Early Cretaceous lepidosaur (sphenodontian?) burrows

Abstract

Scarce fossil tetrapod burrows have been recorded in Cretaceous rocks, which is probably linked to the dominant equable climates that existed for most of this period. The occurrence of Cretaceous tetrapod burrows from Patagonia (Chubut Province, Argentina) dated between 118 and 115 million years ago, gives insights into their paleoecology and paleoenvironment. The rocks containing the tetrapod burrows are of pyroclastic origin and represent eolian dunes and ash-fall deposits, some reworked by fluvial currents and others showing soil development. Fossil burrow casts preserved in a paleosol are composed by a ramp with a slightly curved or straight path in plan-view and lacking bifurcation, a rounded termination with no enlargement, showing a reniform cross-section, and are assigned to the ichnospecies Reniformichnus katikatii. The strongly flattened cross-sectional shape of the burrow casts and comparison with modern lizard burrows suggest that the producers were lepidosaurs (body mass = 50-323 g). Among Cretaceous fossorial lepidosaurs from Patagonia, the best candidate is an eilenodontine sphenodontian. Sphenodontians burrowed in the fossil soils where also arthropods, earthworms and shrubby plants thrived. The rare occurrence of tetrapod burrows in Cretaceous rocks is linked to stressing conditions related to frequent arrival of volcanic ash and a semiarid seasonal climate.

Figure 1

Location, stratigraphic section and exposure of the Puesto La Paloma Member of the Cerro Barcino Formation. (a) General stratigraphic section of the Puesto La Paloma Member at the study area, showing the dated level and the Los Chivos paleosol (LCP) containing the tetrapod fossil burrows. (b) Location map. (c,d) Outcrop view of the intermediate and lowermost section of the Puesto La Paloma Member at the study locality. The arrow in (c) indicates the Los Chivos paleosol. Abbreviations in c and d refer to facies associations. DI: dry interdune. WI: wet interdune. ED: eolian dune. See Table S1 and Fig. S2.

Figure 2

Field occurrence of tetrapod burrow casts. (a,b) Plan view of curved tetrapod burrow casts. (c) Vertical exposure of upper part of the Los Chivos paleosol with two tetrapod burrow casts (arrowed). (d,e) Plan view of fallen block of top of Los Chivos paleosol with several burrow casts (red) and rhizoliths (gray) and interpretative diagram. The inset in (b) is a rose diagram of the dip azimuth of the burrow casts in the block.

Figure 3

Plan view of tetrapod burrow casts. (a,b) Rounded and not enlarged terminations (MPEF-IC 4310 and 4312). Arrows in (a) points to cylindrical protuberances (invertebrate burrows). Scale divisions are 1 cm. (c) Low dipping ramp (MPEF-IC 4311).

Figure 4

Cross-section shape of R. katikatii and inferred body mass of the producer. (a) MPEF-IC 4310. (b) Fs#14. (c) MPEF-IC 4314. (d) MPEF-IC 4315. (e) Fs#11. (f) MPEF-IC 4318. The arrow points to an invertebrate burrow. (g) MPEF-IC 4312. (h) Histogram of inferred body mass obtained using the formula by Wu et al..

Figure 5

Surface ornamentation of R. katikatii. (a–c) Plan view, interpretative diagram and oblique lateral view of MPEF-IC 4310. White arrows and grey areas (in b) indicate invertebrate burrows. Bracket in (c) indicates a set of claw traces. (d,e) Claw traces on roof and smooth bilobed bottom in MPEF-IC 4314. (f) Nearly flat bottom with subtle claw traces (white arrow) and invertebrate burrow (yellow arrow) in MPEF-IC 4318. (g) Smooth bilobed bottom in MPEF-IC 4317. (h) Bilobed bottom with claw traces forming a chevron pattern (arrows) in MPEF-IC 4315.

Figure 6

Massive fill of R. katikatii. (a,b) Polished cross-section of burrow fill and interpretative drawing. Note accretionary lapilli (white arrow) and pumice clasts (yellow arrow) both surrounded by concretionary cementation. (c) Tridimensional model from CT of MPEF-IC 4312 (oblique lateral view) with transparent outline and orange bodies interpreted as denser parts product of cementation. The arrow indicates the subvertical burrow of (d). (d) Detail of quasi-spiral submillimetric burrow with a rounded and enlarged end.

Figure 7

Modern Liolaemus sp. plaster burrow casts (a,c,e,i, and j belong to GHUNLPam 29090; and b,d,f–h to GHUNLPam 29091). (a,b) Side view. The dashed line marks the terrain surface (see also Fig. S3). Arrows indicate the excess plaster poured in the surface to mark the ground surface. (c,d) Bilobed bottom, note that the distal end is smooth. (e,f) Plan view, note distal curvature. Arrows indicate the excess plaster. (g–j) Surface ornamentation and interpretative drawing of the distal part of burrow casts.

Figure 8

Trace fossils associated with R. katikatii. (a–c) Edaphichnium lumbricatum composing subvertical burrows (a,b) and a swelling in (c) (black arrows). Note bifurcation (white arrow in a) and meniscate fill (white arrow in b). (d) Skolithos linearis (black arrows) with rounded end (white arrow). (e,f) Taenidium barretti (margin of burrow arrowed). (g–j) Siliceous rhizocretions (black arrows). Note secondary bifurcation (white arrows in g and j) and concentric rings (in h and i). (g) and (h) are plan views, (i) is a polished section and (j) is a subvertical exposure. (k) Subvertical ferruginous rhizolith, including root cast (white arrow) and lighter halo (black arrows).