I’ve reached my goal of 4 research blogging posts this month, but just barely (and not at all if you’re east of Manitoba)! There are actually two, possibly three, more recent PLoS articles I’d like to talk about, so there will be a bit more research blogging in the near future. Here’s this week’s edition:
Have you ever woken up the middle of the night wondering if Triassic creatures ever got arthritis? Neither have I. But now I know. In fact, the oldest known arthritis sufferer is from the Triassic. It’s an unidentified archosaur – part of the taxon that includes dinosaurs, birds, crocodiles, and their most recent common ancestor – whose fused vertebrae are described in a recent issue of PLoS ONE.
The fossil, consisting of three vertebrae with “osseous overgrowths”, comes from ~245 million-year-old strata in South Africa. The overgrowths—symptoms of arthritis, among other possible conditions—make it impossible to see the joints between the vertebrae, so the authors used a type of CT scan called neutron tomography to generate 3D images of the whole fossil. This technique allowed them to rule out several possible alternative explanations for the bony growths.
The differential diagnosis section is quite technical, and, according to Dr. Mum (who knows a thing or two about bones), a little unusual in how it classifies the types of arthritis. There are many kinds of arthritis, but they can be grouped into two categories. There are the inflammatory kind, which are often autoimmune diseases and diagnosed not just by how the joints are affected but by symptoms throughout the body, including the presence of biochemical markers such as rheumatoid factor (obviously, associated with rheumatoid arthritis). It’s necessarily difficult to diagnose or rule out any of these on the basis of only three vertebrae. But this article notes that rheumatoid arthritis normally affects the spine differently, with little bony overgrowth. Diffuse idiopathic skeletal hypostosis, or DISH, is ruled out for similar reasons, but is not a form of arthritis.
The other main type of arthritis results from general wear and tear. You see it in the elderly as well as in people who repeatedly use certain joints—piano players, athletes, dancers, etc. It’s called osteoarthritis if it affects limb or digit joints, but if it affects the spine it’s usually called degenerative disc disease or facet degenerative change in humans. The latter is what this creature most likely had, but the authors argue that it should be called spondarthritis or spondyloarthritis. They suggest that, given the size of the vertebrae, this animal was probably quite old when it died, and spondarthritis would be much more common in an elderly animal. I’m not sure if they have any further evidence for that; are there no similar larger fossils known from this time and place?
There are two other non-arthritis possibilities that are fairly easily eliminated:
- congenital vertebral synostosis, or fusion of vertebrae from birth, is unlikely, because it normally does not involve bony overgrowths but rather a joining of otherwise normal-looking vertebrae
- scar tissue from a healed fracture normally leaves both fracture lines and immature (regrowing) bone
The process of differential diagnosis for a fossil is interesting, because in many of these diseases one must look for symptoms in parts of the skeleton that we don’t have or in soft tissues (e.g. you’d do a blood test to look for rheumatoid factor). A bit of creativity is required. It’s also unclear how general these diagnostic criteria are for non-human animals. Arthritis in various forms is found in most, if not all, vertebrates, but can the same criteria be applied to, for example, bipedal and quadrupedal organisms?
This article did get me thinking about the evolutionary history of arthritis. Many forms of it, especially osteoarthritis, must be universal among creatures with bony skeletons, because many of them are byproducts of the systems that repair damaged bone and cartilege. Retired racing greyhounds, for example, often have problems with leg and hip joints. But I’m unsure as to whether the autoimmune types occur in other species, or whether entirely different inflammatory arthritis categories can be found.
One final point: I don’t wish to berate the authors for poor grammar and spelling, but this error is too hilarious to overlook (emphasis mine, of course):
The vertebrae under study constitute the remnants of a large, and presumably old, carnivorous reptile. Being affected by this pathology, the individual would suffer an increasing constraint of the movements of its axial skeleton that slowly would hamper its faculty of locomotion. Such condition was surely disadvantageous in a number of activities that may require great physical effort, such as praying and territory defense.