North Gondwana mid-Palaeozoic connections with Euramerica and Asia: Devonian vertebrate evidence

Abstract

Well established distribution patterns for Late Silurian-Early Devonian vertebrates indicate two basic provinces for the Gondwana supercontinent - an East Gondwana Province (Australia - W. Antarctica) with an Early-Middle Devonian endemic Wuttagoonaspis-Pituriaspis fauna, and a West Gondwana Province (broadly equivalent to the Malvinokaffric Province for marine invertebrates), with diverse Siluro-Devonian chondrichthyan-acanthodian assemblages. The northern Gondwana margin remains poorly known, but shows affinity to East Gondwana during the Emsian. The northern blocks of Laurentia and Baltica had a completely different cephalaspid/heterostracan fauna, whilst Asian terranes (Tarim, South and North China, Indochina) shared a distinctive and highly endemic galeaspid-yunnanolepid assemblage. Significant changes in pattern during the Middle-Late Devonian are assumed to reflect major rearrangements of global geography. Range enlargement from Asia-East Gondwana into Euramerica of bothriolepid antiarchs in the late Givetian may involve dispersal along new shallow marine shelf environments established with the Taghanic onlap. A second range enlargement of the East Gondwana wuttagoonaspid-phyllolepid placoderm lineage indicates continental connection to Euramerica via West Gondwana during the Late Devonian, at about the time of Condroz event regressions. Pre-Famennian phyllolepids across the northern Gondwana margin, and their complete absence from Asia, are two predictions of this model; phyllolepids have been found in the Givetian-Frasnian of Turkey and Venezuela, but remain unknown in Asia. In Euramerica they replace the adaptively similar psammosteid agnathans, which become extinct at the end of the Frasnian. A latest Devonian connection between East Gondwana and previously isolated Asian terranes is suggested by the appearance of sinolepid antiarchs (an endemic Chinese group) in uppermost Devonian strata in the Lachlan Fold Belt of eastern Australia. Recent models of Devonian palaeogeographic change based on palaeomagnetic data involve a counter-clockwise rotation of the Gondwana supercontinent. Close connections between East Gondwana and Asian terranes, and between West Gondwana and Laurentia, have been proposed for the Early Devonian, followed by a widening ocean separating Gondwana from Euramerica in the Late Devonian. This model is inconsistent with well-documented distribution patterns for Middle Palaeozoic vertebrates, and other fossil groups.