A recently published article in Historical Biology features the first detailed track and trace-fossil evidence of very large crocodiles from a number of sites north of Tumbler Ridge (Plint et al., 2022). The crocodiles were scraping the muddy bottom with their claws while swimming, creating 'swim traces'. Some of these traces showed remarkable detail, e.g., parallel striations registered by scale patterns on the crocodiles’ feet.

The tracks and traces are from the Dunvegan Formation, from the Cenomanian Stage of the Cretaceous (95–97 Ma). In addition to the crocodile swim traces and tracks, tracks and traces of ankylosaurs, ornithopods and turtles were present.

The tracks and traces were made on a low-lying delta-plain with abundant, vegetated wetlands, shallow lakes, and river channels , located about 100 km inland from the marine shoreline of the Western Interior Seaway (that linked the Gulf of Mexico with the Polar Ocean).  It was possible to document multiple episodes of flooding  and exposure, which determined whether and when the animals walked or swam. This helped explain the variety of tracks and traces that were identified.

The size of the crocodiles can be estimated from the distance between their claws as they swam and left traces. We used the distance between swim-trace impressions to estimate a total body length of about 9 metres, and possibly as much as 12 metres long. A partial crocodile track at one of the sites was 75 cm long, allowing for a similar total length estimate of just less than 9 metres.

Giant crocodiles have been reported several times in the fossil record. In North America the oldest body fossil evidence of giant crocodiles is of Deinosuchus (at ~82 Ma), which has been estimated to have been from 8–12 metres long.  By comparison, the record length of crocodiles living today is 6 metres.

Deinosuchus has been recorded from the U.S.A. and Mexico, but not from Canada. These large swim traces from north of Tumbler Ridge may represent a precursor to Deinosuchus, that lived at least 13 million years before the first reported appearance of giant crocodiles in the North American body fossil record.

The co-existence, on a single surface, of traces made by walking ankylosaurs and swimming crocodiles was intriguing and has not been previously reported. One of the ankylosaur trackways, registered in what was the muddy bed of a dried-up lake, is the smallest thus far described from the Dunvegan Formation.  Its tracks were just 10 cm wide, presumably made by a juvenile.

Many Tumbler Ridge Museum volunteers, like Tiffany Hetenyi and Lisa MacKenzie, contributed extensively to the fieldwork performed at these sites. In the fall of 2020 four large blocks containing some of the finest examples of crocodile and dinosaur tracks and traces were recovered by crane and brought to the Tumbler Ridge Museum, where they are securely stored.

While the Tumbler Ridge area has become well known for its dinosaurs, there is something special about crocodiles. They did not become extinct at the end of the Cretaceous, but have ‘survivor status’, and are still with us today with a recognizably similar body plan. Crocodiles of this gigantic size were likely top predators.

At a personal level, having grown up in Africa, and thus being familiar with and very respectful of crocodiles, it has been an unforgettable experience helping to identify and describe traces of some of the largest crocodiles that have ever existed. This is matched by the privilege of working with authors Guy Plint and Martin Lockley, who are highly experienced  in their respective fields of sedimentology and ichnology.

This publication follows  an article in Cretaceous Research in 2021 that documented exceptionally well-preserved, 112 Ma  swim traces, made by smaller crocodylians at the Quintette Mine site near Tumbler Ridge.

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References:

Lockley, M.G., Plint, A.G., Helm, C.W., Sharman, K.J., Vannelli, K.M. 2021. Crocodylian swim tracks from the Gates Formation (Albian), British Columbia, Canada: comparisons with Cretaceous crocodylian ichnology in western USA. Cretaceous Research 128, 104967. https://doi.org/10.1016/j.cretres.2021.104967

Plint, A.G., Charles W. Helm, C.W., Lockley, M.G. 2022. Crocodylian and dinosaur trace fossil assemblages from crevasse splay/levee and floodplain lake environments: middle Cenomanian Dunvegan Formation, northeast British Columbia, Canada. Historical Biology. https://doi.org/10.1080/08912963.2022.2043294

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