STRATIGRAPHIC CONTEXT OF TIKTAALIK ROSEAE (LATE DEVONIAN): PALEOENVIRONMENT OF THE FISH-TETRAPOD TRANSITION

Discovery of the Late Devonian fish, Tiktaalik roseae, provides a new opportunity to examine the environmental context of the fish-tetrapod transition. Here we present the stratigraphic framework for the 30 m above and below the T. roseae-bearing portion of the Fram Formation. The Fram was described by Embry and Klovan (Bull. Can. Pet. Geo., 1976) as an alternating series of overbank and channel sandstones of a meandering river system – our current analysis corroborates and elaborates on this characterization. The examined Fram consists of 3.0- to 7.0 m-thick packages of stacked, sandy mudstones (individual units are 0.2- to 2.0 m-thick) separated by 0.1 m- to decameter-thick units of massive, thinly laminated, and/or cross-bedded sandstones with erosive bases. Mudstones have sharp or rapidly gradational bases and most are substantially pedogenically altered. These paleosols are generally calcareous with well-developed vertical root traces and mm-scale rhizoliths penetrating to ≤ 0.5 m; some paleosols show evidence of multiple pedogenic cycles. Thin sand bodies can be laterally discontinuous, but the large channel and pointbar sands form resistant, continuous ledges. The thickest of these sands is multistoried with a basal conglomerate and dramatic soft-sediment deformation. With the exception of isolated fish scales in the basal conglomeratic layers of a few sand bodies, vertebrate remains are restricted to a single sandy mudstone up to 0.9 m-thick that shows little evidence of pedogenesis. Multiple individuals from six taxa are heterogeneously distributed throughout the unit in a variety of orientations and plunging angles. Individuals occur as isolated skeletal elements and as semi-articulated individuals; this and variation in bone wear suggests that the assemblage is time-averaged. The presence of semi-articulated body fossils of T. roseae and other taxa suggest some individuals died relatively close to their place of burial. The stratigraphic context of T. roseae indicates pulsed sedimentation and soil development in a fluvial system under dry to seasonally dry conditions with floodplains dominated by bushy vegetation.