Eoastrapostylops riolorense Soria and Powell, 1981, is a primitive meridiungulate mammal known by two specimens from the early Paleogene Río Loro Formation, in NW Argentina. The holotype and most complete specimen is an almost complete skull, mandible, and a few associated postcranial elements. Eoastrapostylops is one of the oldest South American ungulates with a well-preserved skull; although its original description included some cranial characters, its attribution to the endemic South American Order Astrapotheria and the concomitant phylogenetic consequences were based on dental features. New preparation and examination of the holotype (PVL 4216) revealed features not mentioned and/ or incorrectly interpreted in previous studies; particularly important are those of the auditory region. The cranial anatomy is here re-described and compared with that of Astrapotherium Burmeister, 1879, providing the first detailed description of a basal meridiungulate auditory region. The general structure of the preserved elements of E. riolorense resembles more closely that of archaic “ungulates” (i.e., condylarths) and litopterns than that of Astrapotherium or other astrapotheres with known skulls (e.g., Trigonostylops Ameghino, 1897, Astraponotus Ameghino, 1901); the more remarkable differences are: presence of distinct apertures (foramen ovale) for passage of the mandibular ramus of the trigeminal nerve, surfaces on the alisphenoid and squamosal for attachment of the ectotympanic bone, low and crest-like postglenoid process, postglenoid foramen medial to the postglenoid process and not piercing its base, almond-shaped promontorium with a strong caudal tympanic process almost obliterating the post-promontorial tympanic sinus, and the mastoid process exposed laterally and posteriorly. We performed three sets of cladistic analyses based on previously published matrices, including dental, cranial, and postcranial features scored in a wide sample of South American ungulates and archaic “ungulates.” The results suggest that Eoastrapostylops represents a basal meridiungulate lineage that diverged before the differentiation among astrapotheres, pyrotheres, and notoungulates, and thus it can be classified neither within Astrapotheria nor another clade of ordinal rank. Other groups of still uncertain status (e.g., Notopterna, Indalecidae) also would represent independent basal radiations, which would have characterized the early meridiungulate evolution, although this topic needs a more exhaustive exploration.

Trilobites from the Lower Mississippian (late Tournaisian) Redwall Limestone of Arizona are rare. Four identifiable specimens were recovered and studied. These specimens, in conjunction with recorded stratigraphic distributions of lost Redwall trilobites, allows for the assemblage of a range chart. Identifiable trilobite species include: Aprathia sp., Phillipsia peroccidens (Hall and Whitfield, 1877), Cummingella? sp., and Cummingella sp. The Redwall trilobites in this small collection exhibit a much closer taxonomic affinity to the late Tournaisian forms known from Europe than they do to species from strata of similar age in the eastern, central, and southern United States. This suggests that during the late Tournaisian the inferred geographic barrier known as the Transcontinental Arch prominently separated the Redwall depositional location from that of the central and eastern United States. This barrier appears to have contributed to the endemism of trilobites of the eastern United States.

A new late Mississippian trilobite species, Kaskia pitkinensis, new species, is described from the Pitkin Formation of Arkansas. Kaskia pitkinensis, recovered from early Serpukhovian strata, is the youngest species of the Kaskia clade known in the United States. Kaskia c.f. K. wilsoni (Walter, 1924) also is illustrated. Cyrtoproetus kerhini, new species, from the upper Fayetteville Formation of Arkansas is also described. Richterella hindesvillensis Hessler, 1965, from the Hindesville Limestone of Oklahoma, is herein reassigned to the genus Cyrtoproetus Reed, 1943. Cyrtoproetus kerhini, new species, and C. hindesvillensis (Hessler, 1965) are the first known occurrences of Cyrtoproetus in North America.

The large Miocene beavers (Castoridae) Anchitheriomys Roger, 1898, and Amblycastor Matthew, 1918, have been often considered as synonyms (Flynn and Jacobs 2008). Most recently, they have been distinguished as separate genera based on the morphology of the dentary: Amblycastor restricted to North America and Anchitheriormys limited to Eurasia (Mörs and Hulbert 2010). However, the dentary that was cited as evidence for the separation lacks cheek teeth and its specific identification cannot be verified. In addition, a previously undescribed specimen from the type area of “Amblycastor” fluminis Matthew, 1918, demonstrates that the morphologies used to separate these genera are not consistent with the North American specimens; thus, the two genera are synonymous and the North American species should be referred to Anchitheriomys.