Ichnostratigraphy of middle Cenozoic Coprinisphaera from central Patagonia: Insights into the evolution of dung beetles, herbivores and grass-dominated habitats

M. VICTORIA SÁNCHEZ, JOSÉ H. LAZA, EDUARDO S. BELLOSI, JORGE F. GENISE

PALAEOGEOGRAPHY PALAEOCLIMATOLOGY PALAEOECOLOGY

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2010 vol. 297 p. 633 - 648

0031-0182

Dung beetles use the dung of herbivores, particularly large mammals, to provision their underground nests in which they construct brood balls where the egg is laid. These insects are usually linked to grass-dominated habitats (GDH) inhabited by abundant and large mammals, which, feeding on grasses, produce large amounts of the insects´ food: dung. Fossil brood balls of dung beetles, included in the ichnogenus Coprinisphaera, are typical trace fossils of Southern South American Cenozoic palaeosols. This ichnogenus includes five ichnospecies, C.murguiai, C. isp. A, C. kheprii, C. tonnii, and C. kraglievichi, all ofwhich are represented in a pyroclastic fluvial and loessic (eolian) sequence of the middle Eocene-lower Miocene Sarmiento Formation at the classic palaeontological locality of Gran Barranca (Chubut, Argentina). This locality bears a detailed litho and chronostratigraphic framework that is based on several sedimentologic and magnetostratigraphic profiles and numerous radiometric ages. These provide a high-resolution stratigraphy for tracking the changes in dung beetle faunas as reflected in their trace fossils, and accordingly, to record the evolution of GDH. Particular objectives are to analyze: 1) the first Coprinisphaera records; 2) the increase in Coprinisphaera size and its relationship to mammal size; 3) the relative sizes of ichnospecies of Coprinisphaera to infer the potential dung beetle diversity; and (4) themajor changes in the evolution ofGDHas reflected by dung beetle trace fossils.About 1200 specimens of Coprinisphaera belonging to the five ichnospecies and to 15 statigraphical levels were collected, sectioned, identified, andmeasured. The parameters statistically analyzedwere: relative abundance of ichnospecies, density of specimens per m2, and equatorial diameter (ED). From this analysis it was possible to recognize several episodes in the evolution of dung beetle faunas and the associated GDH of southern South America. The first episode, by the latemiddle Eocene (39.8 Ma), records a favorable palaeoenvironmental change that achieved the critical point to support a GDH suitable for the appearance of the first dung beetles. This episode finisheswith the total absence of dung beetle traces due to arid conditions. The second episode, by the early late Eocene (approximately 37.3Ma), records a burst in the density, abundance, and diversity of Coprinisphaera and dung beetles favored probably by the settlement of GDH with large herbivores. This episode concluded near the Eocene–Oligocene transition, with another xeric period without Coprinisphaera records. During the early late Oligocene (approximately 26Ma), the third episode shows similar parameters than the second one recording the return to the optimal palaeoenvironmental conditions of the second episode. The fourth episode (approximately 24.7 Ma), during the middle late Oligocene, reflects changes within a relatively stable ecosystem, recording a significant increase in Coprinisphaera size, along with the disappearance of the smallest specimens, triggered by the appearance of large to huge herbivores. During the Oligocene–Miocene transition, the fifth episode (24.2–20.4 Ma) records stability in palaeoenvironmental conditions, suggested by similar parameters to those of the previous episode. During the middle early Miocene (approximately 19.8Ma), the sixth episode records unfavorable environmental changes for dung beetles and GDH at Gran Barranca. This episode records the first evidence of a deterioration of the dung beetle fauna. Fromthe latest earlyMiocene onward (19.3–18.7 Ma), there is no more evidence of Coprinisphaera, suggesting unfavorable environmental conditions that led to the disappearance of dung beetles from the area of the Gran Barranca.