First report of an aulaceratid stromatoporoid from the Ordovician of Baltica

The aulaceratid stromatoporoids first appeared in the late Middle Ordovician and had achieved a worldwide distribution in tropical to subtropical shallow-marine environments by the Late Ordovician; however, their presence in Baltica has not been documented. Here, we report Aulacera vohilaidia Jeon and Toom sp. nov. from the Upper Ordovician Adila Formation (Pirgu Regional Stage; late Katian stage) of Estonia, which is the first record of an aulaceratid stro­matoporoid in the Ordovician of Baltica. Aulacera vohilaidia Jeon and Toom sp. nov. is char­acterised by three distinct skeletal zones (axial, lateral, and outer), each of which consists of cyst plates with distinctive size and convexity. The axial zone is narrow, featuring large horseshoe-shaped cyst plates with a wavy, imbricated pattern in a line. The lateral zone consists of small cyst plates punctuated by sporadically developed pillars, and the outer zone comprises flat, parallel cyst plates. Ridged structures with cusps are formed by parallel cyst plates in the outer zone, and the growth surface displays a pustular reticulated structure with a polygonal morphology. The occurrence of Aulacera vohilaidia Jeon and Toom sp. nov. in Estonia extends the palaeogeographic distribution of aulaceratid stromatoporoids from peri-Gondwana, Laurentia, and Siberia to Baltica. The arrival of aulaceratid stromatoporoids in Baltica coincided with warming of the climate on the palaeocontinent and the expansion of the range of this group during the Late Ordovician