The Spongeliomorpha Code: Deciphering the Bioglyph Inscriptions on the Walls of the Ancient Tunnels of Murcia

In the quest to determine the maker and meaning of curious trace fossils, the signature of the ancients sometimes is inscribed on the burrow walls for all to see, but the code is difficult to crack. Anastomosing tunnels of Spongeliomorpha iberica in Miocene basins of Murcia and Catalunya, eastern Spain, contain legible bioglyphs, if only the ichno-cryptographer can decipher the message left by the ancient scribes.

What do the inscriptions on the burrow walls tell us about the tunnel maker? Why was the creature burrowing? Was the sea floor soft or firm or hard (indurated)? How deep was the burrow? Was it a pre-depositional excavation that was exhumed by erosion and cast by overlying sediment, or was it a post-depositional burrow created by animals crawling down to reach a concealed firmground interface? Why do many tunnels exhibit a tapering end? What skills were required of the tunnel builders? Who was it who dug the tunnels and left behind their fingerprints to tease us with the puzzle?

Most of these questions can be answered by reading the delicate bioglyphs scratched in the walls of Spongeliomorpha that are preserved in shallow-marine sequences in eastern Spain. The tunnel makers were decapod crustaceans with robust chelipeds, and they were burrowing for purposes of dwelling and possibly also feeding in firm, fine-grained sediment. Although the depth of burrowing is unclear, the burrows were produced pre-depositionally in marl that was scoured and subsequently filled by coarse, fossiliferous sediment. The bioglyph patterns and tapering shape of the tunnel termini suggest that both scraping and grasping behaviors were used, and the tapering ends of tunnels suggest that they may have been excavated during feeding. Unless and until we find a body fossil of the ancient architect within its burrow, its exact identity will remain a mystery.