Early Jurassic trigoniides from southern South America: their recovery after extinction and its bearing on the evolution of the group

ECHEVARRÍA, J.; DAMBORENEA, S.E.; MANCEÑIDO, M.O.

San Luis Potosí

Congreso; 10th International Congress on the Jurassic System; 2018

International Commission on Stratigraphy, IUGS

After a major reduction due to theend-Triassic extinction event (only three surviving genera according to Ros-Franchet al. 2014), the order Trigoniida (Mollusca, Bivalvia) experienced a strongdiversification during the Early Jurassic. In southern basins from SouthAmerica at least eleven genera were recognized between the Hettangian and theToarcian (Leanza 1993, Pérez et al. 2008). Three of them (Trigonia, Prosogyrotrigoniaand Frenguelliella) were Triassicsurvivors, while the other eight genera evolved during the Jurassic, with therespective oldest records for many of them occurring within these basins. Thegenera Groeberella, Neuquenitrigonia and Quadratojaworskiella are exclusive fromSouth America. Thepossibility that Groeberella evolvedfrom myophorid ancestors as suggested by Pérez et al. (1995) implies a fourth surviving lineage from the Triassic. Neuquenitrigonia is clearly related to the Trigonia lineage (Pérez et al. 2008). The genus Quadratojaworskiella contains only twospecies; certain differences between them, nevertheless, suggest relationshipsto different genera for each. The genus Jaworskiellaincludes a few species from South and North America (Poulton 1979, Leanza 1993,Pérez et al. 2008), though they might represent different offshoots from Frenguelliella, perhaps providing anexample of parallel evolution. Myophorella,a widespread and conspicuous Jurassic genus, most likely evolved from somespecies of Jaworskiella withinsouthern South America during the Pliensbachian. Scaphorella seems to be closely related to Myophorella. Psilotrigoniais a genus of uncertain affinities; its oldest record (the Late Sinemurian orEarly Pliensbachian species P. vegaensis)is from northern Chile (Pérez et al. 2008) being found later on in NorthAmerica (Poulton 1979). The list of genera is completed with Vaugonia, most likely originating inJapan during the Hettangian and subsequently arriving to South America in thelate Sinemurian via North America (Poulton 1979), though a wide variety ofspecies have been included within the genus, which deserves detailedphylogenetic revision. The lineage Frenguelliella-Jaworskiella-Myophorella shows one of the mostimportant transitions within the group: the change of flank ornamentationpattern from sub-commarginal to oblique and tuberculate costae. This newcombination of characters is typical of the Myophorelloidea, the most diverseclade of trigoniides during the Jurassic-Cretaceous. The probably parallelevolution among the species of Jaworskiellaand Quadratojaworskiella might beshowing a general evolutionary trend in such direction. Even the genus Prosogyrotrigonia may be part of thistrend, supporting the suggestion by Poulton (1979) of a close phylogeneticrelationship between this genus and Frenguelliella. The high degree of variation ofthese taxa, reflected in, but not limited to, the number of nominal species described,suggests a complex and mosaic evolutionary history. Besides the systematiccharacterization of the group for the Early Jurassic, a detailed stratigraphicanalysis of species and character states within the Myophorelloidea isattempted here in order to understand the early evolution of this major clade.