Lamptonium fabaeforme chaunothorax Riedel and Sanfilippo

Lamptonium(?) fabaeforme(?) chaunothorax Riedel and Sanfilippo, 1970, p.524, pl.5, figs.8-9

DESCRIPTION

Similar to the nominate subspecies, but with larger thoracic pores (Riedel and Sanfilippo, 1970) See Description of L. fabaeforme fabaeforme herein.

DIMENSIONS

Length excluding horn 195-285 m, of cephalothorax 160-235 m. Maximum breadth 120-175 m.

DISTINGUISHING CHARACTERS

L. f. chaunothorax is distinguished from its ancestor L. f. fabaeforme by the larger thoracic pores, from its descendant L. f. constrictum by not having a distinct constriction in the upper part of the thorax, and from L. sanfilippoae by the horn not being covered by a massive layer of rough shell material (Sanfilippo et al., 1985).

VARIABILITY

The spherical cephalis, enclosed by the thick wall continuing upward from the thorax, bears a conical to bladed horn of variable length. The base of the horn also is sometimes obscured by this thickened wall. Thoracic pores are large, less than eleven pores (often as few as eight) around half the circumference, irregular in size, with small thorns projecting inward from the margin. Shell wall thick, and roughened by thorns. The subcylindrical to tapering abdomen is short, with irregularly scattered pores, distally hyaline, and with no differentiated margin (Sanfilippo et al., 1985).

DISTRIBUTION

This species is found in most low- to middle-latitude assemblages of early to middle middle Eocene age, and occurs sporadically. Its morphotypic first appearance lies within the Bekoma bidartensis Zone and its morphotypic last appearance lies within the Thyrsocyrtis triacantha Zone.

PHYLOGENY

L. f. chaunothorax is member of the lineage leading from L. pennatum to L. f. constrictum.

REMARKS

Additional illustrations can be found in Foreman, 1973, pl.6, figs.10-12.

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