Hydrocephalus, BARRANDE, 1846

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 : 166

Hydrocephalus minor ( BOECK, 1827)

Text-figs 3e , 5a, d, f–g

The exoskeleton is micropygous and the overall outline of the trunk is parabolic (posterior width of the thorax ca. 50–55% of the anterior width). The pleural parts are wider (tr.) than in P. paradoxissimus gracilis, being ca. 1.6 times wider than the width of the corresponding axial ring ( Text-fig. 3e ). Several posterior-most pleurae are inclined backward. In the cross-section, the pleurae are considerably vaulted, without any prominent fulcrum, as was described by Whittington (1990) for the other Barrandian species, H. carens BARRANDE, 1846. The flange in the anterior part of each pleura is wide, concave (sag.) and more distinct than in P. paradoxissimus gracilis, with a prominent projection at the antero-lateral corner ( Text-fig. 5a ). Articulating half-rings are short (sag.), reaching ca. 40% of the length of axial rings ( Text-fig. 5d ), with the exception of the first one, which is slightly longer, similarly to H. carens (see Whittington 1990). Pleural spines are flat and claw-like, with a backward projection at their posterior bases ( Text-fig. 5f ). The dorsal surface of the pleural spines bears terrace lines, which are inclined in an angle of around 45 degrees to the sagittal axis of the exoskeleton ( Text-fig. 5g ). On the doublure of the pleural spines, the terrace lines are sub-parallel to the body axis, slightly turned adaxially in the posterior half of the spine ( Text-fig. 5f ). The mutual orientation of the dorsal and ventral terrace lines forms a criss-cross pattern when the pleurae overlap.

Figures

Text-fig. 3. Hydrocephalus minor (BOECK, 1827); Jince Formation, Hypagnostus parvifrons to Paradoxides paradoxissimus gracilis Biozones, Příbram-Jince Basin: a – specimen no. NM-L 46543, from dorsal (a1), oblique lateral (a2) and posterior (a3) view; b – specimen no. NM-L 46544, from dorsal (b1), and posterior (b2) view; c – specimen no. NM-L 46545, from dorsal (c1) and posterior (c2) view; d – specimen no. NM-L 46546, from dorsal (d1) and ventral (d2) view; e – specimen no. NM-L 41157, external (e1) and internal (e2) mold. Abbreviations: op – overlapping pleurae.

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.