'Lepeichnus giberti', a new trace fossil with exceptional characteristics in the palaeontology world

The new work is signed by the experts Zain Belaústegui, Jordi Martinell and Rosa Domènech, members of the PaleoNeoMar group from the Department of Earth and Ocean's Dynamics and the Institute for Research on Biodiversity of UB (IRBio); Fernando Muñiz (University of Huelva) and Maria Gabriela Mangano and Luis A. Buatois (University of Saskatchewan, Canada).

Chasing fossils' prints from the past

Ichnology is a palaeontology discipline which studies trace fossils or prints which have been left by the organisms' activity in the past. The importance of the ichnofossils lies within three main points: most of them are preserved in situ -they provide direct information from the paleoenvironment in which they were generated; they represent the record of the produced animal's behaviour (palaeontology and paleoecology) which is usually linked to the paleoenvironment, and they are often the only record of certain organisms- such as the ones with soft bodies- whose body characteristics enable their no-fossilization.

According to Dr. Zain Belaústegui (UB-IRBio), the main author of the article, "in general, Lepeichnus giberti is a new and exceptional ichnotaxon which consists of two vertical and parallel shafts connected to each other by a horizontal and C-shaped gallery, from which a hook-shaped branch is ratified. This morphological pattern is extremely regular and it always happens again in all studied specimens.

"The exceptional character of the L. giberti -he continues- is due to the -first- findings of the fossil records of each of the evolution stages of a fossil trace, from its origins to the final phase. This has enabled us to describe with a lot of more details how the generation process was 6 million years ago, i.e. its ichnogeny. Actually, 9 different and consecutive ichnogenetic phases have been identified within the L. giberti ichnogeny."

"It is as if we would currently monitor a burrowing organism and took a series of pictures of its burrowing process… This is exactly what we have seen with L. giberti, but instead of pictures, these stages have been kept as fossils" says Belaústegui.

An exceptionally preserved ichnofossil

Ichnotaxons are the terms used by the ichnologists in order to list and classify these trace fossils with defined traits. The preserving state of the L. giberti is exceptional: the sandy silt which filled these burrows -- after being abandoned by their organisms -and the following ferruginization of its walls has helped the preservation of the most delicate details (such as the scars probably done by the organism's appendix). This sandy silt has also helped to its collection; around 90 complete samples with its intact three-dimensional morphology have been collected.

"Everything -said Belaústegui- has enabled us proposing by comparing it to modern burrows. The presence of different sized samples (from 1 to 10 cm of maximum width) and identic morphology have been seen, something which would mean that the producing organism would have the same burrowing behaviour in both young and adult phases. In the restoration and preparation process for the thin layers and sections of the L. giberti specimen, the participation of Dr. Alejandro Gallardo, technic of the mentioned Department of the Faculty of Earth Sciences of the University of Barcelona, should be mentioned.

A sea-linked paleoenvironment

6 million years ago the area where the fossil lies was a bay which was not very deep and was protected from waves and storms, probably affected by tides and with depths rich in organic fields and nutrients. "These would be the ideal conditions to gather big communities of the species (probably a swimming crustacean) which the Lepeichnus dag, because we have found around 93 burrows per square meter. Other organisms, such as anemones, another kind of crustaceans, and even cetaceans would have lived in this area" says Belaústegui.

At this moment there is no knowledge of any current organism which digs a burrow with the same characteristics of L. giberti's. However, a great amount of the morphological traits noticed in modern upogebiid burrows -a kind of crustacean that digs burrows in the sea bottom- are similar to Lepeichnus's.

"Since the similarities are not total -says Belaústegui- we present this kind of decapodes as the probable producers of L.giberti. The fact that there isn't any identical analogue would be due to Lepeichnus being an already extinct burrowing behaviour upogebiid."

Ichnogeny: a new concept in the world of Ichnology

The ichnotaxons are quite common within the fossil records but less than the ones related to animals or plants. The exceptional degree of preservation and fossilization of the different evolution stages of the L. giberti has enabled us proposing the new term "ichnogeny" -of great interest within the Ichnology field.

This new term describes the origins and evolution of a modern or fossil bioturbation structure (burrows, prints, traces, etc. produced in soft non-cemented substrates) or bioerosion (holes, bite marks, etc. produced in hard cemented substrates). Like the authors say, ichnogeny (creation and development of a fossil trace) can be a process depending (or not) on ontogeny (origins and evolution of an organism).

"Let's imagine, for instance, a footprint. The formation process of the footprint, its ichogeny (first the heel, then the sole and last the toes) is the same, be it a baby, teenager, adult or old person. In this scenario, ontogeny and inchogeny are independent from each other. However, it could be possible for them to be related. For instance, now they are known as "flies" (Symplecta genre) that have different burrowing behaviours according to the particular ontogenetic state they are in (larva, pupa or adult). In any of the cases, each inchogeny will be the same, the larva will always generate a certain kind of trace in the same way, and the same happens with pupa and so on" says Belaústegui.

The palaeontology richness in the south of the Iberian Peninsula

The new fossil Lepeichnus giberti pays homage to the town of Lepe -a place with great ichnology interest where ichnofossil was found- and in memory of Dr. Jordi Maria de Gibert Atienza, member of the PaleoNeoMar group of the UB and distinguished leader of the country's ichnology research. The first indicators of this ichnotaxon were found in 1955 by Dr. Fernando Muñiz (University of Huelva) during his doctoral thesis, and he preliminary presented it by himself (without giving it an official name) during the Spanish Society Palaeontology Sessions in 1999.

It is worth remembering that in 2010, due to the ichnology importance in Lepe, the research group PaleoNeoMar of the UB and Dr. Fernando Muñis organized an international conference in this town- about crustacean bioturbation (Workshop on Crustacean Bioturbation -- Lepe 2010). And again, in May 2016, this area will be visited by the participants of ICHNIA, the international conference for Ichnology with the greatest projection worldwide, which will be held from the 6th to 9th of May in the Global Naturtejo Geopark of Idanha-a-Nova (Portugal).