Restudy of the thecideoid fauna from organodetrital residues of Aalenian age from two Cotswold localities revealed the presence of a rare form assignable to the Lacazellinae. Its morphology and shell microstructure enabled it to be identified as the earliest clear representative of the subfamily and the possession of ramuli, jugal pillar, reniform brachial lobes and partially suppressed fibrous secondary shell layer indicate proximity to the rootstock of the Lacazella Munier-Chalmers line of descent in a position possibly ancestral to Praelacazella Smirnova. In the absence of near stratigraphical or geographical relatives the Aalenian specimens are assigned to Protolacazella scripta gen. et sp. nov. The suppression of fibrous secondary shell in P. scripta mirrors the phylogenetic suppression of the secretion of fibrous secondary shell in the Lacazellinae as a whole and may provide an insight into the mechanism by which it was achieved.

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.

The analysis of morphological characters exhibited by the earliest ontogenetic stages of Middle Jurassic, Cretaceous and Holocene thecideoid species, together with the application of techniques that reveal the presence of early juvenile morphological characters buried in the shell fabric of adult representatives of the species concerned, reinforce the idea of a development pattern common to thecideoids from the early Middle Jurassic to the present day. Basic and long-standing differences in the architecture of the brachidial skeleton of thecidellinids and lacazellins may be correlated with the morphology of thecospiroids and indicate that the Thecidellinidae and Thecideidae may have emerged as sister groups as early as the Triassic.

ABSTRACT. Restudy of a hardground fauna from the Middle Jurassic of south-western Utah revealed the presence of thecideoid brachiopods. The absence of separated dorsal valves and the effects of abrasion and diagenesis hampered the description of the specimens. However, serial sectioning of complete shells revealed a thecidellinid which in its morphology and shell microstructure resembled Rioultina Pajaud from the Middle Jurassic of Europe. Important morphological differences clearly separate the new specimens from Rioultina and they are assigned to Stentorina sagittata gen. et sp. nov. However, the close approximation of certain shell microstructural features does suggest a phylogenetic link between the two genera. S. sagittata is also compared with the only other known American Jurassic species, Ancorellina ageri Manceñido and Damborenea from the Lower Jurassic of Argentina. Differences in morphological characters currently of family rank preclude the possibility of Stentorina being descended from Ancorellina stock. It is considered, therefore, that dispersal from Tethyan thecideoid populations to America occurred several times.

Baker, Peter G. 1989 07 15: Location and interpretation of ontogenetic relics in the shell of adult brachiopods. Lethaia, Vol. 22. pp. 241–245. Oslo. ISSN 0024–1164. Careful sectioning of critically orientated adult Ornithella bathonica (Rollier) shells has revealed, close to the umbo in the secondary fibrous shell layer of the brachial valve, relics of early ontogenetic structures which can be directly correlated with those of early juveniles of Zeilleria leckenbyi (Davidson ex Walker MS). It is now confirmed that very early in ontogeny, the brachial valve of O. bathonica possessed a septal pillar identical with that found in Z. leckenbyi and many terebratellacean species. *Brachiopoda. Terebratellidina, Ornithellidae, ontogenetic relics, buried microstructure.