Probably animals—but no particular species has been proved to have dug every giant tunnel. Fossil tunnels in South America preserve claw grooves, distinctive cross-sections and other traces consistent with excavation. Researchers therefore interpret many as paleoburrows, or fossil burrows. Extinct armadillos are plausible builders of some smaller examples, while large ground sloths are plausible builders of the biggest ones. Those are evidence-based attributions, not secure species identifications.
What “paleoburrow” means
A paleoburrow is a fossilized animal tunnel preserved in sediment or rock. The term describes the structure and the interpretation that an animal excavated it; it does not automatically identify the animal. This distinction matters because a tunnel can retain strong evidence of digging while lacking a body fossil that would tie it to one species.
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Researchers evaluate several features together:
- Cross-sectional shape: subrounded or flattened profiles, often wider than high, can reflect the body and limb movements of a burrowing animal.
- Wall and ceiling traces: parallel or grouped grooves may be claw marks. Their spacing, curvature and orientation are compared with the anatomy of candidate diggers.
- Contact with the surrounding deposit: the relationship between the tunnel and the enclosing sediment or rock helps distinguish an excavated cavity from a natural void.
- Surface modification: polished areas, smoothed ceilings and repeated-use zones can indicate animals moving through or resting in the passage.
No single feature is a species fingerprint. The interpretation becomes stronger when morphology, trace marks and geological context point in the same direction.
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South American fossil burrows are not one uniform type. Their dimensions vary by locality, host deposit and the ichnofossil classification used to describe them. The following measurements belong to specific studies and should not be applied to every tunnel found on the continent.
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| Structure or study | Reported dimensions | Length | Context and interpretation |
|---|---|---|---|
| Mar del Plata structures, Vizcaíno, Zárate, Bargo and Dondas (2001) | 0.80–1.80 m in diameter; generally wider than high | Several metres | Structures in Pleistocene deposits; subrounded sections and claw marks support a fossil-burrow interpretation. Scelidotherium and Glossotherium were considered possible builders. |
| Megaichnus minor, smaller forms (Pereira Lopes and colleagues, 2017) | Two reported size groups: 0.6–0.9 m wide by 0.5–0.7 m high, and 1.2–1.5 m wide by about 0.9 m high | At least 30 m | Traces include grouped claw grooves, possible armadillo-osteoderm impressions and possible fur impressions. Candidate producers include giant extinct armadillos. |
| Megaichnus major megatunnels (Pereira Lopes and colleagues, 2017) | About 2 m high and up to 4 m wide | Can exceed 50 m | Scratches, smooth surfaces and large smoothed areas have been interpreted as evidence of animal activity and possible resting or repeated-use zones. Large ground sloths are plausible candidates, but no species-level identification is established. |
The largest figures are therefore not a typical burrow size. A tunnel 4 m wide and more than 50 m long belongs to the exceptional Megaichnus major category described by the 2017 classification paper. The Mar del Plata structures are smaller, although a diameter of 1.80 m is still far beyond the scale of a modern small-mammal den.
Which clues point to animals rather than geology?
Claw grooves and scratch patterns
In the Mar del Plata material, researchers reported claw marks on walls and ceilings. In Megaichnus minor, grouped grooves were also described. A natural fracture can produce linear marks, but repeated, organized grooves in the right position for an animal’s limbs are more compatible with excavation. Investigators still have to rule out later erosion, sediment movement and damage after the tunnel formed.
Impressions of armor, fur and body contact
The Megaichnus paper reports impressions interpreted as resembling armadillo osteoderms and possible fur impressions. These observations suggest that an armored or fur-bearing animal passed close to the tunnel wall while digging or moving through it. They are valuable supporting traces, not diagnostic casts of one named species: similar impressions can be incomplete, distorted or produced by more than one animal with comparable body coverings.
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Smooth and enlarged sections
In Megaichnus major, scratches occur alongside smooth surfaces and broad smoothed areas. The authors suggested that some of these may represent places where an animal rested or where a passage was used repeatedly. A smooth patch can also arise from sediment abrasion, so its meaning depends on its position, continuity and association with other traces.
The absence of a skeleton
The classification study reports no body fossils inside the Brazilian open burrows it examined. That absence does not disprove an animal origin. Burrows are often abandoned, reworked or filled long after the maker leaves, and a carcass may never enter the tunnel. It does mean that the attribution must rely on trace evidence and comparative anatomy rather than a direct skeleton-in-burrow match.
Who could have dug them?
Researchers compare candidate animals using four main axes: tunnel dimensions and shape, geographic and geological setting, the form of scratches and other traces, and whether the animal’s limbs and claws could physically excavate the observed passage. These comparisons narrow the possibilities without proving a culprit.
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| Candidate group | Why it fits some evidence | Where the fit is weaker |
|---|---|---|
| Extinct giant armadillos | Armadillo-like osteoderm impressions and claw-groove patterns have been reported in smaller Megaichnus forms. Their digging anatomy provides a plausible match for broad, relatively low passages. | The traces are not unique to one armadillo species, and the largest megatunnels may exceed what the proposed armadillo body plans could excavate efficiently. |
| Mylodontid ground sloths, including Scelidotherium and Glossotherium | The Mar del Plata study found a reasonable match between tunnel size, claw-mark form and the limb anatomy of these extinct sloths. Very large ground sloths are also plausible candidates for the biggest passages. | No named sloth has been demonstrated as the maker of every large tunnel. Similar claw traces and uncertain tunnel ages prevent a universal species assignment. |
Why giant sloths are proposed for the biggest passages
The largest Megaichnus major examples—about 2 m high, up to 4 m wide and capable of exceeding 50 m in length—require an excavator able to move a very large volume of sediment. Large ground sloths have the body mass, powerful forelimbs and enlarged claws that make this mechanically plausible. “Plausible” is deliberately narrower than “identified”: the tunnel itself does not preserve a label naming the species.
Why armadillos remain important candidates
Smaller Megaichnus forms, especially those with grouped grooves and possible osteoderm impressions, are consistent with giant extinct armadillos. Their armor and digging adaptations offer a logical explanation for the trace combination. The inference is strongest when the tunnel’s width, height and wall marks fit the expected reach of an armadillo-sized animal at the same locality.
How scientists test a proposed builder
- Document the tunnel geometry. Researchers measure width, height, length, curvature and whether the profile is wider than high. A candidate must be physically large enough to occupy the passage but capable of excavating it.
- Map the traces. Scratches are recorded on walls and ceilings, including their grouping, angle and spacing. Possible impressions of armor or fur are described separately from ordinary sediment texture.
- Check the surrounding deposit. The tunnel is tied to a particular formation or sedimentary unit, which constrains the age and the animals that could have lived there.
- Compare anatomy. Claw shape, forelimb leverage, shoulder height and body proportions are compared with fossils of extinct armadillos, ground sloths and other plausible diggers.
- Test alternatives. Investigators ask whether water erosion, collapse, tectonic cracking or human excavation could create the same geometry and marks. A convincing paleoburrow interpretation must explain more observations with fewer ad hoc assumptions.
- State the attribution at the right confidence level. “Animal-made paleoburrow” can be well supported even when “dug by species X” remains unproven.
Are all the tunnels the same age?
No. The Mar del Plata study places its structures in Pleistocene deposits. The Megaichnus classification discusses examples in Plio-Pleistocene deposits, but notes uncertainty about the ages of some Brazilian occurrences. “Plio-Pleistocene” spans a broad interval, and a tunnel’s age is controlled by the deposit that contains it, not by the name Megaichnus alone.
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This matters when matching a tunnel to a candidate. A species must have lived in the relevant region during the time represented by the host sediment. A plausible anatomical match from the wrong age or locality is not a secure identification. For that reason, dimensions, trace morphology, geography and geological age should always be considered together.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How unusual are these excavations?
A 2024 study of South American Plio-Pleistocene fossil burrows reported that some reach “up to four times the size of the largest known non-anthropic, biogenic excavations recorded.” That is a comparative statement about the study’s fossil-burrow sample and the largest previously recorded non-human, biologically made excavations; it is not a claim that every paleoburrow is four times larger, nor that the comparison identifies the builder.
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Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallThe scale does, however, explain why ordinary modern-burrow analogies can mislead. A modern armadillo den or badger sett may show the same general behaviors—digging, sheltering and repeated use—without approaching the dimensions or engineering demands of a 4-m-wide fossil passage.
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Could these be natural caves or human tunnels?
Some natural processes produce cavities, and people have also excavated tunnels. A paleoburrow interpretation is favored when several observations occur together: a consistent, animal-sized cross-section; scratch patterns in plausible digging positions; localized smoothing from body contact; and a clear relationship to fossil-bearing sedimentary deposits. Human mines and modern excavations may show tool marks, spoil piles, entrances or historical context instead.
Even then, a tunnel can be modified after excavation. Collapse may enlarge a section, groundwater may erode the floor, and later sediment can obscure the original marks. Researchers therefore distinguish the inferred original shape from its present exposed form and avoid treating a single damaged surface as decisive.
What the evidence can and cannot establish
- Well supported for some sites: animals excavated at least some of the structures classified as paleoburrows, based on morphology and claw or contact traces.
- Supported as a comparative hypothesis: extinct armadillos likely made some smaller forms, while large ground sloths are plausible makers of the largest megatunnels.
- Not established: one species dug all giant tunnels, every tunnel belongs to the same age, or every scratch can be assigned to a particular taxon.
- Still difficult: identifying a trace-fossil producer without a body fossil preserved in the tunnel. Ichnologists routinely have to work from anatomy and behavior rather than a direct fossil association.
Why the question matters beyond a spectacular measurement
These tunnels preserve behavior that skeletons rarely show. A bone can reveal the size and mechanics of a megafaunal animal; a burrow can reveal how it used those mechanics—whether for shelter, resting, protection from weather or repeated movement through a landscape. Smoothed zones may indicate reuse, while the tunnel’s orientation and relation to sediment can provide clues about the environment in which it was made.
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Quick Recap
A careful way to describe a newly reported giant tunnel
- Call it a possible paleoburrow until morphology and trace evidence support the interpretation.
- Report the measured width, height and length with the locality and host deposit attached.
- Describe claw grooves, impressions and smooth areas as observations before proposing what they mean.
- List extinct armadillos and ground sloths as candidate groups only when their anatomy and known geographic range fit.
- Separate the age of the enclosing sediment from the age of later fills or exposed surfaces.
- Use “probable” or “plausible” for the builder unless a body fossil or uniquely diagnostic trace provides stronger evidence.
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