Paleontology has come a long way from digging up giant bones and believing they were from giant humans – OK, that still happens but now it’s by conspiracy theorists. As technology improves, they can dig deeper and with less destruction of tiny fossils. As DNA testing improves, more distinct species are found that show what an incredible diversity of creatures this planet once hosted. Two discoveries announced this week are excellent examples of these advances in paleontology, and both deserve a “This could change everything!” exclamation. First, 500-million-year-old fossilized brains of predatory arthropods show that they had three eyes and may have been ancient ancestors of modern spiders and insects. Next, at the other end of the size chart, a giant predatory theropod with tiny arms has been found that not only predates its lookalike, Tyrannosaurus rex, but developed its tiny arms completely separately from T. rex. It still pays to let your kids play with plastic dinosaurs.
“While fossilized brains from the Cambrian Period aren’t new, this discovery stands out for the astonishing quality of preservation and the large number of specimens. We can even make out fine details such as visual processing centers serving the large eyes and traces of nerves entering the appendages. The details are so clear it’s as if we were looking at an animal that died yesterday.”
That’s Joseph Moysiuk, a University of Toronto (U of T) PhD Candidate in Ecology and Evolutionary Biology, getting excited about half-a-billion-year-old fossils of Stanleycaris hirpex, a radiodont (an extinct species of arthropods) found in the Stephen Formation near the Stanley Glacier Burgess Shale locality – the Stephen geologic formation in the Canadian Rockies of British Columbia and Alberta consists of thin-bedded limestone, siltstone and the so-called Burgess Shale which is famous for the exceptional preservation of soft-bodied fossils from the Middle Cambrian era. Moysiuk, lead author of the study published in the journal Current Biology, was more excited than usual because the Burgess Shale preserved not one, not two, but 84 excellent Stanleycaris hirpex fossils showing their brains and nerves. These new fossils show that the brain of Stanleycaris was composed of two segments, the protocerebrum and deutocerebrum, connected with the eyes and frontal claws, respectively.
“We conclude that a two-segmented head and brain has deep roots in the arthropod lineage and that its evolution likely preceded the three-segmented brain that characterizes all living members of this diverse animal phylum.”
Take Moysiuk’s word for it – this is a big paleontological deal because it reveals how the segments of the brains lined up and how they evolved into the brains of the arthropods we see today – insects, crabs, shrimp, scorpions, spiders.
“These fossils are like a Rosetta Stone, helping to link traits in radiodonts and other early fossil arthropods with their counterparts in surviving groups.”
However, the best – OK, the creepiest – part of this discovery is that Stanleycaris hirpex had a huge third eye in the front of its head – a characteristic never before seen in a radiodont. Dr. Jean-Bernard Caron, ROM’s Richard Ivey Curator of Invertebrate Palaeontology, and Moysiuk’s PhD supervisor, explains in the press release that this shows the earliest arthropods had already evolved a variety of complex visual systems like their modern counterparts – think the strange eyes of crabs and spiders. Caron calls this “a crucial jump forward in understanding what they looked like and how they lived.” And what exactly did they look like? (Photos of the fossils here.)
“With large compound eyes, a formidable-looking circular mouth lined with teeth, frontal claws with an impressive array of spines, and a flexible, segmented body with a series of swimming flaps along its sides, Stanleycaris would have been the stuff of nightmares for any small bottom dweller unfortunate enough to cross its path.”
Fortunately, the Stanleycaris hirpex was only 20 cm (7.8 inches) in length – scary to Cambrian Period creatures, but not to our next new prehistoric dinosaur, the Meraxes gigas. Yes, before you ask, Meraxes was named after the dragon from George R.R. Martin’s fantasy novel series “A Song of Ice and Fire” – but it is no one’s fantasy. Discovered in what is now the northern Patagonia region of Argentina, this particular M. gigas died when it was about 45 years old, about 11 meters long, and weighed more than four tons. And, as we mentioned before, very little of that weight was arms. M. gigas had the same tiny forelimbs as T. rex but the big surprise to the paleontologists studying the fossils is that M. gigas and T. rex were not related – M. gigas went extinct 20 million years before T. rex began thundering around. That means both species evolved their tiny arms independently in different parts and times of the prehistoric world. And, despite the fact that they were so disproportionate to the rest of their body, as Juan Canale, the project lead at Ernesto Bachmann Paleontological Museum in Neuquén, Argentina, and lead author of the study published in Current Biology, explains in the press release.
“The skeleton shows large muscle insertions and fully developed pectoral girdles, so the arm had strong muscles.”
So, the arms of M. gigas and T. rex did not atrophy due to lack of use. However, previous research shows that as the T. rex’s head got bigger, its arms got smaller, and that seems to be the case with the M. gigas. However, the Carcharodontosauridae genus, which the M. gigas belongs to, were larger than members of the Tyrannosaurus genus. Both evolved these small arms for something. Canale has his own theory.
“They may have used the arms for reproductive behavior such as holding the female during mating or support themselves to stand back up after a break or a fall.”
Seeing a huge male M. gigas holding a female with its tiny arms would be something worth traveling back in time to watch – it’s probably too offensive for a Jurassic Park movie – but that’s the theory. The head of the M. gigas supports this idea – the skull the researchers found has crests, furrows, small hornlets and other ornamentations used to attract potential mates.
“Sexual selection is a powerful evolutionary force. But given that we cannot directly observe their behavior, it is impossible to be certain about this.”
Giant prehistoric therapods using their tiny arms for sex. Tiny arthropods using their giant third eyes to find or attract mates. Paleontology is a lot kinkier than it looks.