Stemec

Stemec was a relatively small plotopterid, comparable in size with its Miocene relative Plotopterum. It is only known from a single right coracoid belonging to an adult bird, as proven by the smooth surface of the shaft, although two fragmentary tibiotarsi found in the same locality and similar to those of modern cormorants might have belonged to the genus, or to another small-sized plotopterid. The holotype coracoid is 74.8 mm long, 12.8 mm wide, and fairly complete, although erosion had damaged part of its surface. It shared more similarities with the known coracoid of Plotopterum than with those of the larger Tonsalinae plotopterids like Copepteryx and Tonsala. It was although mostly differenced from Plotopterum by several osteological specificities, including the raised and flat platform formed by the facies articularis humeralis, and the absence of a prominent groove formed by the sulcus supracoracoideus typical of the later genus.^[1]

During the Oligocene of Vancouver Island, Stemec lived alongside the Desmostylian Cornwallius, the Aetiocetid Chonecetus and the Pelagornithid Cyphornis in a shallow marine environment rich in sessile fauna.^[1] The discovery of Stemec and its close relative Plotopterum in coastal deposits indicate that the latest and smallest plotopterids had costal habits, when the larger and older tonsalin plotopterids where pelagic feeders. Elements of their osteology indicates that they were not proficient swimmer like their larger, open sea specialist relatives, which may explain both the apparent lack of competition between larger and smaller plotopterids and the later survival of the latter form.^[2]