THE EARLY DEVONIAN ARMOURED AGNATHANS OF PODOLIA, UKRAINE

VICTOR VOICHYSHYN

In the marine Přidolian to early Lochkovian environments of Podolia, vertebrates were represented almost exclusively by thelodonts and acanthodians. Starting from the mid Lochkovian (Chortkiv Formation), the heterostracans increased their abundance and diversity.Diverse macroscopic remains of the cyathaspidid heterostracans, the first small−size pteraspidids and osteostracans appeared in the late Lochkovian (Ivanie Formation). The Old Red−type ecosystem of Podolia gradually emerged with retreat of the sea. Simultaneously,various agnathans, with ecological preferences to brackish or fresh waters, appeared. At the transition to the Pragian (red−bed Dniester Formation), when coarse terrigenous quartz sand gradually replaced fine carbonate silt and clay, the armoured vertebrate community reached its peak diversity, especially regarding the osteostracans and pteraspidid heterostracans. Many local lineages developed but some ties of the heterostracan faunas with those of western European and the osteostracan faunas with Spitsbergen ones are traceable. Most evolutionary series recognised show increase in body size, both regarding the heterostracans and osteostracans. Taxonomic diversity decreased with coarsening of the sandy sediment and increase of sedimentation rate, culminating in complete disappearance of fish fossils before the Emsian. The sensory line system of the pteraspidids, with supraorbital commissures developed on the rostral plate and arrangement of dorsomedial canals on the dorsal shield, is proposed to have high diagnostic value. Some 33 species, belonging to 20 genera of the heterostracans and 16 species of 9 genera of the osteostracans, are described. Althaspis tarloi sp. n., Djurinaspis secunda sp. n., Palanasaspis chekhivensis gen. et sp. n., Podolaspis danieli sp. n., Semipodolaspis slobodensis gen. et sp. n., and Zenaspis kasymyri sp. n. are proposed.