Imagine an armoured crocodile, 3 times the length of a human adult, and with a bulky head filled with razor sharp, bone crunching teeth. Now imagine that thing was very, very real. But thankfully, only more than 160 million years ago.
Thanks to a new fossil discovery, one of the major extinct groups of crocodile-ancestors received a small re-write of their evolutionary history. The geologically oldest discoveries of the now extinct Notosuchia help solve a long-standing riddle about their evolutionary origins.
A bit of background science first
Back in the Cretaceous, between 66 and 145 million years ago, the ancestors of modern crocodiles formed two major groups. The neosuchians were often small-bodied, lived in shallow lagoons or coastal environments, and would eventually go on to give rise to modern species.
The other group, notosuchians, were utterly bizarre animals. Known mostly from Africa and South America, these were built like small tanks, with thick armour-plating covering their bodies. Some looked more like armadillos than crocodiles! They also lived in inland environments, and oddly had a diet composed mostly of plants. Their short, stout heads had mouths full of little teeth, similar to some herbivorous dinosaurs. These were great for slicing up tough plant matter as opposed to meat.
Notosuchians and neosuchians are what we call sister taxa – they are the closest relatives to each other, and share a common ancestor. This means that they originated at the same time, from the same ancestral stock of animals. Together, they form a bigger group known as mesoeucrocodylians.
Where are all the Jurassic notosuchians?
But what is weird is that we have found fossils of neosuchians all the way back into the Middle Jurassic. This is around 40 million years before we find the oldest notosuchian fossils, which are all known from the Cretaceous period. We call this phenomenon a ‘ghost lineage’, where we know that notosuchians must have also existed in the Middle Jurassic, but we just haven’t found them yet.
This has actually puzzled palaeontologists for quite some time now. Where are all the Jurassic notosuchians?! Part of this is probably because Middle and Late Jurassic sedimentary rocks of the type that we would find notosuchians in are actually very rare in the southern hemisphere. Most of our knowledge of neosuchians from this time comes from Europe and North America, where the geological record is quite different.
Now, a new discovery from the Middle Jurassic of Madagascar has helped fill in this ghost lineage. We knew the fossils had to be out there, and now finally they have turned up!
History of discovery
Nicknamed ‘Razana’, the new specimens belong to an existing, but previously very poorly understood, animal called Razanandrongobe sakalavae.
Razana was actually first described back in 2006 on the basis of just a few teeth and cheek bones. Due to the scrappiness of the material, researchers could not really identify with any certainty whether it was actually from a meat-eating theropod dinosaur or a weird croc. All they knew was that it was very big and very predatory, and for a decade, that’s how it stayed.
The new specimens come originally from a private collection. Researchers were able to identify as being the same as Razanandrongobe by comparing the bits of the specimens that overlapped. Discovered originally between 1972 and 1974, no-one knew their potential until now as they remained locked away in private hands. They were first collected by the assistant director of technical services of Société Sucrière de la Mahavavy, D. Desouens, and eventually transferred to the Muséum d’Histoire Naturelle de Toulouse in France in April 2012. Here, they came under the scrutiny of Cristiano Del Sasso and colleagues, and were published with some great illustrations in PeerJ.
What do we know about this croc?
Fact sheet:
- Location: Hills west of Ambondromamy, Mahajanga Basin, northwest Madagascar
- Age: 167-164 million years old, the Middle Jurassic period (Bathonian)
- Geological strata: Sakaraha Formation
- Group: Mesoeucrocodylia
- Diet: meat
Based on these new specimens, researchers were able to show that Razana cannot possibly have been a theropod. They identified key anatomical features of the skull and jaws that do not match those of theropods in any way, but are in fact much more similar to mesoeurocodylians. They confirmed this by running a phylogenetic analysis including the new specimens of Razana, which confirmed its new status as a notosuchian.
Evolutionary implications of Razana
Razanandrongobe tells us much about the Middle Jurassic evolution of crocodyliforms. It is the oldest known Jurassic notosuchian, and therefore closes the long ghost lineage between them and neosuchians by around 42 million years!
What is perhaps even weirder is that Razana might be the first known notosuchian, but it doesn’t have the most primitive position in the evolutionary tree. The analysis of its phylogenetic position found it to be closely related to a rather advanced group of notosuchians called sebecosuchians. Razana probably represents a very early sebecosuchian. What this means is that the origin times for the other ‘deeper’ notosuchian groups are pushed even further back. In many cases, the origins of other groups are now hyopthesised to be in the Early Jurassic. So in the future, we can probably expect to find even more weird notosuchians as we unlock new places to discover fossils.
With Razana being the oldest known member of its group, we can infer that notosuchians probably had their evolutionary origins in the southern landmasses of the southern hemisphere, which during the Jurassic were called Gondwana. With still some 30 million years of ghost lineage remaining, it is possible that this evolutionary origin could be over-turned in the future.
Feed me!
The biggest teeth of Razanandrongobe were 15cm long, like large steak knives. These were designed for punching from hard tissues such as bone and tendon. The serrations, or denticles, on these teeth were larger than even those found in the largest predatory theropods, including T. rex. This is extremely unusual, as other members of Notosuchia typically had teeth useful for a herbivorous diet of plants.
Razan might have even been the biggest land dwelling mesoeucrocodylian of the Jurassic! It was certainly the largest of its notosuchian kin, and with those giant chompers would have been one of the top predators around at the time, and was certainly king of its ecosystem.
As always, new discoveries raise more questions too! Why did notosuchians start so big but then evolve smaller bodies over time? Why did they switch from carnivory to herbivory over millions of years? Did they compete with theropod dinosaurs in their early evolution?
As always, palaeontologists rally to the cry of “We need more fossils!”, and only time will tell.
References
(2017) Razanandrongobe sakalavae, a gigantic mesoeucrocodylian from the Middle Jurassic of Madagascar, is the oldest known notosuchian. PeerJ 5:e3481 https://doi.org/10.7717/peerj.3481
Source: PLOS.
