Papagaios, Life & Times on a Speculative Archipelago - The Mystery of the Chisel-Toothed Lizard

Text taken from ‘The Baffling Jaw’ a post on the blog ‘Science Detective’ by @sciencelockholmes, real name Amir Rexhepi, 12 October 2016.

Some mysteries in science can’t be solved because we don’t yet have the technology to solve it. Others are marked by passionate arguments from many sides and answers that cannot be known for certain. My favourite kind are those where an undisputed, accepted view is shaken by new evidence. A big example of this would be the appearance and lifestyle of dinosaurs which went from sluggish, stupid swamp monsters to dynamic animals with complex behaviours.

I prefer small-scale stories myself, and my favourite example of our evolving understanding of the past begins in a museum in Germany. At least, that was where I first learned of it.

On a family trip to Germany in 1998 we spent a day at the Paläontologisches Museum München. All my siblings loved fossils, but while my sister ran off to find a T. rex or some other equally large specimen, and my brother dragged our mother to see the world-famous Archeopteryx specimen, I methodically trawled over the hundreds of smaller specimens in glass topped cabinets. Among the myriad fragments of bones and shells I found an unassuming specimen labelled Palusodon dunkerii.

Palusodon dunkerii was discovered quite by chance by German malacologist (snail guy) Dietrich Heidelberg in 1861. A student of Wilhelm Dunker, himself a world expert in malacology, Heidelberg was travelling back to Europe from South America and somehow convinced the ship’s captain to stop off in the Papagaios Islands so he could collect some of the islands’ endemic snails, knowing his mentor had none of their shells in his collection.

(Heidelberg and Dunker were part of a weirdly large community of German academics who had a side hustle as malacologists and traded shells with each other like Victorian Yu-Gi-Oh cards.)

It was in the mountains of Ninho Island that Heidelberg spotted a skull staring up at him from a rock he was lunching on. Being a palaeontologist by trade (remember the snail thing was a side gig), he whipped out his hammer and chisel and took the rock home with him.

The skull, 4cm long, was from a lizard, albeit it with novel features. The tip of the premaxilla was curved upwards, while the front six teeth of the lower jaw were significantly longer, squashed together and angled forwards at 45° relative to the other teeth. Heidelberg likened it to a palisade wall around a fortress, naming the lizard Palusodon (Palisade tooth), the species name dunkerii being for his mentor.

The purpose of the teeth was hard to determine, especially without the rest of the skeleton. Heidelberg noted the thicker enamel at the front of the teeth and angled tips and thus proposed they were a shovel or chisel to break into ant hills.

This was the accepted view for over a hundred years, until construction workers on Milagres Island found a bed of petrified wood and amber. Most pieces were small and unremarkable, with the usual insects trapped in them. One larger piece, however, held something truly remarkable: a lizard, around 10cm long, and almost entirely preserved. It was significantly smaller than P. dunkerii, but its skull bore the same cluster of chisel shaped teeth. It was declared as new species in the Palusodon genus: P. heidelbergii.

P. heidelbergii, being more complete, revealed much more about the genus and its lifestyle. Most notable were its proportionately long feet with both large toe pads and small hooked claws - strong evidence of a climbing lifestyle and a mark against a terrestrial, ant-eating diet. Two camps soon formed with new hypotheses.

One camp, noting that Ninho at that time was a young and rocky island, imagined a creature that crept into nests and used the tooth comb to crack eggs before lapping up the insides. The second camp saw the teeth as perfect for scraping away bark, perhaps to reach boring invertebrates, or to lap up oozing sap.

Which option was correct was up in the air throughout the late 20^th Century, and even in 1998 when I first saw the original fossil in Munich. It would be another twelve years until the truth was revealed.

Sara and Peter Carmichael were Canadian honeymooners, enjoying a nature walk on Verde Island when Peter spotted a funny green and red lizard headbutting a tree. Sara, a photographer by profession, was quick to capture dozens of images and videos of the amusing reptile. A few weeks later, back at home, they uploaded the files to social media where they went mildly viral. As it spread, people began asking what this lizard was doing, and indeed, what it was. Experts were unsure, and expeditions were sent to Verde to track down more of them to study. Before long, the lizard was identified as a living member of Palusodon, a third species – Palusodon verdensis. And finally, we had an answers as to what those strange teeth were for.

P. verdensis used its tooth comb to dig deep notches in the bark of trees. It would typically dig nine to fifteen notches before wandering away. A day later it returned and lapped up the sap oozing from the tree’s wounds with a long, thin tongue. This type of diet is called gummivory and is not too common as specialised microorganisms are needed to breakdown the compounds in the sticky gum.

Since the discovery of living Palusodons, two other species have been found, very small and living high up in the laurel forests of Verde, an apparent refuge for a group that was once much more extensive in the archipelago.

Mystery: solved