Early Cambrian trilobite larvae and ontogeny of Ichangia ichangensis Chang, 1957 (Protolenidae) from Henan, China

Journal of Paleontology, Jan 1999 by Zhang, Xi-Guang, Pratt, Brian R

In general, ontogenetic trends exhibited by I. ichangensis can be summarized as follows:

1. Cephalon-The protaspis bears an expanded anterior glabellar lobe, but with growth, the posterior glabellar lobes and occipital ring widened. The convexity of the glabella increased.

Axial, glabellar, and occipital furrows deepened with growth, and the three shallow transverse glabellar furrows became lateral glabellar furrows as the glabella inflated.

Even the smallest protaspis bears weakly elevated eye ridges, which extend transversely to connect the palpebral lobes located on the lateral border. The palpebral lobes became slightly more elevated during late meraspid or early holaspid periods, but the eye ridge is always prominent.

The anterior border is ridgelike and defined by short anterior border furrow. The protaspis displays a pair of deep fossulae, which disappeared during late meraspid stages when the anterior border furrow widened and flattened to become the preglabellar field, which during the holaspid period became longer than the anterior border

The facial suture is invisible in all small protaspides. Probably librigenae are located on the lateroventral sides, but too minute to be preserved. With increasing size, the facial suture moved dorsally to adopt an anterolateral to posterolateral trace.

The smallest protaspis possesses two pairs of fixigenal spines, the anterior pair reducing in size gradually and finally disappearing during the late meraspid period; the posterior pair became short nubs on the posterior border in the early holaspid period.

2. Pygidium-The smallest protaspis has no protopygidium. Rather, it seems to develop from the narrow border between two posterior fixigenal spines and behind the occipital ring. It may include a corresponding doublure on the ventral side. A pair of marginal spines developed with the addition of a new axial ring behind the occipital ring. Appearance of the transverse joint coincided with the development of the second axial ring and second pair of marginal spines. During subsequent growth stages, the transitory pygidium assumed more axial rings and corresponding paired marginal spines. It seems that the release of thoracic segments is not co-ocurring with the increase of axial rings, because transitory pygidia in our collection display three or four axial rings. The tail became wider with growth, but the axial furrows defining the newly added axial rings shallowed. The conical marginal spines became flat and short, and in mature holaspides only the anterior two or three pairs of spines remained.

3. Thoracic segments-Because no complete exoskeletons have been collected, the number and morphology of the thoracic segments of 1. Ichangensis are unknown. However, Ichangia is likely to have 13 thoracic segments as other known ichangnids, such as Hsueaspis and Zhuxiella (Zhang et al., 1980, p. 237).

4. Prosopon-The protaspis bears fine polygonal ridges with tiny granules. Similar prosopon has been seen on some meraspides, but the holaspis is mainly smooth. Probably this is due to flaking off of the outermost integument. Terrace lines are present on the doublure of some of the smallest protaspides (Figure 5.2, 5.3), and similar lines were also found on hypostomes (Fig. 7.11, 7.13-7.15).