2020 年 108 巻 p. 11-21
Vertebrate burrows from the Mesozoic of North America have been scarcely known. We report two different burrows (burrows A and B) produced by small animals in the Lower Cretaceous (Albian–Cenomanian) Mussentuchit Member of the Cedar Mountain Formation, Utah, USA. The burrow-bearing bed of the Mussentuchit Member consists of poorly-drained paleosols, and the burrows are infilled with light-colored carbonate probably due to a rise in a regional water table. Both burrows were found in-situ and inclined downwards. The burrow A is 60 cm long, and terminated in an expanded distal chamber, whereas the burrow B is 100 cm long and branched, with some small expanded chambers in the middle of the tunnel. Both tunnels have the width to height ratio larger than 1.3. In the burrows, the external walls lack scratch marks, but do show localized, prominent bulges in the burrow A and divots at local expansions of the tunnel in the burrow B. These are unlike those reported from Triassic and Jurassic vertebrate burrows. The estimated weight of the excavators is 3.1 g for the burrow A and 6.8–17.8 g for the burrow B based on the area of each tunnel, indicating that both tracemakers were small animals. A bulge in the burrow B was possibly left by tip of the excavator’s head, as seen in the burrows of modern fossorial squamates. The discovery of a potential squamate burrow from the Cedar Mountain Formation of Utah is consistent with the oldest body fossils of skinks and snakes from the Early Cretaceous.