It differs from H. eos in having a sub−triangular lateral outline and in the presence of a more prominent central lobe. The central lobe is broken but seems to show a sinusoidal morphology. It has a narrow gap between the anterior and posterior lobes. We find no significant morphological differences between H. eos and H. ichnusum , and regard H. ichnusum as a junior subjective synonym.

Stratigraphic and geographic distribution.—Upper part of the traditional lower Cambrian in New Brunswick, eastern Canada (Hanford Brook Formation, Protolenus elegans Zone ; Siveter and Williams 1997); lower part of the traditional middle Cambrian (Łeba Formation, Eccaparadoxides insularis Zone ) in the Łeba area (Białogóra 2 core) of northern Poland ( Bednarczyk 1984); lower part of the traditional middle Cambrian (probably Acadoparadoxides oelandicus Superzone ) in the Almbacken drill core (depth: 30.75 and 30.47 m) of Scania, southern Sweden; probably upper lower Cambrian (uppermost Bilbilian Stage) of Sardinia ( Elicki and Pillola 2004).