As you’ll note from the logo over there, this marks the first issue of “blogger half-assedly opining about peer-reviewed publications when, really no one asked in the first place anyway.” I’ll use this logo whenever I…well you get the picture. Feel free to borrow the logo for your own half, or even whole-assed efforts.
Invert-workers are always carping1 about vertocentrism, and of course they have a point: tardigrades are like a gajillion times radder than tyrannosaurs and it’s a shame that Discovery Channel programming doesn’t reflect this fact. I guess they’re busy vetting questions for Cash Cab or whatever. But it occurs to me: it’s actually unfair to resent vertebrates as a whole for this injustice because, let’s face it, aside from lamnids no one gives a swimming crap about fish.
I know, I know, cladistically speaking, tetrapods like Sue and me are just aberrant terrestrial fish. Aside from our freakish lineage however, the silent majority of “values” vertebrates (i.e fish) might as well be ostracods for all the press attention they get. I mean, ostracodes. Whatever. (Actually ostracods/es have a pretty good PR person these days).
A few cases in point2: Funisia, an ediacaran with all the charisma of a sodden mop head, got major press attention thanks to some good old fashion sexing up by the media. Likewise, Martialis heureka, the recently discovered basal ant, is already generating major buzz3 well before the peer-reviewed paper announcing the discovery has even hit the presses. And don’t get me started about Aptostichus stephencolberti.
Meanwhile, the discovery of a new and extraordinarily bizarre fossil fish, Hsianwenia wui, announced in last week’s issue of PNAS (Chang et al. 2008), sank with less of a splash than a 49 kg Chinese diver making a perfect entry4. And that’s unfortunate, because if the public has an inordinate fondness for things with hydroxylapatite endoskeletons, well Hsianwenia is about as bony as they come.
Hsianwenia (which I’m pronouncing “shee-An-Wen-ya” until someone corrects me) was discovered in Pliocene lake sediments from the Qaidam basin on the north side of the Tibetan plateau. Hsianwenia belongs to the largest family of freshwater fishes the Cyprinidae which also includes minnows, carp and goldfish among many others. The uplift of the Tibetan plateau over the past several million years has created multiple small, isolated lakes and waterways. This in turn has driven the evolutionary radiation of an endemic suite of Cyprinids. These 100 or so species in 15 genera are grouped together the subfamily Schizothoracinae, known to the more poetically-minded as “snow trout” or “snow carp” (Qi et al. 2006).
While FishBase reports that the flesh of living schizothoracines is “much relished”, eating Hsianwenia would have been a chore. That’s because unlike its relished relatives, Hsianwenia is characterized by a peculiar thickening of the skeleton. This “pachyostosis” is so extreme that the authors state that the bones appear to leave little room for muscle.
While no known living fish possess a similar super-skeleton, another extinct fish, Aphanius crassicaudus—from Miocene sediments on the northern margins of the Mediterranean—apparently independently evolved extremely thick bones. Multiple specimens from both species demonstrate that the pachyostosis is not evidence of disease or disorder, but was a natural feature in each fish. More over, this condition was amplified through the course of ontogeny with fish becoming progressively stouter as they aged.
What factors could have selected for this unusual evolutionary quirk not once but twice? The sediments containing the two fish species—though separated by space and time—share some provocative mineralogical clues: gypsum and calcium carbonate. Both of these minerals are calcium salts and their presence as inorganic precipitates suggests that the bodies of water these fish lived in had extraordinarily high concentrations of dissolved calcium and other minerals.
The authors of the recent paper suggest that the hypertrophied skeleton of Hsianwenia (and Aphanius) was a novel solution for ridding the body of excess calcium5. By thickening their bones, these fish were able to sequester calcium before it built up to toxic levels within its tissues. Chang et al. also speculate that the saline waters were toxic to other vertebrate species given the absence of other vertebrate fossils. So, these strange fishes may have had no need to escape from predators and could afford to reduce muscle space and add bulky bone. Pollen and, yes, ostracods/es provide circumstantial support for generally arid and saline conditions in and around the lake while the fish were thriving.
Hsianwenia’s solution to it’s hard-water environment worked pretty well for 200,000 years or so, allowing it to thrive in waters where no other fish could. Of course, Mother Nature’s a vindictive bitch, and all evolutionary solutions are by definition, temporary. A thick evaporite deposit capping the fish-bearing layers speaks to our tale’s tragic end: the aridification of the Qaidam basin continued, the lake dried up, the freaky thick-boned fish died, the end.
So there you have it: tectonics, climate, aqueous geochemistry, evolution, morphological novelty and million-year-old fossil fish bones scattered across the high desert. A fish story worth telling.
And you thought fish were boring.
Qi, D. et al. 2006. “Mitochondrial cytochrome-b sequence variation and phylogenetics of the highly specialized schizothoracine fishes (Teleosti: Cyprinidae) in the Qianghai-Tibet Plateau.” Biochemical Genetics 440: 270-285.
1 As we’ll soon see, this is a hilarious pun.
2 There is one, sort of. Be patient.
3 I suppose I’m mixing hymenopteran metaphors here.
4 Credit where it’s due: a German science blogger has already written about Hsianwenia here (in German).
5 One hypothesis to explain the “explosive” evolution of organisms with hard parts in the Cambrian holds that changes in seawater chemistry (perhaps linked to tectonic activity) drove organisms to begin precipitating minerals to prevent toxic buildup inside their cells. Subsequently these structures were exapted into shells and carapaces and bones and teeth ultimately triggering an adaptive arms race. While this hypothesis is speculative and controversial recent research does support the importance of seawater chemistry in setting the patterns of biomineralization among various lineages (Porter 2007).