RADIATION OF THE EARLIEST CALCAREOUS BRACHIOPODS LEONIDI. POPOV, LARSE. HOLMER& MICHAELG BASSETT Alt-Union ceotogicat Research lnstitute (VSEGEI), Srednii Prospekt 74,199026 St Petersburg, Russ¡a; lnstitute of Earth Sciences, Historical Geology and Palaeontology, Norbwägen 22, S-75236 Uppsala, Sweden; Department of Geology, National Museum of Wales, Card¡ff CF1 SNP' Wales' UK ABSTRACT- The Cambrian rad¡at¡on, and morphological variation, of five calcareous-shelled brachiopod orders, ¡ncluding laxa assigned 1o both aniculates and inarticulates in the past, is reviewed, and analysed clad¡stically The oldest known calcareous-shelled brachiopods are early Atdabanian obolellids. Chileides arose ¡n the Botom¡an, and kutorgin¡ds in the Late Aldabanian or eafy Botomian The oldest prolorthoids l'articulates'] appear in the middle Eafy Cambr¡an, but the group did not radiate until Middle Cambrian t¡mes INTBODUCTION Phylogenetic analys¡s rgano- phosphatic brachiopo shell chemistries from early recent discover¡es of Early and Middle Cambrian brach¡opod faunas in lsrael, Jordan, North AÍrica, Kirgizia, China and Australia show thât both the taxonomic and morpholog¡cal diversities of the earliest calcareous stocks are considerably greater than & Jell, 1990; o summary bY 'inarticulates' be esPec¡allY important for the understanding of the earliest phylogeny of this group; prev¡ously, the oldest known representatives of the non-ped¡culate, calcareous'¡nart¡culaled' stocks (crani¡des, craniopsides, trimerellides) were from the Ordovician' and it was generally assumed that they had originated from organo- phosphatic ancestors at about that t¡me (e.9. Williams & Rowell, 1965, fig 141) This paper gives a brief account of the morphological variation within the five Orders of Cambrian calcareous brach¡opods -the Obolellida, Ciileida' Kutorg¡nida' Orthida, and Graniopsida, and investigates the¡r earliest radiation in the light of a phylogenetic (cladistic) analysis at the Family level. The detailed phylogenetic relationship between the calcareous and phosphatic brachiopods ¡s outs¡de the scope of this review; following our earlier stud¡es, we consider that this divergence took place at an early stage ¡n the history of the Phylum (Popov et al 1 993; Holmer et al. 1 995) primary shell; the latter conclusion requires Turther study for conf¡rmation, because all available spec¡mens oI Bicia are either silicified or preserved as moulds ln Trematobolus and some genera of the Fam¡ly Naukatidae, the dent¡cles are composed ent¡rely of secondary shell, and in the latter group they are supported by an arcuate plate, the so-called anterise (Popov, 1992, fig.1) The dorsal sockets are poorly developed to absent ¡n most obolellides, but they are present in Naukatidae and some species oÍ Trematobolus MORPHOLOGY OF CAMBBIAN CALCAFEOUS BRACHIOPODS Obolellida Th¡s Order compr¡ses the oidesl known calcareous brachiopodsr from the early Atdabanian (Pelman' 1977) The group includes both forms that lack art¡culation, and those that have pr¡mitive articulatory structures, cons¡sting of paired ventral denticles and dorsal sockets (Popov, 1992, fig.1). such structures were acquired f¡rst with¡n the Family Obolell¡dae, in which the Botomian genus B¡cia has a pair of small, rudimentary denticles alongside the narrow, open delthyrium (e.g. Ushatinskaya, 1988, fig 1, pl 9) The denti- cles are direct extensions oT lhe inner s¡des of the ventral proparea, as demonstrated by the fact that the growth lines can betraced cont¡nuously fromthe proparea lo the distal tips of the denticular nubs This suggests that the denticles were formed along the mantle marg¡n and were composed partly of An open delthyrium is present only in the earliest Obolellidae (e g Obotelta, Bîciaand lvsh¡nellal; in all other obolellides it is covered by a concave 'pseudodeltidium' A morphologically s¡milarstructure is known otherw¡se only in the Eichwaldlidae; however, ¡n this latter group the structure ¡s part of lhe arti- culatory mechanism, and there is no evidence of homology with the obolellides The obolellide muscle system is closely comparable with that of other 'articulated' brach¡opods in having anterior and poster¡or adductor scars, whìch form a quadr¡partite muscle f¡eld in the dorsal valve of Trematobolus and some related genera, but are radially arranged in other obolellides; a single pair of oblique muscles (the internal oblique) was attached dorsally to a small area at the bottom of the notothyrial cavity ln some 'articulated' obolellides, the attachment scar of the internal oblìque muscles ¡s located posterior lo the ax¡s of rotation, suggesting that these may have served as diductors ln the Obolell¡dae and Trematobolidae, the mantle canal system is baculate with dorsal vascula med¡4, but the mantle canals of Naukatidae are not known Chileida Thechileides first appeared in the Bolomian, and the Order includes the eafiest known calcareous brachiopods with a strophic shell; they lack any trace of articulatory structures along the posterior margin The dorsal valve in all chileides lacks a pseudointerarea and is characterised invariably by hemiper¡pheral growth. This suggests the existence of a single generative zone along the posterior margin of the mantle, so that it is unlikely thal the mantle lobes were completely separated. lf th¡s were so, then the axis of rotat¡on was pos- sibly fixed ent¡rely by fused mantle lobes (Popov & Tikhonov, 1990; Popov 1992, fig 1) Taxa in this group also have an unusually large ventral umbonal perforation, wh¡ch ¡s enlarged by resorption and somet¡mes covered poster¡ofy by a plate The plâte ¡s more or less identical in morphology to the colleplax descr¡bed by Wright (1981) in eichwaldiids. The function of the ventral per-foration is not understood fully, but it is unlikely that it served as a pedicle opening, both because of its anter¡or position and the fact that the chile¡des also have a delthyrium lf it is homo- logous with a colleplax, it is poss¡ble that it was the site of an organic pad secreted by the outer epithelium, as proposed by Wright (1981) Popov & Tikhonov (1990), and Popov (1992) speculated that the perforation may have served as paÌt of an hydraul¡c shell-opening mechanism 209