I’ve touched on the tension between genetic and morphologic data in this post and the comments it attracted. As I stated then both sets of data are the products of a single history and should therefore be more or less in accord. When they are not, well then it is our job as scientists to find out why. Gone are the days of simply giving a hurumph and declaring one’s personally preferred set of data to be correct and the other erroneous. I want to feature a 2007 paper by Hurley et al. as an excellent model for the unification of both sources of data. Although a few years old now, it remains one of my favourite bits of palaeontological research. And it doesn’t even feature dinosaurs. No, it is about the phylogeny of ray-finned fish. But before I can get to the paper in question I will need to bring my readers up to speed with the relevant issues in this post before turning my attention to the paper itself in my next post.
For those not aquainted with fish taxonomy, I’ll first give a quick rundown of the groups involved. All vertebrates that replace their cartilaginous inner skeletons (endoskeleton) with bone are known as Osteichthyes (literally “bony fish”). Note that not all fish with bone are osteichthyans. Several extinct groups like placoderms and osteostracans covered their bodies in large plates of bone. However this bone is derived from skin (ectodermal) tissue and remains superficial to the endoskeleton which remains cartilaginous. Also note that we tetrapods also replace our endoskeletons with bone: we are bony fish (albeit highly terrestrially adapted bony fish). Anyway the bony fish clade divides into two great clades: Sarcopterygii (including tetrapods) and Actinopterygii. It is the Actinopterygii, or ray-finned fish that concern us here. Actinopterygians are a clade and can be diagnosed by a bunch of characteristics, the most obvious one being that there is just one dorsal fin. Of course the path of evolution is rarely simple and we find that many modern actinopterygians divide their single dorsal fin into an anterior spiny portion and a posterior portion with soft rays. Sometimes the dorsal fin is bilobed with a dip in the profile between the spiny and soft sections but more often there is a finless gap, thus two dorsal fins are secondarily acquired.
The evolutionary success of ray finned fish cannot be overstated. In terms of diversity they outnumber all other vertebrate clades put together 2:1 and exhibit a jaw-dropping array of specializations, some of them nightmarishly bizarre to our eyes. Living ray-fins can be divided into five clades, the vast majority of ray-fins belong to just one of these the Teleostei. The other four groups are the cladistians (bichirs), chondrosteans (sturgeons and paddlefish, not to be confused with the chondrichthyans which are the sharks, rays and ratfish), ginglymods (gars*) and halecomorphs (bowfin).
Polypterus, the bichir, a living cladistian from Central Africa. Image from wildwoods.co.uk
Pallid sturgeon, a chondrostean. Image from wikipedia.
Lepisosteus, gar, a living ginglymod from North America. Image from wikipedia.
Amia calva, the bowfin. The sole surviving halecomorph. From North America. Image from wikipedia.
The staggering diversity of modern ray-fins is actually a relatively recent phenomenon compared to the venerable age of the total actinopterygian group. The first ray-fins appeared no later than the Late Silurian, when we find the first fossils of their sister group, the Sarcopterygii (e.g. Psarolepis in China) whereas the steep rise in diversity and disparity didn’t really start until the Late Cretaceous and is restricted to the teleost clade. One intriguing feature of living teleosts is that their entire genome has been duplicated. Some have suggested that this duplication event may have been the impetus for the teleost diversification. With two copies of every gene it is easy to imagine that duplicates of essential genes that have little freedom to vary without compromising the organism would be free to vary, perhaps allowing a more rapid exploration of morphospace.
It is universally agreed on both morphological and molecular data that the cladistians are basal to all other rayfins, but the relationships of the other clades remain contentious. Morphological analyses support a ladder like arrangement with chondrosteans, ginglymods and halecomorphs forming serially closer outgroups to the teleosts. The clade including ginglymods, halecomorphs and teleosts is known as the Neopterygii and is characterized by, amongst other features, loss of the clavicle bone, a vertical hyomandibula (jaw suspensorium) and a mobile maxilla (but not in modern ginglymods). Molecular data based on sequences of mitochondrial genes however gathers these three groups into an ‘ancient fish clade’ that is the sister group of teleosts. The following diagram from Hurley et al. (2007) illustrates these two different arrangements well.
The shape of the tree is not the only source of molecular vs. morphological tension in ray finned fish evolution. Estimates of the timing of the divergences between these clades is also dramatically different when using the different data sources. For example the oldest crown-group teleost (reminder: the crown-group is the group descended from the most recent common ancestor of all extant species) in the fossil record dates from the Late Jurassic (151 million years), whereas mitochondrial data estimates the divergence date at either the Early Permian (285 million years) or Early Carboniferous (334 million years) depending on the method used. That’s a difference of 134 to 183 million years. Now we can always expect that a clade will not show up immediately after its divergence from its sister group, due to low diversity and low abundance but this difference is extraordinary. Other molecular divergence dates are also strongly discordant with the fossil data, for instance the bowfin-teleost split (which would provide a minimum age of crown-group Neopterygii) is estimated to range from 417-390 million years (Late Silurian to Mid Devonian) using mitochondrial data whereas the palaeontological date of neopterygian crown-group is 245 million years (Early Triassic). However the picture isn't quite as bad as it once seemed and we shall look at how improvements in our understanding of both sets of data is straightening out this problem in the next post.
*Note to Australian fisherpeople, the ‘garfish’ you will be familiar with are part of the vast teleost radiation and are not ginglymods of any sort.
Hurley, I.A., Lockridge Mueller, R, Dunn, K.A.,Schmidt, Friedman, M., Ho1, R.K., Prince, V.E., Yang, Z., Thomas, M.G. and Coates, M.I. (2007)A new timescale for ray finned fish evolution. Proc. R. Soc. B 274, 489–498
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