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

ABSTRACT-Exceptionally rich and well-preserved silicified exuviae comprising numerous immature growth stages allow the tracing of detailed morphological and morphogenetical features of the reduced-eyed phacopid species Cryphops? ensae (Richter and Richter, 1926) from the latest Famennian of Thuringia (Central Germany). Ontogenetic features of this species are more similar to those of Phacops than to Trimerocephalus.

INTRODUCTION

THE ONTOGENY of Devonian phacopids remains poorly known. Hitherto, data on only a few isolated protaspides, meraspides, and early holaspides of Phacops Emrich have been published. These include protaspides from the Lower Devonian of New York (Whittington, 1956); protaspides and early meraspides of Phacops spedeni from the Emsian of SW Australia (Chatterton, 1971); early meraspides and early holaspides of Phacops (Omegops) accipitrinus from the late Famennian of Erfoud, SE Morocco (Alberti, 1972), and early holaspides of Phacops zinkeni from the early Devonian of Kellerwald, Germany (Jahnke, 1969). So far the best insight into the ontogenetic development of phacopids has been obtained from growth series of the early Famennian Trimerocephalus: complete specimens of meraspides 3, 5-11, young holaspides of Trimerocephalus dianopsoides from the Holy Cross Mountains, Poland (Osm*lska, 1963); meraspides 9-10, early holaspides of Trimerocephalus lelievrei from Tafilalt, SE Morocco (Cronier and Feist, 1997; Cronier et al., 1998); and late meraspides and early holaspides of Trimerocephalus mastophthalmus from South Urals (Maksimova, 1955).

The discovery of silicified larval and early postlarval growth stages assigned to Cryphops? ensae (Richter and Richter, 1926) from the latest Devonian of Thuringia (Central Germany) gives us the opportunity to elucidate the ontogenetic development of the latest reduced-eyed phacopid that occurred just before the end-Devonian extinction of the group.

The material used for this study has been collected by K. Bartzsch and D. Weyer from the Upper Devonian Clymenia Beds from West Fischersdorf and East Breternitz near Saalfeld, Thuringia (Fig. 1).

The trilobite bearing sequence (Fig. 2) is mainly composed of dark-colored argilites with intercalated micritic limestone nodules and thin beds of turbiditic quartzites and tuffs. The trilobite association of exclusively reduced-eyed and blind phacopids and proetids such as Chaunoproetus and Drevernannia characterizes the deeper off-shore environment of an external platform (approximately 200 m depth). It also represents one of the deepest Late Famennian outer shelf communities from the European North Gondwana margin.

Various immature growth stages belonging to Cryphops? ensae were obtained from several horizons of the latest Famennian Upper Clymenia Beds (Fig. 2). These immature trilobites are silicified and were extracted, after decalcification in formic acid (10 percent), from body chambers of cephalopods in which they had accumulated, apparently without preferential orientation. The extraordinarily dense concentration of immature shells contrasts with the low density of adults preserved in calcite, which are present laterally in the same beds, outside the body chambers. This probably reflects a preference of these organisms to molt close to a food source and in a particularly undisturbed environment.

The material described and figured in this work is housed in the collection of the Museum far Naturkunde, Invalidenstrasse 43, 10115 BERLIN, Germany (No: M.B.T. 3721-3759).

SYSTEMATIC PALEONTOLOGY

Terminology.-The morphological terminology of phacopid trilobites is based on the works of Hupe (1953), Harrington et al. (1959), Chlupac (1977), and Clarkson (1992). The symbol CL is depicted as cephalic length (sagittally from the front of the glabella to the posterior margin of the occipital ring); CW as cephalic width (distance between outer margins of the genal angles); PL as length of pygidium including articulating half ring; PW as maximum width of pygidium (L = length, WM = maximum width, Wm = minimum width, C = cephalon, G = glabella, R = rachis, P = pygidium); Sag. = sagittal; Tr. = transversal.

Family PHACOPIDAE Hawle and Corda, 1847 Subfamily PHACOPINAE Hawle and Corda, 1847 Genus CRYPHOPS (Richter and Richter, 1926) Type species.-Phacops cryptophthalmus Emmrich, 1844, Upper Devonian, Rhineland.

CRYPHOPS? ENSAE (Richter and Richter, 1926) Figures 3-5, 9, 11-15

Phacops (Cryphops?) ensae RICHTER AND RICHTER, 1926, p. 164, pl. 9, figs. 61-64.

Diagnosis.-labella overhanging anterior border; glabellar furrows S2 and S3 poorly defined; preoccipital furrow S1 continuous (Tr.); lateral preoccipital lobes low, poorly differentiated; ocular lobe small, less prominent, shifted to the front; visual surface of elliptic outline with few lenses; palpebral furrow faint, orientated inwards; posterior band of the cephalic doublure twice as wide (Sag.) as vincular furrow; dense, homogeneous tuberculation of cephalon. Pygidium relatively wide (Tr.) and short (Sag.); prominent axis gradually tapering posteriorly with five rings and four ribs; ring furrows and pleural furrows deep anteriorly and shallowing posteriorly.