On Friday I gave you a bit of a spot-the-difference with these two skulls, wanting to know if they were two individuals from the same species or if they were from two different species:
I must say that it was a bit of a tricky identification without the added complexity of a between specimen comparison, yet you all did remarkably well.
As usual Jake was the first to comment, correctly identifying that the specimens are both rodents and squirrels at that. He also recognised that both were adult animals, although one was probably older than the other when it died, based on the degree of wear on the teeth (assuming the diet was similar). The squirrel identification was also supported by Will, henstridgesj, Dave Godfrey, Jamie Revell and Barbara Powell.
Barbara also picked up on the feature that made me consider that these specimens may have been from different species – the sutures between the premaxilla, maxilla, nasals and frontal bones that make up the rostrum (the nosey bit). This is something that Lena and Jamie Revell also commented upon.
The position of the sutures (or junctions) between the various bones that make up the rostrum can certainly be useful in diagnosing differences between species – it’s a handy one for distinguishing between Lions and Tigers for example:
However, in this case I don’t think that the differences between the sutures are all that diagnostic, I think the differences may simply be down to either sexual dimorphism (that’s where males and females of the same species develop differently) or differences between the ages of the individuals. In fact, given that the specimen with the more heavily worn teeth is smaller and less robust than the other specimen I wouldn’t be surprised if it was an older female and younger male of the same species that are being compared.
One of the reasons I don’t think the sutures are diagnostic comes down to timing of their fusion. According to Wilson & Sánchez-Villagra, 2009 the pattern of closure of the cranial sutures in rodents follows a fairly standard pattern, with the rostral elements being amongst the last to fuse. This suggests that those sutures are more likely to vary between animals of different ages and between animals with different life histories. That said, there are geographical variations in this species, so these specimens may represent individuals of different subspecies from different parts of the range – something I can’t check because there is no locality information with them (at least not that I’ve found yet).
With the spot-the-difference dealt with, I will leave you with the correct species identification as made by henstridgesj, these are the skulls of Continue reading