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 Post subject: the great fossil enigma
PostPosted: Sat Jul 22, 2017 7:12 pm 
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Simon Knell’s book on conodonts is interesting but a little disappointing. Knell is a historian of science, not a geologist or paleontologist (although his specialty is history of geology); he notes in his preface that he’s not presenting himself as a scientist or posing scientific theories. His approach is entirely historical and heavily biographical; the people who worked on or with conodonts are presented in chronological order, from Christian Pander in the 1820s to the Leicester and Birmingham groups in the 2000s (the book was published in 2013). Along the way, he sometimes seems to be namedropping everybody who had anything to do with conodonts, no matter how remote; the list includes dinosaur expert jack Horner (because he worked at the Bear Gulch quarry in Montana, where a putative conodont fossil turned up) to Pleistocene mammal expert Bjorn Kurten (because he had a conversation about conodonts with somebody in Sweden). This approach has its merits; you really do get a feel for the messy way science is done, with back-and-forth arguments among the workers, blind alleys, false hopes, brilliant analyses that turn out to be wrong, and personal vendettas. The problem with the strict historical approach, though, is it took a long time to figure out what conodonts actually were, why they were important, and how they caused unique taxonomic problems, and with Knell’s strict chronological approach you don’t learn these things until well into the book. If you already know the answers to these questions – i.e., if you are a paleontologist or at least a geologist, that’s OK, but if you’re an interested layperson – one of the reviewers described the book as “semi-popular”, trying to illustrate the dilemma – you’re going to be more confused than enlightened.

So, I’ll do it. There are previous threads on conodonts that you can search for. In summary, they are tiny little things that usually look like teeth. The argument over whether they really were teeth takes up a good part of Knell’s book; the general consensus now is that they are “teeth”, in that they are involved in feeding, but it’s still debatable whether they are homologous to vertebrate teeth. They belonged to an animal that was almost certainly a chordate and very probably a vertebrate (although there’s still some argument about that too). When I say “belonged to an animal” that requires some explanation; conodonts were found in enormous numbers in sedimentary rock, but the animal they belong to (and a few people thought they were parts of a plant) was unknown until very recently, because they were its only easily fossilized body part.

Conodonts are very useful in stratigraphic correlation. Suppose you have some black shale in Ohio and some pale grey limestone in Oklahoma. Do the Ohio Shale and the Oklahoma Limestone correlate – i.e., were they deposited at the same time? If you happen to have a convenient volcanic ash bed – by now a bentonite or metabentonite – in both, you can date that using radiometric methods and establish a common age. In the absence of a volcano, however, you can only do biostratigraphic correlation – and this was the only way correlation could be done until the invention of radiometric dating. Biostratigraphic correlation – biostratigraphy – involves looking at the fossils in the rocks and seeing if they are the same species. There a catch, though – a lot of fossils are facies dependent – although the organisms that formed them may have lived simultaneously, they lived in different environments. The Ohio Shale was likely deposited in anoxic deep water (that’s why it’s black, the organic carbon in it was never oxidized), while the Oklahoma Limestone was probably deposited in clear, shallow, well-oxygenated water. The Ohio Shale will likely have no benthic fauna at all – no oxygen – while the Oklahoma Limestone may have a plethora of brachiopods, bryozoans, corals, trilobites, and the whole gamut of epifauna. Thus there are no benthic fossils to compare. The solution is to use fossils that are facies independent – that were planktonic or nektonic, lived in the water column, and only settled to the bottom for fossilization when they died. The fossils you use for biostratigraphy are organisms like this – free-swimming cephalopods, floating graptolites, and conodonts. Conodonts have the additional advantages that they are tiny – Knell’s book cover illustration is four of them on the head of a pin – and thus you can bring up lots of them in a drill core; and that they change quickly with time, thus allowing high correlation resolution. So if you find the same conodont species in the Ohio Shale and the Oklahoma Limestone, you can say the two formations correlate to within the time stratigraphic range of that particular species. Conodont biostratigraphy became vitally important to the petroleum industry (it was later discovered that they had another valuable property for petroleum exploration; they change color in a predictable fashion based on the maximum temperature they have been exposed to, allowing them to be used to estimate if a particular rock unit had been heated to much or too little to produce petroleum). Knell estimates there were about five times as many geologists using conodonts for biostratigraphy as there were directly interested in their biology; I’d guess the ratio was actually larger than that.

Which brings us to the problem of conodont taxonomy. In the 1930s, it was realized that conodont “species” were not isolated types; instead conodonts grouped into units called “conodont assemblages” or “conodont apparatus”, composed of some number of “conodont elements” that had previously been considered individual species; i.e., four of Species A, four of Species B, and eight of species C. Knell notes it took a considerable time for this to catch on; critics claimed what seemed to be “conodont assemblages” were actually fecal pellets from an animal that had eaten a number of different conodont species. The counter observation was that these were fecal pellets, whatever animal was doing the defecating had always eaten four of Species A, four of Species B, and eight of Species C. This sort of thing happens all the time in paleontology; somebody finds some fossilized leg bones and gives them a species name; years later somebody finds a fossilized spinal column and gives it a species name; years later somebody finds a fossil skull and give that a species name; and finally somebody finds a more-or-less complete skeleton, shows that the legs and spinal column and skull all belong to the same animal. Under the International Code of Zoological Nomenclature rules, the very first published name – in this case the leg bones - has priority and it’s what the complete animal is now known as; the former names for the spinal column and skull are discarded (although they will get listed as synonyms in the literature in case somebody has to look up older works). Although for most taxonomic groups this doesn’t matter very much, since there aren’t that many people who are interested, for conodonts it caused (and still causes) huge problems, since the individual conodont elements in a conodont assemblage had all been given individual species names, and those species names were vitally important to petroleum exploration. This lead to something of a war – still not resolved – between taxonomists who wanted to follow the ICZN rules and petroleum geologists who wanted to keep familiar species names. Knell, who is often critical of petroleum geology in general and American petroleum geology in particular, calls the later approach “illegal”, which is pretty strong; violating the ICZN rules does not result in a SWAT team of taxonomists breaking into your lab and confiscating your specimens. At the same time, he notes that this would be no problem at all in paleobotany; since you almost never find a complete fossilized plant with roots and stem and leaves and flowers and pollen and fruit all preserved at once, paleobotanists are perfectly comfortable working with “form genera” and “form species” that only apply to one part; the paleobotanists assume it will all get straightened out eventually and in the meantime why worry.

Which now brings us to what the conodont animal is. (Knell notes that some conodont specialists don’t like the term “conodont animal”, pointing out that we don’t talk about a “lion animal” or a “giraffe animal”, and just use “conodont”, which may be strictly correct but isn’t very useful for people outside the field. People had been trying to find the animal associated with conodonts since Pander first named them in the 1820s, with no luck at all. Even Konservat-Lagerstätte, like the Burgess Shale or Hunsruck Slate or Mazon Creek formation, that preserved other soft-bodied organisms didn’t show any sign of conodont animals. Then at the 1969 North American Paleontological Conference, University of Montana geologist Bill Melton showed up with an unusual fossil that a student had found in the Carboniferous Bear Gulch Limestone. On examination, the thing seemed to have a conodont apparatus in life position in the gut; the immediate suggestion was this was the conodont animal, and the conodont apparatus was not teeth but some sort of support structure (this had been suggested before, based on the observation that conodont “teeth” never seemed to show wear or breakage). After a considerable amount of armwaving among conodont experts, it was finally concluded that the Bear Gulch creature had eaten a conodont animal (as it turned out, the Bear Gulch creature, eventually given the name Typhloesus wellsi, was itself pretty weird; it was apparently an active predator but doesn’t seem to have eyes or any other sensory apparatus, and seems to have a blind gut. And it ate conodont animals but no conodont animals were discovered in the Bear Gulch quarry). There were a couple other false alarms of conodont animals, including one from Burgess Shale; all turned out to be incorrect. Finally a pretty convincing one turned up from the Carboniferous of Scotland; it was a small vermiform creature with a conodont apparatus at the head end, paired muscles groups, and eyes; leading to the conclusion that it was a vertebrate. Once people knew what to look for a couple more putative examples turned up, from the Silurian of Illinois and the Ordovician of South Africa; however, although conodonts are know from the Cambrian to the Triassic there are still less than 20 conodont animal fossils known. Given the diversity of conodont elements, an analogy might be if you had to try and figure out mammalian diversity from millions of teeth but only a few fossils – a giraffe, a beaver, and a seal, maybe.

Here’s what some conodont apparatus look like; note that these are reconstructions based on microscopic smashed up fossils, you never find them this well organized.

Image

So all-in-all Knell’s book is interesting enough but could use a less biography and name-dropping and more explication.


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 Post subject: Re: the great fossil enigma
PostPosted: Sun Jul 23, 2017 10:37 am 
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As frequently happens, something new. I had never heard of a conodont.


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