Guest Blog Post by Lara Sciscio and Christian A. Meyer
This study, published in Fossil Record, reports a large bowl-shaped depression from Late Cretaceous fluvial deposits in Bolivia, associated with sauropod trackways. Its morphology and scratch-like traces suggest a biogenic origin, possibly representing titanosaurid digging behaviour or an abandoned nest.
A good mystery always starts with a clue of something that was or shouldn’t have been…
The sun beats down on the Bolivian Altiplano as we walk across the rocky grey expanse of a now ancient riverbed, following a trail of fossil sauropod dinosaur tracks. We are in the Maragua region, near Sucre, in the heart of Bolivia.
Studying the trackways, we know they were made by titanosaurs, one of the largest long-necked dinosaurs to have walked the Earth. Then we stop. Within these trackways there lies a large bowl-shaped depression, seemingly carved into the rock. It is ~4 metres wide and over half a metre deep with a frayed, ragged margin.
It is unlike anything our ichnologist eyes have seen before.
Stepping closer, we realise that this is a rare oddity cut into the palaeosurface, and a slew of questions arises: Is this hollow formed by weathering, carved by modern waters? Or is it something older, potentially dug by the largest animals to ever walk the Earth? And then frozen in time?
These questions led us on a scientific detective hunt that stretched back 67 million years, to when titanosaurs walked this part of the Earth. And just short of the asteroid impact that would bring the “Age of Dinosaurs” to an end.
Weathering… or Dinosaurs?
Usually, when we encounter oddly shaped depressions in the rock record, ~99% of the time, the explanation for their formation is weathering and erosion. Nevertheless, you gather evidence and then use it to determine how and what happened. These were our first priorities on encountering the Mullurina Mayu bowl-shaped depression.
As we examined the site in detail, those potential abiotic explanations soon began to fall apart. Hydraulic scours usually have smooth walls and are aligned with the flow of water; weathering pits leave characteristic weathering textures and are grouped in smaller concentrations. This bowl structure had neither, and instead, we noticed many parallel grooves grouped in bundles and oriented around its walls and near its base.
It became evident that abiotic mechanisms were unlikely, and these potential abiotic tool marks (a type of sedimentary structure) were biotic in nature. The presence of these scratch-like marks, their shape, orientations, and distribution indicated that something had been digging.
Fingerprints
Our evidence was not in the shape of the depression itself, but the multitude of scratch traces around its perimeter. The walls carried bundles of parallel grooves, ~1 cm wide, several cm’s long, and other isolated claw traces preserved in what would have been mud. These traces were not random but oriented in two directions and indicate movement of an animal’s autopod.
Scratch-like traces associated with sauropod tracks are features known to ichnologists, although a rarity, and have been encountered at several sites, from Colorado (USA), Spain, to Mongolia. Recent discoveries in Mongolia have shown that sauropod feet could leave detailed impressions of their keratinised scales (“fingerprints”). These produce distinct striations, and their patterning and the elongate incisions are evidence that their feet moved through or across the sediment.
The grooves in Bolivia closely resemble those skin-drag traces made by a scaled foot as it repeatedly scraped its feet through and across the muddy riverbank sediment. We infer this as behavioural evidence of an animal, and the most likely is the titanosaur which left its footprints nearby. We had found fingerprints of a titanosaur, excavating, but excavating what?
Building a nest?
This is a question that many may ask: could the titanosaurs be trying to construct a nest? The answer – maybe.
There is a limited body of evidence of nesting scrapes in the fossil record. They are described as depressions which cut into the original sediment (tick), have a distinct rim (tick), and would later be infilled with different sediment (tick!). The strongest evidence for a nest structure, however, would be the presence of eggs, eggshell, or embryos, and this is still missing at the Mullurina Mayu depression. This makes our assumption of nesting less confident.
Instead, we can use the evidence we have to say it was most likely a sauropod dinosaur, using its scaley feet, that excavated the ground, and one possible explanation is that this is an abandoned nesting attempt. Ichnologists advance only because we make careful probabilities rather than claiming certainties where evidence is absent. Nonetheless, this is an exciting discovery for what it shows and for where it leads our imaginations!
What were they doing?
Body fossils (bones) tell us what animals look like, tracks tell us how they moved, how big they may be, and traces like the Mullurina Mayu depression give us a small glimpse of what they were doing. The ichnological (trace) fossil record often captures moments of an animal’s life. Here, we can see a giant titanosaur scraping, digging, and interacting with the environment rather than just looking at its bones.
Whether it was nest scraping, digging for fresh water or mineral salts (like modern elephants) or carrying out some other behaviour we have yet to recognise, what we do know is that the most interesting fossils are not of the animals themselves but the traces of their behaviours, short time shots of their lives, frozen and fossilised in the rock record.
Cover image:
Artistic reconstruction of the Mullurina Mayu bowl-shaped depression during the Late Cretaceous, depicting the probable tracemakers responsible for the Calorckosauripus trackways and possibly the bowl-shaped depression. Illustration by Joshua Knüppe (commissioned 2025).
Original source:
Meyer CA, Heymann JH, Ibarra EC, Flores EL, Sciscio L (2026) Digging Sauropods: excavation behaviour or evidence for nest building? An example from the Late Cretaceous El Molino Formation (Maragua, Bolivia). Fossil Record 29(2): 519-532. https://doi.org/10.3897/fr.29.198794
For more articles on palaeontology, visit the Fossil Record website and follow the journal on BlueSky and Facebook.
