UC Riverside

Geochronologically calibrating the latest Cambrian and earliest Ordovician is critical for further investigations into the cycles of evolutionary diversifications and extinctions during this interval. The peri-Gondwanan terrane, Sibumasu (western Thailand, eastern Myanmar, northern Malaysia, and western Yunnan, China) is globally the only tectonic province known to have multiple zircon-bearing volcanic ash beds interbedded with fossiliferous strata from both the latest Cambrian and earliest Ordovician. The terrane thus has great potential for producing a geologically constrained biostratigraphic succession. Well-resolved taxonomy is foundational for reliable biostratigraphic correlations, without which absolute dates have limited value beyond the locality from which the dated material was collected. Trilobites are particularly useful fauna for correlations during this time interval as their diversity, environmental pervasiveness, and good preservation potential mean that they are found on most tectonic provinces and can be correlated with reasonably high precision. Hitherto the trilobites of Sibumasu have not been taxonomically well-resolved. This study uses field collections made during three excursions (2008, 2016, and 2018) to southern Shan State, Myanmar and Ko Tarutao, Thailand to revise Sibumasu’s Cambro-Ordovician trilobite fauna. It is the first description and illustration of trilobites from Cambrian Myanmar and substantially expands the trilobite record of the fauna of Thailand’s Tarutao Group. The latter, though the subject of three separate studies (1957–1988), had been left with most species in open nomenclature. Collections from both localities have tightly constrained stratigraphic context where possible. Detrital zircon and volcanic ash samples were collected at the same time as the fossils for use in dating and paleogeographic reconstructions. Three species, one of which is new, are recorded from Myanmar. 42 species were recovered from the Tarutao Group, 18 of which are new; following the most recent revision in 1988, only 18 species were recognized. Two new genera were also recovered from the Tarutao Group.