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.
Some 67 million years before creationists believe their god created Earth during its mythical six-day “Creation Week”, a group of enormous, long-necked dinosaurs walked across the soft mud beside a river or lake in what is now Bolivia. At least one large animal may also have stopped to scrape repeatedly at the ground, leaving a great bowl-shaped depression surrounded by grooves and possible claw marks. Whether it was preparing a nest, digging for water or mineral salts, or doing something else entirely is uncertain, but the marks it left behind provide a rare glimpse of dinosaur behaviour shortly before the end of the Cretaceous Period.
The evidence comes from the Mullurina Mayu locality in the Maragua region, about 23 kilometres west of Sucre, Bolivia. There, an exposed surface of Maastrichtian-age claystone in the Lower Member of the El Molino Formation preserves several trackways attributed to titanosaurian sauropods. The tracks have been assigned to the trace-fossil species Calorckosauripus lazari, believed to have been made by comparatively non-derived titanosaurs.
Now, in a paper published in the journal Fossil Record, Christian A. Meyer, Jose Hugo Heymann, Edwin Chuca Ibarra, Eddy Laura Flores and Lara Sciscio have described an unusual structure preserved among those trackways. It is an irregular, bowl-shaped hollow measuring approximately 4.2 by 3.2 metres and reaching about 60 centimetres in depth. Around its walls are bundles of parallel grooves, roughly one centimetre wide and up to about 30 centimetres long, together with broader marks on the floor that may have been made by claws.
As Lara Sciscio and Christian Meyer explain in an accompanying article on the Pensoft blog, the first task was to exclude more mundane geological explanations. A hollow produced by flowing water would normally show features such as smoothed walls, tool marks and an orientation corresponding to the direction of flow. A weathering pit should show characteristic weathering textures and would more usually occur as one of several similar hollows. None of these features is present at Mullurina Mayu.
Instead, the arrangement of the grooves suggests repeated movements of an animal’s feet through soft sediment. They resemble the “skin-slip” striations known from other sauropod footprints, where the animal’s scaly foot dragged against the mud as it moved. Because the depression lies among titanosaur trackways, the researchers regard excavation by a sauropod as the most economical explanation, although they acknowledge that another animal cannot be ruled out.
One possibility is that the hollow represents an abandoned attempt to construct a nest. It cuts down into the original sediment, has a distinct rim and was subsequently filled with material different from that into which it was dug—all features associated with some known dinosaur nest scrapes. However, no eggs, eggshell fragments or embryos have been found, so the authors properly present nesting as a plausible hypothesis rather than an established fact. Other possibilities include digging for water or mineral-rich sediment, as modern elephants sometimes do.
Whatever the precise purpose of the excavation, the site preserves far more than a collection of footprints. It records a sequence of events on an exposed Late Cretaceous land surface: dinosaurs walked across damp sediment; an animal repeatedly scraped or dug into it; the surface dried sufficiently to develop desiccation cracks; the hollow was gently filled with marl; and soil-forming processes produced caliche before further sediment accumulated and the entire succession eventually became rock. Geological processes later folded, uplifted and eroded those rocks to expose the ancient surface at an altitude of about 3,450 metres in the modern Andes.
The formation is particularly useful because it preserves repeated changes between submerged, shoreline and exposed conditions. Modern work interprets the Potosí Basin principally as a vast, fluctuating lake system with intermittent marine connections, reconciling earlier marine and lacustrine interpretations. The background is supported by the 2026 Fossil Record paper, modern cyclostratigraphic research and the Bolivian dinosaur-track review.
The El Molino Formation. The El Molino Formation is an extensive sequence of sedimentary rocks exposed in the central Bolivian Andes, particularly within the Potosí Basin and around Sucre and Torotoro. It forms part of the larger Puca Group and records changing environments during the final part of the Cretaceous and the beginning of the Palaeogene.This is not the signature of a single, violent, global inundation. A current energetic enough to scour continents and deposit creationists’ supposed worldwide Flood sediments would be expected to destroy delicate footprints, centimetre-wide scratch marks and the sharply defined sides of a hollow in soft mud—not preserve them together with evidence of drying, gentle burial and soil formation. Nor could the animals have walked about, excavated the ground and perhaps attempted to nest on an exposed riverbank while that riverbank was supposedly beneath a world-covering ocean.
The formation is divided into Lower, Middle and Upper members. The dinosaur tracks and unusual excavation at Mullurina Mayu occur near the top of the Lower Member, which has been dated to approximately 73–67 million years ago. The El Molino Formation as a whole extends across the Cretaceous–Palaeogene boundary, recording environmental changes before and after the asteroid impact and mass extinction about 66 million years ago.
Geological setting
When the formation was deposited, the Andes had not yet risen to anything resembling their present height. The Potosí Basin was a broad, shallow depression on the eastern side of the developing Andean volcanic arc. At times, its enormous lake system may have approached the area of the modern Caspian Sea.
Today, the rocks occur high in the Andes because later tectonic compression folded, faulted and uplifted them. Some track-bearing beds, most famously at Cal Orck’o near Sucre, now stand at very steep angles even though they were originally deposited as approximately horizontal lake margins, mudflats and floodplains.
What kinds of rock does it contain?
The El Molino Formation consists predominantly of:
- mudstone, claystone and marl;
- fine-grained sandstone;
- micritic, bioclastic and oolitic limestone;
- stromatolitic carbonate deposits;
- ancient soils and caliche horizons; and
- beds containing mud cracks and other evidence of periodic exposure and drying.
These rocks were deposited under a succession of different conditions rather than during a single event. They record shallow water, exposed mudflats, river channels and overbank deposits, soil formation, evaporation, renewed flooding and further sedimentation.
Lake, sea or both?
The depositional environment has been debated. Fossil sharks and other fishes with marine or estuarine affinities led some researchers to interpret much of the formation as restricted marine deposits. Other evidence—including freshwater gastropods, ostracods, charophytes, catfish, turtles, pollen, stable isotopes and evaporitic minerals—indicated a predominantly lacustrine environment.
More recent research suggests that these interpretations need not be mutually exclusive. The Potosí Basin probably contained an immense, predominantly freshwater to saline lake system whose depth and extent fluctuated with rainfall, evaporation, subsidence and global sea level. Intermittent connections with the ocean could have allowed marine organisms to enter the basin during some intervals.
A remarkable record of dinosaur life
The El Molino Formation contains one of the richest dinosaur-track records in the world. Track-bearing localities include Cal Orck’o, Humaca, Niñu Mayu, Mullurina Mayu and numerous sites in and around Torotoro National Park. Together, their tracks record:
- several kinds and sizes of titanosaurian sauropod;
- small and large theropod dinosaurs;
- armoured ankylosaurs;
- ornithopods;
- small birds; and
- possible swimming, slipping, turning, herding and digging behaviour.
Body fossils are not always present, but footprints and other trace fossils reveal where the animals walked and sometimes how they behaved. At Humaca, parallel trackways have been interpreted as evidence of a group of young titanosaurs travelling together. At Mullurina Mayu, the tracks and associated scrape marks may record a titanosaur digging into soft ground, possibly during an abandoned nesting attempt.
Why this is not a “Flood deposit”
The formation preserves repeated episodes in which sediment was deposited, exposed to the air, crossed by animals, allowed to dry, altered by soil-forming processes and then buried beneath later sediments. Some localities contain numerous separate track-bearing horizons stacked one above another.
A single global flood could not produce that orderly succession. Nor could dinosaurs walk, travel in groups, dig into exposed ground and leave delicate impressions while supposedly submerged beneath a violently moving world-covering ocean. The El Molino Formation records a changing landscape over millions of years—not one year of biblical catastrophe.
Sources: Meyer et al. (2026), Fossil Record 29: 519–532;
Tasistro-Hart et al. (2020), Geological Society of America Bulletin 132: 1931–1952;
Meyer et al. (2021), Journal of South American Earth Sciences 106: 102992.
The creationist escape claim that the dinosaurs made these traces during Noah’s Flood fares no better. The rocks record alternating episodes of exposure, animal activity, drying, low-energy sedimentation and pedogenic alteration, with additional track-bearing surfaces occurring at other levels in the formation. Those are successive environmental conditions requiring time, not different moments in one continuous, year-long catastrophe.
To science, Mullurina Mayu is a fascinating Late Cretaceous “cold case”: strong evidence that a large animal—probably a titanosaur—dug into a muddy Bolivian floodplain about 67 million years ago, while its exact motive remains unknown. To creationism, it is yet another crime scene at which the geological evidence refuses to corroborate the biblical suspect’s story.
In their open access blogpost, two of the papers authors, Lara Sciscio and Christian A. Meyer, describe their discovery and investigation of this bowl-shaped depression. Their blog is reprinted here under a Creative Commons license, reformatted for stylistic consistency:
A Cretaceous Cold Case: The mysterious affair at Mullurina Mayu (Bolivia)
This study reports a large bowl-shaped depression from Late Cretaceous fluvial deposits in Bolivia, associated with sauropod trackways.
A good mystery always starts with a clue of something that was or shouldn’t have been…
Drone overview of the outcrop in the western part of the Maragua Syncline, Bolivia. Can you spot the palaeosurface with titanosaurid trackways? 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.
Drone overview of the outcrop in the western part of the Maragua Syncline, Bolivia. Can you spot the palaeosurface with titanosaurid trackways?
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?
Drone view of the Mullurina Mayu surface with the circular structure. Parallel sauropod (titanosaurid) trackways diagonally cut across the surface. Two people are the base of the outcrop are for scale.
Excavating loose sand from the structure before taking close-up photos for 3D modelling. In the foreground are titanosaurid tracks.
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
Close-up of scratch-like marks within an imprint inside the depression (scale bar: 20 cm).
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.Publication:
Whether the Mullurina Mayu depression was an abandoned nest scrape, a water hole or the result of some other activity remains uncertain, and the researchers make no attempt to disguise that uncertainty. What the evidence does show is that a large animal repeatedly moved its feet through soft sediment, probably excavating the hollow. Its association with titanosaur tracks makes a sauropod the most likely culprit, but science distinguishes carefully between what the evidence demonstrates and what it merely suggests.
That uncertainty about the animal’s purpose has no bearing on the age or geological significance of the site. About 67 million years ago—almost 67 million years before the supposed “Creation Week”—dinosaurs were walking across exposed mud in Bolivia, while at least one animal appears to have stopped and dug into it. The surface subsequently dried, the depression was gently filled, soil-forming processes altered the overlying sediment, and further deposits buried the whole assemblage before it became rock.
This ordered sequence is irreconcilable with creationist Flood mythology. A violently moving global flood could not simultaneously scour and reshape the planet while preserving footprints, centimetre-wide skin-drag grooves, claw marks, desiccation cracks and a sharply defined hollow in soft mud. The familiar claim that the dinosaurs made the tracks during the Flood merely replaces one problem with several more: it requires exposed land, calm conditions, wandering and possibly nesting animals, drying sediment and developing soils while the entire world was supposedly beneath a catastrophic ocean.
The rocks of the El Molino Formation therefore tell a coherent story, but it is not the one related in Genesis. They record changing lakes, rivers, floodplains and exposed soils inhabited by successive communities of animals over immense periods of time. Once again, creationists are left trying to compress a complex geological history into a single imaginary catastrophe, while the evidence remains exactly where those dinosaurs left it—some 67 million years before their god was allegedly preparing to create the world.
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