Larval morphology and ontogeny of a late Devonian phacopid with reduced sight from Thuringia, Germany

Journal of Paleontology, Mar 1999 by Cronier, C, Bartzsch, K, Weyer, D, Feist, R

Discussion.-The characteristic features of the genus Cryphops were discussed by Richter and Richter (1926) and by L*tke (1968), especially the configurations of the eye and the pygidium: the reduced eye with few lenses displayed on an ellipsoidal visual field ("cryptophthalmus mode" of Richter and Richter, 1926) and a relatively short pygidium with few axial rings.

The species P. ensae from the Famennian has been assigned tentatively by its authors to Cryphops because it shares characteristic features of both Phacops and Cryphops. It approaches the contemporaneous "granulatus" group of Phacops in terms of the configuration of the cephalic ventral side, i.e., by the presence of a relatively large area posterior to the vincular furrow; however the configuration of the dorsal cephalic features, especially the eye, is analogous to that of the type species Cryphops cryptophthalmus (Emmrich, 1844). This species differs from the contemporaneous Cryphops? wocklumeriae (Richter and Richter, 1926) by the less prominent ocular lobe, the visual surface with fewer lenses, the less distinct palpebral furrow, the homogeneous tuberculation, and the wider (Tr.) cephalon. Moreover, in Cryphops? wocklumeriae, the cheeks are more strongly inflated, the dorsal furrows deeper, the lateral border furrows relatively angular, and several prominent tubercules are present on the occipital ring.

Material.-The material is composed of disarticulated exuviae comprising a few postlarval instars (holaspides) and many more larval instars; an uninterrupted growth series ranging from the early larval to the early post-larval stages is represented. Only a small number of specimens occur as articulated sclerites.

Numerous transitory pygidia-pygidia (82), cranidia (40 heads without doublure; facial suture functional), cephala (three heads with doublure; facial suture ankylosed), isolated shed doublures (53) and protaspides (seven) have been sampled. Sets of protaspides, cephalic doublures, cranidia-cephala, and transitory pygidia-pygidia have been investigated separately.

The material includes different ontogenetic instars, essentially meraspides. Among the transitory pygidia, ontogenetic stages of numerous exuviae are morphologically distinguishable when remaining unarticulated proto-thoracic segments are present. Among cranidia, according to modes of molting, cephalic pieces are without a doublure (early meraspid stage) or with a doublure (late meraspid and holaspid stages). Indeed, Cryphops has two successive modes of molting during its ontogeny. In the early larval period, it molts like most other trilobites via the functional marginal cephalic suture, with the line of weakness facilitating ecdysis. This mode of molting has also been observed in the early instars of Phacops (Jahnke, 1969; Chatterton, 1971; Alberti, 1972) and Trimerocephalus (Maksimova, 1955; Osmolska, 1963; Cr6nier and Feist, 1997). In later instars, after ankylosis of the facial suture, the ecdysial opening occurs between the cephalon and the thorax. At this step, the neck joint opened to facilitate exuviation when the whole dorsal shield of the cephalon was shed during ecdysis. This special molting behavior ("Salterian mode of moulting," Richter, 1937), which differs from that of most other trilobites, has been described in detail by Henningsmoen (1975), Chlupac (1977), Speyer (1985), Maksimova (1955), Osmolska (1963), and Cronier and Feist (1997).