Hesperornithiform Birds from the Late Cretaceous (Campanian) of Arkansas, USA

The Late Cretaceous Hesperornithiformes constitute a geographically expansive order of early birds, spanning the Northern Hemisphere from Asia to North America. Within North America, hesperornithiforms have been reported from as far north as Ellesmere Island in the Arctic to the Gulf Coastal Plain of southwest Arkansas, a distribution spanning the entire Western Interior Seaway. This paper presents two new specimens of Hesperornis from the middle to late Campanian of Arkansas: an isolated phalanx from the third toe and a section of the shaft of a tibiotarsus. These are the second and third occurrences of Hesperornis from the southeastern-most portion of the range in North America. An expanded description of the only other hesperornithiform material from Arkansas, an isolated partial tarsometatarsus, is also provided. Morphometric comparisons with other hesperornithiforms show that these specimens are some of the largest known individuals of Hesperornis.