The Synthetoceratinae (Mammalia, Tylopoda, Protoceratidae). Bulletin of the AMNH ; v. 145, article 2

Supplemental Materials
Journal Title
Journal ISSN
Volume Title
New York : [American Museum of Natural History]
"The subfamily Synthetoceratinae, a group of uniquely horned North American Tertiary ruminants, marks the evolutionary culmination of the family Protoceratidae. In addition to a pair of widely curved postorbital horns this family is characterized by the development of protruding bony plates on the upper surface of the maxillaries that phyletically unite to form a common-trunked rostral horn, a feature that has no parallel among other horned ruminants. Members of the North American endemic subfamily are distributed from South Dakota to Texas, eastward across the Gulf Coast to Florida, and north into New Jersey. Temporally, they ranged from the late Arikareean of the northern Great Plains to the early Hemphillian of the Gulf Coastal Plain. Taxonomic changes within the subfamily, Synthetoceratinae, include (1) the transfer of Syndyoceras from the monotypic subfamily, Syndyoceratinae, to the Synthetoceratinae, (2) the placement of Stirton's genus, Lambdoceras, in synonymy with Prosynthetoceras but retaining it as a subgenus (Prosynthetoceras (Lambdoceras)), and (3) the synonymy of Prosynthetoceras siouxensis dawesensis Frick with Prosynthetoceras (Lambdoceras) siouxensis Frick. As now defined, the subfamily includes three genera: Syndyoceras Barbour, Prosynthetoceras Frick, and Synthetoceras Stirton. Syndyoceras, the earliest occurring genus, is the first protoceratid to possess a single-trunked rostral horn. Monotypic Syndyoceras (S. cooki) is found only in the northern Great Plains. Prosynthetoceras, on the other hand, is by far the most widely dispersed genus in the subfamily, occurring in the Great Plains, and in the Gulf and Atlantic coastal plains. The genus is herein divided into two mostly allopatric subgenera: Prosynthetoceras (Prosynthetoceras), including P. (P.) texanus and P. (P.) francisi, and Prosynthetoceras (Lambdoceras), including P. (L.) hessei, P. (L.) siouxensis and P. (L.) trinitiensis. The type subgenus is restricted to the Gulf Coastal Plain, whereas P. (Lambdoceras) is distributed in both the northern Great Plains and in the Texas Coastal Plain. The latest occurring and most advanced member of the lineage is the monotypic genus Synthetoceras (S. tricornatus), which is restricted to Texas and the Gulf Coastal Plain. Evolutionary changes in the Synthetoceratinae involved (1) increased total body size, (2) increased length of the shaft of the rostral horn, (3) lengthened diastemata, (4) reduced premolars, (5) taller-crowned molars, and (6) reduced lateral metapodials. Whereas early synthetoceratine evolution took place in the Great Plains, an early dispersal, (?early Hemingfordian), brought the ancestors of the Prosynthetoceras lineage to the Texas Gulf Coastal Plain where they eventually spread eastward into Florida and northeastward along the Atlantic Coastal Plain. The P. (Lambdoceras) group, however, remained in the Great Plains during most of its evolutionary history and is recognized only in early Barstovian deposits on the Texas coast. Synthetoceras, the direct descendant of Prosynthetoceras, flourished throughout the Gulf Coastal Plain; some populations spread north into the southern Great Plains, where they are preserved in the Clarendonian of the Texas panhandle, and others persisted, at least in Florida, until the early Hemphillian"--P. 216.
p. 123-218, [2] folded leaves of plates : ill., map ; 27 cm.
Includes bibliographical references (p. 216-218).