The Origin Ofchubutolithes Ihering, Ichnofossils from the Eocene and Oligocene of Chubut Province, Argentina

The distinctive trace fossil Chubutolithes gaimanensis n. ichnosp. occurs in Casamayoran (early Eocene) and Colhu6haupian (late Oligocene) alluvial rocks of the Sarmiento Formation in eastern Chubut Province, Argentina. Though known for nearly 70 years, its origin has remained obscure. Examination of new specimens and comparisons with modem analogs demonstrate that specimens of Chubutolithes represent the fossil nests of a mud-dauber (Hymenoptera: Sphecidae). Virtually identical nests are constructed today by mud-daubers in areas as disparate as southern Santa Cruz Province, Argentina, and Nebraska, confirming that quite similar trace fossils can be produced by several different taxa in a higher taxonomic clade. No satisfactory ethological term exists for trace fossils that, like Chubutolithes, were constructed by organisms above, rather than within, a substrate or medium. The new term aedificichnia is proposed. Chubutolithes occurs in alluvial paleosols and is associated with a large terrestrial ichnofauna. These trace fossils include the nests of scarab beetles, compound nests of social insects, and burrows of earthworms. INTRODUCTION of Casamayoran (early Eocene) age, and not in the considerably C HUBUTOLITHES IS a name coined by Ihering (1 922) for small, YOUWer (Miocene) ''Patagonian" sensu strict0 as thought by enigmatical fossils found by him in Eocene rocks of PatSchiller (1 925). In the Gaiman area, these rocks are currently agonia, Argentina. Specimens of Chubutolithes were originally included in the Pan de AzGcar Member of the Sarmiento Fordescribed (but not named) by Windhausen (1921) from their mation (Mendia and Bayarsky, 1981). One specimen is now type locality, just west of the town of Gaiman, Chubut Province known from the Trelew Member of the Sarmiento Formation (Figure 1). Additional specimens were soon discovered in the (Colhu6haupian = late Oligocene). "fomaci6n Patagonia," north of Comodoro Rivadavia, by In the course of geologic investigations in the Gaiman area Schiller (1925, fig. I), who was the first to figure them. Although in 1983 and 1987, Bown obtained several specimens of this he gave a scientific name to his finds, Ihering (1922) provided fossil from its type locality on the Pan de Azficar (Simpson, no specific designation, considering them to be the remains of 1935, figs. 3,4), and concluded that it is almost certainly a trace an unknown invertebrate. fossil. Further studies by both of the authors confirm this deChubutolithes has been known in Argentina for well over half termination and offer a solution to its origin. Because Chubua century; however, there has been no consensus about its origin. tolithes is a very distinctive form that is quite abundant locally Windhausen (192 1) believed it to be a mammal coprolite; howin rocks of Casama~oran age, and because the name Chubutoever, Schiller (1 925, p. 36) was very impressed by the fine detail lithes has long priority, this trace fossil is given a new ichnospeof the surface structure, which is faithfully reproduced in every cific name, but Ihering's (1922) designation is retained as the specimen. He stressed that the structure is an extremely notable ichnogeneric name. problematic form that he and his colleagues were unable to Abbreviations for institutions are: MACN, Museo Argentino explain. After more reflection, Schiller thought it possible that Ciencias Naturales "Bernardino Rivadavia," Buenos Aires; it was a "trace" of a coelenterate. Like Windhausen, Frenguelli USGS, U.S. Geological Survey, Denver, Colorado. (1 927, p. 239) also found Chubutolithes in the section at Gaiman and believed that it represents the internal mold of a cavity SYSTEMATIC PALEONTOLOGY produced by some "inferior" organism that could have been a coelenterate, or possibly a tunicate. Ichnogenus CHUBUTOLITHES Ihering, 1922 Simpson (1 935, p. 13-1 5) provided the first detailed descripType species. -Chubutolithes gaimanensis n. ichnosp., type tion of Chubutolithes and, although he was reluctant to speculate and only known ichnospecies. about its origin, he observed (p. 15) that they ". . . probably are Diagnosis. -Cylindrical to fusiform compound structures, organic in at least a broad sense of the word." Simpson also roughly 3-7 cm in length by 1.5-4.5 cm wide; made up of one first detailed the stratigraphic occurrence of the fossil, recording to several hemicylindrical cells adjacent and parallel to one that it is abundant in his (1935, p. 13 and fig. 3) "Section 2, another; one side unstructured, other side exhibits from one to stratum h" near Gaiman (Simpson's Gaiman Nuevo). Both several superficial parallel grooves, parallel to the long axes of Windhausen (1921) and Frenguelli (1927) observed that Chuthe cells and partitioning the compound specimen into two or butolithes at Gaiman was found associated with a nodular bed more raised, curvilinear cells; between the transverse lines, and ("bancos con nodulos" and "10s bancos de Chubutolithes," reforming relief on the raised cells, a second pattern consists of a spectively-see Figure 2, this paper), a fact not explicitly stated series of intersecting curvilinear lines that, though curved, are by Simpson but almost certainly observed by him. The occurtangentially perpendicular to the long axes of the cells; these rence of Chubutolithes at Gaiman was also noted parenthetically lines are also incuse, and numerous small, lozenge-shaped areas by Bordas (as "chubutolites," 1937, p. 2 17). between them and bounded by their intersections are slightly Simpson (1 935) was able to document that all specimens of raised in reliec a few well-preserved specimens exhibit a partlyChubutolithes known during the time of his study (those from filled aperture, about 5 mm in diameter, to one side of one end near Comodor Rivadavia and from Gaiman) occur only in rocks of the long axis (dorsally); interiors of cells unstructured.