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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.

The type specimen of a large, flightless marine bird, Hesperornis regalis, was collected by O.C. Marsh in 1871. Due to the practices of the time, and the lack of accurate maps, the type locality was initially recorded by Marsh in a letter simply as the “upper Cretaceous of Western Kansas,” and then later reported as “the gray shale near the Smoky Hill River in Western Kansas.” Since the Smoky Hill Chalk Member of the Niobrara Formation is exposed along the Smoky Hill River in present day Wallace, Logan and Gove counties over a distance of more than 75 miles, this vague locality information encompasses about 5 million years of depositional history. An 1880 account by Marsh narrows the locality down to the south bank of the Smoky Hill River about 20 miles east of Fort Wallace in Logan County. The results of a recent field investigation of the eastward route traveled by Marsh's 1871 Yale College Scientific Expedition sheds light on the probable locality of both the type specimen (YPM 1200) of Hesperornis reg...

The Smoky Hill Chalk Member of the Niobrara Chalk was deposited in the Western Interior Sea during a span of about five million years extending from the upper Coniacian through the lower Campanian. Coincidentally, this period also encompasses much of the early evolution and radiation of the family Mosasauridae. Thousands of mosasaur specimens have been collected from the Smoky Hill Chalk Member of western Kansas since the first Yale College Scientific Expedition in 1870. Early workers viewed the entire Smoky Hill Chalk Member as the “Pteranodon beds” and were not able to provide accurate stratigraphic information. Even though mosasaurs are represented by numerous complete and well preserved specimens, the ranges of the various species could not be defined further without better stratigraphic data. In 1898, S. W. Williston first described the general distribution of mosasaurs within the Rudistes and Hesperornis beds of the Smoky Hill Chalk Member. Seventy years later, D. A. Russell grouped mosasau...

Despite the abundance of hesperornithiform fossils from the Western Interior of North America, particularly the Niobrara Chalk of Kansas, few specimens are known of Parahesperornis, a toothed, foot-propelled diving bird somewhat smaller than Hesperornis. Numerous specimens of three species of Hesperornis as well as those of Baptornis advenus are known from the Smoky Hill Chalk of western Kansas, but only two specimens of Parahesperornis have been collected to date. This paper presents a new specimen of Parahesperornis and a detailed description of the tarsometatarsus of this genus. Minor differences in morphology of the isolated tarsometatarsus presented here prevent its inclusion in Parahesperornis alexi, however these differences are not sufficient to warrant the designation of a new taxon at this time.