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On the Track of Ichnology

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We can do nothing . . . that does not leave its impress behind, for good or for evil, for a blessing or a curse,..[] Our footprints are left in whatever we do. . . . The traces of our actions, good or bad, have life, and they will testify for or against us.
Adam Gifford, prosecutor in a murder case in 1864

Sherlock Holmes was a gifted reader of tracks and surely he would be delighted by the achievements that naturalist have accumulated in this art over the last 200 years.
Ichnologythe examination of traces is considered a relatively young branch of the earth sciences. To the public the most known ichnofossils are surely vertebrate footprints (especially dinosaurs), but ichnology encompass an incredible broad array of traces left by the activities of organisms: borings of molluscs in rocks, burrows of animals in mud, nests, middens and shelters of vertebrates, petrified casts of plant roots, also petrified excrements and last but not least human artefacts (in the form of sculptured rocks).

Fig.1. Isochirotherium infernii – natural cast of the hind limb footprint. The attribution of an ichnospecies to a real animal species is tricky, only in rare cases the trackmaker is fossilized together with its traces. Based on the known fossil species coming from the same geologic formation or epoch and comparing the anatomy of a foot to the shape of a footprint it is however possible to formulate an educated guess: Isochirotherium is attributed to a Triassic archosaur of large size.

Like common fossils the traces left by organisms on various substrates (especially vertebrate footprints on the ground) were noted already in antiquity and many old legends recognized footprints as results of the gait of a once living organism, however the trackways were incorrectly attributed to giant humans, monstrous birds or other mystical creatures. Even in modern times some ichnofossils were tied to very strange trackmakers, like the (supposed) Triassic midden to the Kraken and the Holocene footprints found at Bluff Creek to the ichnospecies Anthropoidipes ameriborealis.

It was only at the beginning of the 19th century that the first widespread scientific inquiries started and only in the second half of the 20th century ichnology begun to emerge as a systematic and from other scientific fields separated discipline. First public interests in fossil tracks started in 1800 to 1830, with the discovery of large tridactyl footprints in Jurassic sediments of the Connecticut River Valley (Massachusetts), interpreted at the time as marks left by “Noah’s Raven” or other giant birds (and beeing in fact from dinosaurs this explanation is not all to wrong) – Noah’s flood was still a popular explanation for all sorts of strange discoveries.
The first scientific work on tetrapod ichnology, about Permian tracks found on sandstone slabs from Scotland, was published in 1831 by Rev. Henry Duncan. Soon descriptions of hand – like spurs, denominated Chirotherium (meaning appropriately “hand mammal“) and other ichnofossils of Permian and Triassic sediments followed.

There are however rare examples of naturalists – much earlier than the 19th century -studying fossil tracks. Il “Rinascimento” (The Renaissance) was a cultural movement emerging in the 14th century in Italy, dedicated to the collection of empirical knowledge. Also in Italy there are almost everywhere easily accessible outcrops of Mesozoic and Cainozoic sediments, in many cases rich in fossils and sedimentary structures of all kind.

The Cretaceous “Marne a Fucoidi (Fucoid – marls)” are named for their abundance of Fucoides – a dark-coloured branching structure found in these marls. These strange marks are so abundant in the sediments that even historic travel guides mention them. Based on Italian material the French geologist Adolphe Brongniart described in 1823 this supposed fossil as the remains of algae – only in 1881 the ichnofossil character of these ancient burrows of some still unknown organisms was recognized and the modern term Chondrites adopted.

Fig.2. Examples of Chondrites, as found in the “Marne a Fucoidi”. Late Cretaceous marls, Bottaccione Gorge near the city of Gubbio, Umbria.

Another kind of common fossil trace, found often in the Italian geological formations of Cainozoic age, is Zoophycos – described by Villa Antonio in 1844 and named in 1855 by the naturalist Abraham Massolongo.

Fig.3. Zoophycos found in the Trubi Formation of Zanclean age (Pliocene) at Scala dei Turchi, in the Rossello composite section (Sicily).

300 years earlier (!) these unusual marks on rocks were observed and described with great curiosity by two of the greatest naturalists of the Renaissance.
The universal genius of Leonardo da Vinci (1452-1519), based on recent observations of animals behaviour, developed speculations about fossils and geology far ahead of his time. It is known that Leonardo rejected the idea that fossils (his “”nicchi“, shells as he calls them) in sediments were of inorganic nature or even the evidence of a large, single flood. He recognized the similarity to modern shells of marine organisms and that the deposition of these “nicchi“  inside the sediments needed much more time and occurred incessantly: a first glimpse of the process of fossilization.

There is to be seen, in the mountains of Parma and Piacenza, a multitude of shells and corals full of borings, still sticking to the rocks. When I was at work on the great horse of Milan, certain peasants came into my workshop and brought a large sackful of them to me

Leonardo did not only study the preserved shells, but was also interested in the impressions left on the surfaces of some of the layers. Based on observations of marine life found on a modern sand coast, Leonardo recognizes that the individual layers are sedimentation phases, divided by phases with no or slow sedimentation and when the surface of the sand was colonized by living animals:

…among one and another rock layer, there are the traces of the worms that crawled in them when they were not yet dry

Leonardo also recognized ichnofossils documenting a possible predatory behaviour:

The trace of the course [of the moving animal] is still preserved on the shell that has been consumed in the same manner of woodboring beetles

However Leonardo never published his observations and so his knowledge had to wait for three centuries in his secret notebooks to be rediscovered.

These were still dangerous times, the naturalist Ulisse Aldrovandi (1522-1605), born only three years after Leonardo’s death, spent his last years of life in house arrest, accused of heresy for his naturalistic speculations.

Aldrovandi was a son of the Renaissance: he studied law and philosophy and other arts, but was also interested in zoology, botany, medicine and geology.
In one of his most important work as naturalist, the “Musaeum Metallicum” published posthumous in 1648, he describes, classifies and represents hundreds of “fossilia“, a term including all things found in the underground and excavated – like rocks, soils, fossil resins, minerals and trace fossils – and also some monsters and other oddities.

to understand plants and animals there is no better way than to depict them from life

Although an advocate of personal observation, Aldrovandi still combines, like other naturalists of this transition phase, the empiric approach of later Galileo with theoretical and philosophical principles of former philosopher.
He compares some fossils to anatomical parts of modern animals, implying a connection, but for other fossils, displaying no superficial resemblance to living animals, he still relies to a mysterious force modelling the rocks. Nevertheless this tentative classification of rocks is an incredible great achievement – the true nature of fossils, intended as remains and tracks of ancient animals, will be accepted universally only in the late 18th century.

This ambivalent approach can be also observed in the ichnofossils: Aldrovandi refers to a rock with holes and hollows as “Silicem dactylitem“, because the apparent resemblance to of the holes to the borings of the modern species Pholas dactylus, a lithophagous bivalve found living inside the rocks on the shores of the Mediterranean Sea. This affiliation is intentional and Aldrovandi describes in great detail observed modern bioerosion caused by these molluscs.

Fig.4. View of the plate “De Silice”, pictured in Aldrovandi’s “Musaeum Metallicum” (1648) and interpreted as borings of mollusks (=Gastrochaenolites). Below a rock with strange snake-figures covering its surface, Aldrovandi didn´t correctly identify this as an ichnofossil (Cosmorhaphe). This illustration is however remarkably realistic, thanks to its extreme detail (note the semicircular furrows in the trackway) and to the use of hatching to restore three-dimensionality and volume to the subject.

For other, as we now know, ichnofossils he simply states “that [it] resemble snake figures“, obvious misinterpreting, in difference to Leonardo, the nature of this invertebrate tracks (=Cosmorhaphe).

Fig.5. Example of invertebrate tracks in the Triassic Gröden-Formation, resembling the snake-figures as depicted by Aldrovandi.

The “Stelechites” described by Aldrovandi as straight, subcylindrical structures:

[I]n nature we also find a stone that resembles the trunk of a tree and shows the hardness of iron.

From the figures and the description however it is not clear what Aldrovandi is exactly referring to, maybe these fossils are fragments of invertebrate burrows (=Thalassinoides).

Fig.6. “Stelechites” from the “Musaeum Metallicum” (1648) Aldrovandi uses this term to indicate subcylindrical elements of various origin. Some specimens of “Stelechites” most probably represent trace fossils, but most are only concretions or inorganic minerals.

Fig.7. Fragments of a burrow infill emerging from marls of the Trubi Formation and in the overlying Monte Narbone Formation at Punta Piccola (Sicily) – the ichnofossil fits the description given by Aldrovandi as tree-like structures; maybe the notion of the “hardness of iron” is related with the apparent resistance against erosion by the ichnofossil in comparison to the surrounding rocks.

Many of the accomplishment of Aldrovandi and even Leonardo were for long time neglected and became forgotten. Only 300 years later the knowledge of these two personalities’ was rediscovered and only now we begin to appreciate what traces can tell to us if we carefully examine them (to be continued…).

Bibliography:

BAUCON, A. (2008): Italy, the Cradle of Ichnology: the legacy of Aldrovandi and Leonardo. Studi Trent. Sci. Nat., Acta Geol., 83: 15-29
BAUCON, A. (2009): Ulisse Aldrovandi (1522-1605): The Study of Trace Fossils During the Renaissance. Ichnos, 16: 4, 245 – 256
BAUCON, A.. (2010): Leonardo da Vinci, the founding father of Ichnology. PALAIOS 25: 361-367
LEONARDI, G. (2008): Vertebrate ichnology in Italy. Studi Trent. Sci. Nat., Acta Geol., 83 (2008): 213-221
LOCKLEY, M. &  MEYER, C. (2000): Dinosaur Tracks and other fossil footprints of Europe. Columbia University Press: 323
MAYOR, A. & SARJEANT, W.A.S. (2001): The Folklore of Footprints in Stone: from Classical Antiquity to the Present. Ichnos 8(2): 143-163
WAGNER, E.J. (2006): The Science of Sherlock Holmes – From Baskerville Hall to the Valley of Fear, the Real Forensics Behind the Great Detective’s Greatest Cases. John Wiley & Sons: 244

David Bressan About the Author: Freelance geologist dealing with quaternary outcrops interested in the history and the development of geological concepts through time. Follow on Twitter @David_Bressan.

The views expressed are those of the author and are not necessarily those of Scientific American.





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