Having been treated to a dinosaur last time in taxon of the week, this week I’ve decided to go for an obscure Palaeozoic fish. This fish is pretty obscure even for a Palaeozoic fish; so obscure, in fact, that it doesn’t even have its own Wikipedia article. I speak, as you’ll know if you bothered to follow that link, of the Devonian jawless fish Euphanerops.
Loathe as I am to introduce even more palaeontological nudity to The Dinosirs, Euphanerops is a so-called ‘naked anaspid’, a group that look similar, but not the same, as modern lampreys. The naked anaspids consist of a handful of taxa that are united by the fact that they look a bit like members of a larger group, the anaspids, but don’t have their distinctive bony scales. Both groups look like they lie somewhere on the gnathostome stem-lineage, which we met when we looked at heterostracans a few weeks ago. As we saw then, these stem-gnathostome groups are extremely important in understanding vertebrate evolution as they provide our only morphological information on how characters like jaws and paired fins evolved in vertebrates.
The specific area onto which Euphanerops sheds light is the evolution of paired fins. Jawed vertebrates today all possess paired fins, whether as traditional ‘fins’ or as limbs in tetrapods (although some, like snakes, have secondarily lost them). Modern jawless vertebrates, the lampreys and the hagfish, have no paired fins, they have only single ‘median’ fins down their midline. Unfortunately this gives us no information regarding how paired fins evolved, for all we know they may have just popped into existence. One avenue of investigation in solving the puzzle is evolutionary developmental studies (evo-devo), where the development of modern animals is studied for clues. This gives us intriguing hints, for example paired fins seem to develop in similar ways to both gill arches and median fins, suggesting a possible shared evolutionary origin. They can also be induced to develop along a lateral line along the sides of the body. However, if we want to know how the structure changed morphologically we have but one recourse. To the fossil record!
Fossils clear up the picture quite a lot. The earliest ‘paired fins’ we see are in groups like the thelodonts and the (clothed) anaspids, but these are all paired ‘fin-folds’, fin-like structures that lie paired along the body in vaguely the same place as pectoral fins, but which have no skeleton supporting them. Some heterostracans, such as Pteraspis, also have bony paired fin-like projections from their head-shields, but it looks like these are just convergent adaptations rather than actually being fins. Osteostracans, a group with a wide array of fancy head shields, have the first paired fins as we know them today, with a skeleton on the inside (like your limb bones). They only had pectoral paired fins (ie. the front fins of a fish, or your arms), and the skeleton was made of cartilage, but they were paired fins nonetheless, although some groups of osteostracan secondarily lost them again. Later, in jawed groups such as the placoderms, we finally see the full complement, of pectoral and pelvic paired fins.
Combined with the molecular evidence this progression from no paired fins to paired fin-folds to paired fins with skeleton has been used to propose an evolutionary mechanism. The story runs that lampreys and hagfish have a single stripe of ‘fin-competency’ down their midline, where fins can develop. This stripe was somehow double and spread onto the sides of groups such as anaspids, allowing paired fin-folds to develop on the sides as well. These paired ‘fin-competency’ zones then began to interact with the developmental tissue that bone is formed from, leading to skeletonisation of the fin in later groups, and eventually to the complex fin skeletons we see today.
This seems like a good explanation, and fits the evidence well. Euphanerops makes the picture a bit more complicated however. The genus has been known for a long time, and is also interesting in that some fossils appear to preserve a lamprey-like gill skeleton, which would imply that this is ancestral for gnathostomes. Our interests lie however in the realm of paired fins, and in 2013, a fossil was described as having paired fins. In itself this is not unusual, as the similar anaspids sometimes have paired fins in a position roughly where pectoral fins are in modern fish. Euphanerops however has paired fins where the anal fin is in a modern fish, ie. between the anus and the tail. This state of affairs is not known in any modern fish, or in any other extinct ones we know of.
This has interesting implications for the evolution of paired fins. It seems that rather than just following a progression from no fins to paired fins, evolution had a period of ‘experimentation’ with paired fins in different places before settling on the paired fins we see today. This fits in with our overall picture of evolution: while it can be tempting to think that it ‘progresses’ towards an aim, in fact it occurs piecemeal, leaving behind a mosaic of different ‘experiments’ with morphologies. It’s also worth noting that lines of fin competency idea of fin evolution mentioned before isn’t overturned by Euphanerops and its paired fins, but our picture of paired fin evolution is broadened.
So, far from being merely an unclothed anaspid and a Twitter handle that rolls off the tongue, Euphanerops and its paired fins have much to tell us about both paired fin evolution and wider evolutionary principles. Like Limusaurus last week it also illustrates how palaeontological and molecular methods can be used in a synthesis to better understand evolution. Pretty impressive all round for a fish that not even Wikipedia loves*.
*But I love you Euphanerops!.
Sansom et al (2013) Unusual anal fin in a Devonian jawless vertebrate reveals complex origins of paired appendages.
Janvier et al (2006) Lamprey-like gills in a gnathostome-related Devonian jawless vertebrate.
Yonei-Tamura et al (2008) Competent stripes for diverse positions of limbs/fins in gnathostome embryos.
Johansen (2010) Evolution of paired fins and the lateral somatic frontier.