ABSTRACT
Micromorphic brachiopods of the Superfamily Megathyridoidea have a long and successful evolutionary history extending back more than 130 million years. A morphological comparison of the Late Jurassic kingenoid brachiopod Trigonellina pectunculus (von Schlotheim) from southern Germany and Praeargyrotheca hexaplicata Smirnova, one of four stratigraphically oldest species of the Superfamily Megathyridoidea from the Lower Cretaceous of Crimea, Ukraine, suggests a probable close ancestor-descendant relationship between the two taxa. Compared to T. pectunculus, Lower Cretaceous (and younger) megathyridoids are micromorphic yet they are undoubtedly adult forms and sexually mature, but at a size comparable to a very juvenile growth stage in T. pectunculus. The known stratigraphic ranges of both T. pectunculus and P. hexaplicata are consistent with a macro-to micromorphic ancestor-descendent relationship and, accepting the integrity of these stratigraphic ranges, then the adult size discrepancy between the two species may be viewed as a striking example of the heterochronic phenomenon of paedomorphosis. A link between increased rearing temperatures and smaller body size has been established experimentally in a number of present day ectotherms. Thus, a prime paedomorphic influence favouring selection at smaller size-at-maturity (and precocious sexual maturation) in descendent populations of Trigonellina may have been the development of elevated sea water temperatures in the circum-equatorial Tethys seaway during Late Jurassic to Early Cretaceous time. Given the morphological disparity between Praeargyrotheca and the other two sympatric megathyridoid genera (Evargyrotheca, Krimargyrotheca), the latter may not have evolved directly from Trigonellina but from one or more other Jurassic macromorphic ancestors. If proven, then the Superfamily Megathyridoidea, as presently constituted, would be a polyphyletic group.
