Many species of iguanas dig shelters for cover towards predators, fires and excessive climate. Most additionally make underground nests. Anthony J. Martin, CC BY-SA

A pregnant iguana dug right into a vegetated sand dune about 115,000 years in the past on a small island in a series of islands that in the future can be referred to as the Bahamas. As soon as she buried herself and was surrounded by free sand, she scraped out a chamber and laid her eggs in it. On her manner out of this underground nursery, she packed sand behind her, forming distinctive layers that marked her progress to the floor.

As soon as again within the sunshine, she tamped down the highest to hide the nest. Over many centuries, a skinny layer of soil developed over the previous nesting burrow, and minerals from that soil fashioned between the sand grains, turning the dune into limestone, which preserved the construction of the nesting burrow.

In December 2013, whereas exploring a roadcut on San Salvador Island within the Bahamas with 19 undergraduate geology college students from Emory College, considered one of us (Anthony) seen this uncommon construction within the rock. It seems the street excavators had unwittingly uncovered a bit of historical sand dune, containing this iguana burrow from way back.

An iguana from way back fastidiously constructed this nest layer by layer.

Anthony J. Martin, CC BY-SA

Over the following six years, with contributions from undergraduates and the Emory Middle for Digital Scholarship, we had been in a position to conclude that we’d discovered not solely the primary recognized fossil iguana nesting burrow, but additionally the primary hint fossil attributed to an iguana. Based mostly on its geologic setting, we estimate the burrow is about 115,000 years previous, inserting it within the Late Pleistocene Epoch, which is finest recognized for its ice ages and megafauna, like mammoths and large floor sloths.

Hint fossils

A hint fossil is oblique proof of historical life made whereas an organism was alive. The research of this iguana burrow and different hint fossils, equivalent to tracks, nests, tooth impressions and feces, fall below the science of ichnology.

Hint fossils are vital as a result of they straight mirror historical conduct. Additionally, not like bones or shells, which are sometimes moved after an animal’s loss of life, most hint fossils are discovered precisely the place they had been created.

Hint fossils provide a novel window to the distant previous. When a dinosaur sat down alongside a lake shore in modern-day Utah through the Early Jurassic Interval about 200 million years in the past, adjusted its stance, stood and walked away, that dinosaur’s conduct was recorded within the sediment beneath it. Equally, when a fish swam alongside a lake backside in modern-day Wyoming greater than 50 million years in the past, it left not solely trails from its fins, but additionally impressions from its mouth alongside the lake backside whereas feeding. And human footprints from about 12,000 years in the past in modern-day New Mexico informed of a younger grownup carrying a toddler throughout a foot path shared with mammoths and floor sloths.

Hint fossils, not like shells and bones, present snapshots – and even brief documentary movies – of animals dwelling of their authentic environments.

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The Bahamas burrow

Regardless that we discovered no physique components or eggs, there’s ample proof the construction we found within the Bahamas was an iguana burrow. The wind-blown layers within the former sand dune had been clearly interrupted and blended, displaying the construction was made whereas the sand was nonetheless delicate. It matches the width, depth and form of contemporary iguana nesting burrows. And close by, considered one of us (Melissa) discovered a fossilized land-crab burrow, insect burrows and root traces preserved within the outcrop, displaying it was certainly an inland dune – precisely the place an iguana would make a nest.

The burrow deconstructed into its behavioral components. NC? is a hint of the attainable nest chamber, CZ (1-6) exhibits the compaction zones, the place the mom iguana packed sand behind her on her manner out, and the arrow exhibits her total route of motion whereas exiting.

Anthony J. Martin, CC BY-SA

Probably the most convincing clue, nevertheless, is the sequence of compacted sand layers contained in the construction. These are the locations the place the expectant mom packed sand together with her legs and head on her manner out with a purpose to conceal her eggs and hatchlings from predators.

Apparently, we didn’t discover any proof that the hatchlings dug their manner out, by way of the layers, to the floor – as iguana hatchlings do. This means a failed nest. Though hatching success in fashionable iguanas is often excessive, nest failures occur, and are probably when soil moisture is simply too excessive, equivalent to after a heavy rain.

The endangered San Salvador Island rock iguana (Cyclura riyeli riyeli).

Anthony J. Martin, CC BY-NC-SA

Iguanas on San Salvador

The oldest recognized iguana physique fossils from San Salvador date from lower than 12,000 years in the past. The invention of this burrow, from 115,000 years in the past, enormously extends the pure historical past of iguanas on this location.

The iguanas that dwell on San Salvador Island as we speak are among the many rarest lizard species on this planet – the San Salvador Island rock iguana (Cyclura riyeli riyeli). This species and others had been frequent all through the Bahamas earlier than 1492, when Europeans launched rats, pigs and different invasive species that preyed on eggs and iguanas of all ages. Now, fewer than 500 people persist on remoted cays offshore from the principle a part of San Salvador.

We hope our research generates consciousness and appreciation of Bahamian iguanas and their lengthy historical past on this space. We additionally hope it conjures up their continued safety.

The authors don’t work for, seek the advice of, personal shares in or obtain funding from any firm or group that may profit from this text, and have disclosed no related affiliations past their educational appointment.