The development of Clonograptus tenellus and Adelograptus hunnebergensis

Abstract

1974 01 15: The development of C/orlograpltrs leridris and Addograplirs hir~r~lebergerr~is. I.e/haia, Vol. 7, pp. 79-92. 040. ISSN 0024-1 164. E m The development of Cloriograp/iis ier~elliis Linnarsson and Adelogrupirrs Irri~il~ebergerisis (Mobel-g) is described for the first time. The material comprises well-preserved isolated specimens from the Shincton (Tremadoc) Shales of Chcrme's Dingle and Lady Covert, neai-Leighton, Shropshire. Two new morphological terms, the cauda and the conus, have been introduced for the adequate description of the prosicula. This material demonstrates that the nema is a hollow tube with a passage into the apex of the prosicula, and an open growing end. The tcrm nematocaulus has been applied to the soft tissue which secreted the nema. The mode of development and the internal stolon system suggested by Bulman (1949) have been confitmed. Early growth stages of this material are compared with those of Rhabdopleirra conrpacla Hincks. Jarin E . H i t / / , Sedgwick Mrtsermi. Dowwirig Streel, Catttbridgc~. U . K . (Presetlr adtlress: 28 Rockfield Drive, ~,/arrilrrdrlo, Caerrrar~~ortshire, Wales), 5/11 April, 1973.LETHAT 7 (1Y74) nebergensis is difficult unless a large portion of the rhabdosome is available, and the differing branching patterns can be seen. C. tenellus can, however, be distinguished on the basis of older growth stages, that is, after the first crossing canal has developed; of these growth stages, about 60V0 are referable t o C. tenellus. As the present material comprises mainly early growth stages and distal fragments, only a small proportion of the total specimens can be identified positively.This study has concentrated on the detailed morphology and development available with isolated material; description of the gross morphology of the two species was adequately covered by Stubblefield (1929). Because of the difficulty in distinguishing C. tenellus from A . hunnebergensis in the present material, a general description applicable t o both species will be given. Ack,rowledgenzeri/s.-This work is the first stage in a project to investigate and evaluate the La2 fauna of Victoria, Australia, which is generally believed to be Upper Tremadoc age. Further investigation of the main components of the underlying (Lower Tremadoc) anisograptid faunas was clearly necessary, since there is no strictly comparable Upper Tremadoc fauna elsewhere. The project has been financed by a grant from N.E.R.C. which is gratefully acknowledged by the author and the senior investigator, Prof. 0. M. B. Bulman, under whose general supervision this work was done. sity of Cambridge, for their helpful suggestions and discussions. All specimens are deposited in the Sedgwick Musuem, Cambridge. O I I Iriver/ebrute Palueoritology V. Gruplolirhiriu with seetioris or1 Eriteroprreiis/ii orid Pterobrurichiu. (2nd Edition), I-XXXII, 163 pp. Geol. Soc. Amer. and Bulman, 0. M. B. 1972: A new Dic/yorieniti fauna Univ. Kansas.

Tremadoc faunas from the Carmarthen district, South Wales

“…Flexuous, denticulate stipe fragments typical of A. hunnebergensis are numerous on some bedding planes, but good proximal ends are scarce. NMW 77.2G.20a figured here agrees closely with the descriptions of Stubblefield (1929) and Hutt (1974), and little can be added. The production of the first secondary stipe at the second theca of the primary stipe is characteristic.…”

Structure and Secretion of the Graptolite Prosicula, and its Application for Biostratigraphical and Evolutionary Studies

“…Notably lacking is any sign of longitudinal ridges; in fact, the only longitudinal lines are present along the nema. No siculae of mid Tremadoc age were examined during the present study; the prosiculae of Adelograptus, described from this interval by Hutt (1974), appear, however, to be very similar to our early Tremadoc forms. Late Tremadoc prosiculae, belonging to genera such as Kiaerograptus and Aorograptus (see Williams and Stevens 1991) have a more 'advanced' appearance in terms of the overall form, which is typically an elongate 'bell-shape' with a highly constricted neck where the diaphragm separates a short, narrow conus from the cauda, which makes up over 90 per cent.…”

“…Furthermore, only a few detailed descriptions of chemically isolated prosiculae have been previously published (e.g. Kraft 1926;Kozlowski 1971;Hutt 1974;Bates 1996). The present study aims to address this deficiency in part through the detailed examination of chemically isolated lower, middle and upper Ordovician (Tremadoc to Caradoc) prosiculae.…”

“…Hutt (1974) also figured the diaphragm from middle Tremadoc graptolites and introduced the terms 'conus' and 'cauda'; the conus was defined as the cone-shaped portion of the prosicula located towards the nema (termed the 'nema-prosiculae' by Kraft 1926 andRickards 1996), while the remaining, largely parallel-sided, part of the prosicula that joins onto the metasicula was referred to as the cauda. Bates (1996) described the diaphragm as composed of condensed fibres; Hutt (1974) concluded that it was secreted once the prosicula was complete since some graptolites lacked one. Despite the number of studies utilizing wellpreserved, chemically isolated material in recent years, no new observations of the diaphragm (with the exception of Bates 1996) appear to have been made since Hutt's publication.…”

Development of the graptoloid rhabdosome

“…Both dextral and sinistral development have long been known in Dictyonema flabelliforme, Clonograptus tenellus and Adelograptus hunnebergensis (Stubblefield, 1929;Bulman, 1936a); all figured specimens of the latter two species given by Hutt (1974), however, show dextral development and it seems that even among these dendroid species, dextral development may be the norm.…”