You are probably refering to this article in Nature (which I am sure you never read)
Tetrapod trackways from the early Middle Devonian period of Poland
Grzegorz Niedźwiedzki, Piotr Szrek, Katarzyna Narkiewicz, Marek Narkiewicz & Per E. Ahlberg
Tetrapod trackways from the early Middle Devonian period of Poland : Abstract : Nature
From the paper:
"The Zachemie trackways show that very large stem-group tetrapods, exceeding 2 m in length, lived in fully marine intertidal to lagoonal environments along the south coast of Laurussia during the early Eifelian, some 18 million years before the earliest-known tetrapod body fossils were deposited. This forces us to infer much longer ghost lineages for tetrapods and elpistostegids than the body fossil record suggests (Fig. 5a, b). (Ghost lineages are those that must have existed at a particular time, according to the phylogeny, but which are not represented by fossils at that time.) Until now, the replacement of elpistostegids by tetrapods in the body-fossil record during the midlate Frasnian has appeared to reflect an evolutionary event, with the elpistostegids as a short-lived transitional grade between fish and tetrapod morphotypes (Fig. 5a). In fact, tetrapods and elpistostegids coexisted for at least 10 million years (Fig. 5b). This implies that the elpistostegid morphology was not a brief transitional stage, but a stable adaptive position in its own right. It is reminiscent of the lengthy coexistence of non-volant but feathered and winged theropod dinosaurs with volant stem-group birds during the Mesozoic."
Note that the paper refers to "stem-group tetrapods," not reptiles.