Heterochrony, a pervasive mechanism in generating evolutionary trends in brachiopods: Some examples from Ordovician and Silurian rhynchonelliforms

BENEDETTO, J.L.

Puerto Madryn

Congreso; Reunión de Comunicaciones de la Asociación Paleontológica Argentina 2018; 2018

Asociacion Paleontologica Argentina

Evolutionary lineages preserved in the fossil record are critical to interpreting evolutionary processes (e.g. gradual vs. punctuated phyletic change) but interpretation of the underlying mechanisms is challenging. Among the many causes invoked to produce evolutionary trends ?defined as a persistentdirectional change in one or more character states through time? heterochrony has been recognized as a crucial factor not only as an agent in speciation but also in macroevolution. All of the Ordovician successions of Argentina (NW basin, Famatina Range, Cuyania terrane) provided welldocumented examples of evolutionary trends in orthoid, plectorthoid and plectambonitoid brachiopods. Such trends involve size, shell morphology, ornamentation and, more importantly, changes in the development of such internal structures as cardinalia, which are often considered to be ?stable? and thus taxonomically (and phylogenetically) informative. The lineage found in the Santa Victoria Group leading from the basal plectorthoid Protorthisina (Furongian) to the lower to upper Tremadocian ?nanorthids? Kvania, Gondwanorthis, Lampazarorthis and Tarfaya is a good example of a long-term trend (ca. 10 Ma) directed mainly by heterochronic processes, but perhaps the best documented hypothesis of heterochrony (peramorphosis) is the Panderina?Productorthis lineage recorded in the Floian-Dapingian Famatina Group. There is also compelling evidence supporting that the Silurian rhynchonellide Clarkeia (Uncinuloidea) evolved from Harringtonina (Rhynchotrematoidea) by peramorphosis. More striking is the case of the plectambonitoid Ahtiella famatiniana (Strophomenata), which on the basis of empirical and cladistic evidence evolved from the orthoid Monorthis transversa (Rhynchonellata) showing that in brachiopods heterochrony may have played a substantial role in originating higher taxa (macroevolution).