The early ontogeny of Jurassic thecideoid brachiopods and its contribution to the understanding of thecideoid ancestry

Abstract

A collection of very early brephic juveniles from the Upper Aalenian of the Cotswolds, England, has provided the first opportunity to study the ontogeny of thecideoids prior to the recognizable development of a median septum. Traced towards their origin, the enduring characters of thecideides appear to be cementation, a delthyrium closed by a pseudodeltidium, an almost circular dorsal valve with a prominent erect bilobed cardinal process, well-defined inner socket ridges, widely spaced lateral adductor muscle fields, tubercles, fibrous secondary shell lining both valves and probably endopunctae. The subperipheral rim and median septum so characteristic of adult thecideoids are undeveloped in the earliest juveniles. Initiation of the development of the free ventral wall in the ventral valve is identified as an important event in thecideoid ontogeny. Further important discoveries arising from the study are that the crura-like outgrowths which form the brachial bridge in thecideides are not structurally homologous with the crura of spiriferides or terebratulides and that during their earliest ontogenetic development thecidellinids and thecideids are indistinguishable. Also, the identification of morphological characters correlative with both palaeontological and neontological approaches to thecideoid phylogeny has important implications for thecideide taxonomy. Interpretation of the morphology exhibited by the Cotswold specimens introduces the probability that during their earliest ontogeny moorellinin, thecidein and lacazellin dorsal valves follow the same development pattern and the described ontogenetic sequence has been corroborated by evidence from Jurassic rocks of Argentina, North America and France, from Cretaceous rocks of England, Central Europe and Czechoslovakia, also from extant species from the Atlantic, Indian and Pacific oceans, and to an extent which suggests the pattern of ontogenetic development revealed has been typical of thecideoids throughout their history.

Post‐metamorphic allometry in the earliest acrotretoid brachiopods from the lower Cambrian (Series 2) of South China, and its implications

“…Moreover, the varying body size and variable morphology during ontogeny is vital for understanding how heterochronic processes may relate to evolutionary change (Beecher ; Gould ; Rudwick ; Klingenberg ). The study of ontogeny offers the opportunity to observe varying morphology with increasing size and facilitates the recognition of divergence or convergence of characters during ontogenetic development (Baker & Carlson ; Baker & Logan ). Thus, detailed studies of ontogenetic development are needed in order to investigate the evolutionary and phylogenetic relationships of fossil brachiopods, and to understand the functional changes of different structures during different developmental stages.…”

The development and shell microstructure of the pseudodeltidium and interarea in thecideide brachiopods

“…Williams and most other workers conducted their studies on adult specimens of Lacazella and Thecidellina , which had the well‐developed straight hinge lines characteristic of most adult thecideid genera. More recent studies (Baker and Carlson 2010, Baker and Logan 2011) have drawn attention to the very short hinge lines of early juveniles and raised the possibility that the development of the interarea (Fig. 6B, D) is subordinate to the development of the pseudodeltidium.…”

“…As the interarea develops, the hinge line and hinge axis remain coincident until the cardinal margin is reached. Because, in fossils, the early juvenile dorsal valve is almost circular (Baker and Carlson 2010), the hinge line is short. Beyond the teeth, the edges of the dorsal and ventral valves diverge, so that the postero‐lateral flanks of the ventral umbo have an in‐sunk, incipiently palintropic configuration like that of rhynchonellide and terebratulide genera (Cohen 2007).…”

“…k3, p. 301) also noted that during early development of Lacazella , the umbo of the ventral valve was in the form of a palintrope. Study of Enallothecidea Baker, 1983 (Baker and Carlson 2010, p. 658) shows that the delthyrium was closed by an apical pseudodeltidium, although whether the plate developed early or late is not known. The morphology of Enallothecidea is considered (Baker and Carlson 2010, p. 659) to be close to the ancestral morphotype.…”

“…Study of Enallothecidea Baker, 1983 (Baker and Carlson 2010, p. 658) shows that the delthyrium was closed by an apical pseudodeltidium, although whether the plate developed early or late is not known. The morphology of Enallothecidea is considered (Baker and Carlson 2010, p. 659) to be close to the ancestral morphotype. Research by Baker and Wilson (1999) on their new genus Stentorina indicated that the early juvenile ventral umbo was palintropic.…”