What is a ‘dwarf’? Popular images conjured are those of small, marching, bearded miners, or perhaps more recently impish and alcoholic pseudo-politicians from Westeros.
In palaeontology, or even biology, a ‘dwarf’ is a very specific, and peculiar, thing. Just because something is small, it does not mean it is a dwarf. It could just be well, small, or it could be younger specimen or animal. In modern animals, this is pretty obvious to see, as we can track the life histories of individual animals and tell when something is small because it is small, or small because it is young.
A ‘dwarfed’ organism is one which has undergone a size reduction from a larger ancestor, due to factors such as resource availability. It has to happen over generational time, not the life of an individual. Tiny island elephants are one fascinating example of what we call insular dwarfism.
This is pretty difficult to see in the fossil record though. Fossils are frozen in time at the point of death. If they kept growing, that would be kind of weird. Fortunately for us, they preserve their growth stages in their bones. So how do you tell if fossils are small because they are small, or because they are young, or because they are something else, like a dwarf?
Well, individual fossils also preserve their growth history in their bones. I don’t want to say just like a tree, but yeah, it’s pretty much just like a tree. If you slice open the bones of oh say for example a fossil crocodilian (crocodiles, alligators, caiman and gharials and false gharials), you see tiny growth lines inside the cross-section.
These are called ‘lines of arrested growth’, or LAGs. Their form is influenced by the physiology, growth rate, and age of an animal, which is reflected in their microstructural properties. So combining crystallography with palaeontology, you can learn some pretty neat stuff about even extinct animals! LAGs indicate times of pause in growth in animals, and in crocodiles usually indicate a cyclic growth period.
A new study from an international team of experts led by Juliana Sayão decided to see if they could work out whether a cool little crocodilian ancestor called Susisuchus was a dwarfed croc, or just a small, immature specimen. To do this, they obtained cross-sections of the rib and the ulna (lower arm bone, use your imagination for how) of a specimen of Susisuchus and checked it out under a microscope.
The forearm of Susisuchus had 17 of these LAGs in the ulna, which suggests it was pretty old, at least a sub-adult, but way past sexual maturity. Goodness only knows what a moody teenager crocodile looks like.. But this means we know at least that the croc was 17 years old when it died. If anyone feels like throwing a very late crocodilian birthday party, now is the time.
These lines of arrested growth are arranged in a very specific parallel-fibered way, which we can use to deduce the rate of growth and bone deposition of the animal. Sherlock himself would be proud! In this case, parallel fibers indicate that the bone was deposited relatively slowly, implying a slow rate of growth.
Based on this, and contrary to previous assertions, this means that Susisuchus was a pretty small critter, even when fully mature. It just grew slowly, for some reason, perhaps due to some physiological factor, or due to ecological relationships with other animals around at the time.
Importantly, Susisuchus possesses what we might think of as a combination of ‘primitive’ and ‘advanced’ anatomical features. If Susisuchus is indeed a dwarfed taxon, we might expect it to retain more of these ‘primitive’ features despite being relatively advanced. This discovery might help to figure out where Susisuchus should be placed in the crocodilian tree of life, as this combination of characters can lead to some confusion when assessing it alongside other crocs. Recently, Susisuchus was moved from a position very close to the origin of modern crocodiles to a more basal position, outside of the clade known as Eusuchia, which includes all modern crocodylians. But this might just be due to mis-interpretation of some of its features. Only further analysis and time will tell!
Susisuchus was actually a fairly ordinary croc as far as these Mesozoic weirdos go. It probably lived a fairly sedate semi-aquatic lifestyle, perhaps as an ambush predator. The relatively dense bone which Susisuchus possesses is probably indicative of the lineage returning to the waters after being fully developed for life on land. Spongy tissues within the bone which the researchers also identified in cross section show that Susisuchus was actually probably fairly buoyant in spite of this, and could probably control its movement with little paddles, just like we see in some modern crocodiles.
What’s really cool is that we can compare Susisuchus to some living crocodilians! Osteolaemus is considered to be a dwarf crocodile, with several species known from Africa. Similar to its extinct counterparts, it’s also super cute, but perhaps most importantly by comparing species we can learn more about the growth of crocodiles and the evolution of dwarfism.
Citation: Sayão JM, Bantim RAM, Andrade RCLP, Lima FJ, Saraiva AAF, Figueiredo RG, et al. (2016) Paleohistology of Susisuchus anatoceps (Crocodylomorpha, Neosuchia): Comments on Growth Strategies and Lifestyle. PLoS ONE 11(5): e0155297. doi:10.1371/journal.pone.0155297