Paradoxides

Laibl, Lukáš, Esteve, Jorge & Fatka, Oldřich, 2016, Enrollment And Thoracic Morphology In Paradoxidid Trilobites From The Cambrian Of The Czech Republic, Fossil Imprint 72 (3 - 4), pp. 161-171 : 163-164

Paradoxides paradoxissimus gracilis ( BOECK, 1827)

Text-figs 2a, c , 5c, e, h–j

In overall morphology, the exoskeleton of P. paradoxissimus gracilis is conspicuously micropygous, with thorax slightly narrowing posteriorly (the posterior width of the thorax reaches about 60% of its anterior width, Text-fig. 2c ). The structure of the thorax consists of comparatively narrow (tr.) pleural parts, which are roughly the same width as the axial lobe of the same tergite ( Text-fig. 2c ). The pleurae are flat, non-fulcrate and horizontally oriented, as is clearly visible in the tergites that are perpendicular to the main compression pressure ( Text-fig. 2a ); they therefore are not flattened dorso-ventrally. This is in contrast with Whittington’s assumption that pleurae sloped downward abaxially, being gently curved distally ( Whittington 1990). Only the slightly wider (tr.) pleurae in first two thoracic tergites seem to be slightly curved ventrally in their distal parts ( Text-fig. 2a ). The posterior margin of all pleurae bears an inconspicuous backward oriented projection at the base of pleural spine ( Text-fig. 5c ). In the anterior edge of each pleura, a wide (exsag.) but shallow flange with a small projection at the antero-lateral corner ( Text-fig. 5c ) is developed. This structure is inserted below the posterior part of the previous pleura. The articulating half-rings are comparatively short, reaching about 40% of length (sag.) of the axial rings ( Text-fig. 5e ). The pleural spines are slender, comparatively long (sag.), directed postero-laterally in the anterior part of the thorax and more posteriorly toward its rear part. Dorsal surface of pleural spines is smooth, without any surface sculpture, with the exception of a few terrace lines sub-parallel to the antero-lateral margin of spine in the first two thoracic tergites ( Text-fig. 5h–i ). The pleural spine doublure bears a furrow running from the tip of the spine, sub-parallel with the posterior margin of the spine ( Text-fig. 5j ). Terrace lines on the doublure are apparently restricted only to the area anterior from this furrow ( Text-fig. 5j ), and are sub-parallel to the main axis of the exoskeleton.

Figures

Text-fig. 2. Paradoxides paradoxissimus gracilis (BOECK, 1827); Jince Formation, Paradoxides paradoxissimus gracilis Biozone, Příbram-Jince Basin: a – specimen no. NM-L 4373, from dorsal (a1), oblique lateral (a2) and ventral (a3) view; b – specimen no. GBA 1896/004/0057, dorsal view; c – outstretched specimen no. NM-L 41167, dorsal view. Abbreviations: rp – rostral-hypostomal plate.

Text-fig. 5. Thoracic structure, articulating and coaptative devices of H. minor (a, d, f–g), P. paradoxissimus gracilis (c, e, h–j) and A. cf. sacheri (b): a – specimen no. NM-L 46550, internal mold of pleurae; b – specimen no. NM-L 4776, internal mold of pleura; c – specimen no. NM-L 46551, internal mold of pleurae; d – specimen no. NM-L 46550, internal mold of axial rings and articulating half-rings; e – specimen no. NM-L 46552, internal mold of axial rings and articulating half-rings; f – specimen no. NM-L 46550, detail of pleural spine; g – specimen no. NM-L 41157, external mold of pleural spines; h – specimen no. NM-L 46547, external mold of pleural spines; i – specimen no. NM-L 46548, external mold of pleural spines; j – specimen no. NM-L 46549, pleural spines. Abbreviations: ahr – articulating half ring; alp – antero-lateral projection of flange; bp – backward projection at base of pleural spine; dtl – dorsal terrace lines; fl – flange; fu – fulcrum; ptl – petaloid-like terrace lines; vf – ventral furrow of pleural spine; vtl – ventral terrace lines.