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

4) Hypostome. An early meraspid hypostome present in our material is approximately 0.52 mm long and 1.02 mm wide. It is similar in form to late meraspid and early holaspid hypostomes, which are moderately inflated and subtrapezoidal. The posterior margin has a median spine and two pairs of distal spines that point backwards. Middle, lateral and posterior furrows are shallow and complete. The posterior and lateral borders are narrow. The anterior wings are directed sub-abaxially. There is no surface sculpture. The doublure is narrow and sub-parallel to the ventral surface and turns upwards, merging into a pair of small posterior wings. The middle furrow separates a crescentshaped posterior lobe from a slightly larger, moderately inflated anterior lobe. The anterior border is curved downwards. The holaspid hypostomes differ from those of the earlier stages in that they are more elongated; the anterior border spines are reduced to gentle swellings and the anterior wings are longer and more laterally directed.

5) Thorax. The early thoracic segments have border spines at the distal ends equivalent to those developed in protothoracic segments of transitory pygidia. In later stages, the segments become more rounded distally and the Organs of Pander are more prominent.

Comparison.-Morphometric features such as the relation of length to width of meraspid cranidia-cephala (Fig. 6.2) of Cryphops? ensae are significantly different from those of Trimerocephalus dianopsoides Osmolska, 1963 (as seen by the different slopes of the relationship between these two dimensions). For a given instar, the individuals belonging to Cryphops? ensae are also smaller than those of Trimerocephalus dianopsoides. On the other hand, the length/width ratios among transitory pygidia of Cryphops? ensae are more similar to those of Phacops (Omegops) accipitrinus Alberti, 1972, than to Trimerocephalus dianopsoides (Fig. 7.3). For the same width, the transitory pygidia of Phacops (Omegops) accipitrinus are longer (Sag.) than those of Cryphops? ensae. Moreover, the degrees 4 and 7 of transitory pygidia belonging to Cryphops? ensae are respectively smaller than the corresponding transitory pygidia of Phacops (Omegops) accipitrinus.

For a given instar, the meraspid cranidia-cephala of individuals belonging to Cryphops? ensae are smaller than those of Trimerocephalus dianopsoides.

CONCLUSION AND DISCUSSION OF ONTOGENY

The growth series from larval to postlarval stages exhibit a relatively simple pattern of instar development, with discrete instars during early ontogeny.

Changes that apparently took place between the small meraspid cranidium and mature holaspid cephala include: 1) cranidium relatively wider in older stages due to smaller convexity and lateral extension of the glabella; 2) decrease of the length/ width ratios of the cranidium and the glabella, whereas the WM/ Wm ratios of the glabella first increase and then decrease at the end of the larval period; 3) increase in size of the visual surface and its slight migration posteriorly; 4) increase in lens number up to and rarely more than seven; 5) ankylosis of the cephalic doublure that takes place during the late meraspid period, later than in the ancestral Phacops; 6) reduction of the glabellar furrows S2 and S3 into short branches before their disappearance; 7) decrease in size of the occipital spine; 8) decrease of the spinose sculpture, which becomes reduced to granules in later stages; and 9) allometric reduction of the genal spines, which seems to be a general feature in phacopid early development (Osmolska, 1963; Jahnke,1969; Chatterton, 1971; Alberti,1972; Cronier and Feist, 1997).