Date: 15 Apr 94 02:31:51 To: All Subject: Conodonts LONG (was Re: Help! Transitional form

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Date: 15 Apr 94 02:31:51 From: Andrew MacRae To: All Subject: Conodonts - LONG (was Re: Help! Transitional form lost) From: macrae@pandora.geo.ucalgary.ca (Andrew MacRae) Date: Fri, 15 Apr 1994 02:31:51 GMT In article <2ojlsc$27f@paperboy.gsfc.nasa.gov> jgacker@news.gsfc.nasa.gov (James G. Acker) writes: > Kari Tikkanen (ktikkane@phoenix.oulu.fi) wrote: > : I have seen in some (finnish) newspaper an article few years > : ago where it said > : that a new transitional form between inverterbrate and verterbrate has > : been found. > : Later I tried to 'hunt that article with my dogs and cats' > : but I never found it. > : > : Do you know anything about it or am I just dreaming ? > > Are you thinking of conodonts? The conodont organism had > the first "bone" (same mineral composition as bone), but was basically > a marine worm with a hard part. I believe Stan Friesen knows much > more, but this organism is referred to as the first in the Phylum > Chordata, due to the presence of this structure, from which the > vertebrate bone structure apparently derived. > > > I don't play a marine worm with a hard part ANYWHERE. > Big mistake mentioning microfossils - you should know better :-) Ah, conodonts. Big subject for micropaleontologists. This is a bit long, but it is a really interesting story, and is a good example of the sort of interesting detective work that paleontologists do, even if it does not seem as "exciting" as dinosaurs (because microfossils are so tiny) :-) [And, well, ok, I have done some work with them too :-)] Conodonts are about mm-size phosphatic (mostly Ca-phosphate) spiny, blade, or plate like elements that look similar to tiny teeth. They were first described by Pander in the late 1800s, and have been recovered from marine rocks of late Cambrian to Triassic age. Normally a rock is broken down using acids (for limestone, buffered acetic acid) or wetting agents (for the clays in shale) that will not dissolve the phosphatic material. The remaining residue is then separated by density to concentrate the phosphatic material and the conodont elements are then hand-picked with a microscope. Conodonts are very useful for biostratigraphy (relative dating) because their morphology is distictive, they preserve well (partially recrystallized ones even occur in metamorphic rocks), and their morphology changes rapidly through geologic time. Because of the recovery method, the elements are usually studied as single elements. The function and affinities of conodonts have been an open subject experiencing lively debate for a long time - more than 100 years - despite their empirical usefulness for biostratigraphy. People have suggested everything from plants to annelid jaws to fish for affinities, and everything from gill rakers to copulatory spicules to teeth for their function. One of the objections to the fairly obvious possibility of a tooth function is that they have such delicate spines, and more importantly, there is convincing evidence that they grew by accretion on the outer surface, and that they could be repaired upon breakage. This implies that the entire conodont, including the supposed surface being used for a tooth function, was surrounded by soft tissue that could deposit the phosphate (contrast with our teeth, for example, which grow internally by accretion, and then are erupted to the surface, to wear away for most of the rest of our lives - in conodonts, the teeth always grew, and on all surfaces). Another clue is the occurrence of conodont assemblages - sets of about a dozen (varies considerably) conodont elements with _different_ morphology (e.g., two plate-like ones, two blade-like ones, and several pairs of spiny elongated ones), arranged in bilaterally-symmetrical patterns on rock bedding planes, thought to represent the assemblage of elements found in one "conodont animal". This was a bit of a surprise when first found in the 1930s and 1940s, because until then it was assumed that distinctive morphologies represented separate species, genera, and even higher taxonomic levels. When this was discovered, conodont nomenclature was substantially revised, and the current taxonomy deals exclusively with "conodont assemblage" species, containing several morphologies of elements. The geometry of the assemblages suggests that there was a specialization of the conodont elements for different purposes in the same animal. This geometry has been figured out in "3D" by looking at compressions of assemblages in different orientations (I have done some of this using 3D modelling on a computer). An interesting pattern through time is that the earliest assemblages consist entirely of curved cone-shaped elements very similar in morphology (e.g., Cambrian and Ordovician), and the more specialized and differentiated assemblages occur later (Silurian and later). Once the geometry of the assemblage was understood, the big question was the characteristics of the rest of the "conodont animal", by now clearly a soft-bodied creature, and very unlikely to be preserved. Several candidates were proposed, including one from the Burgess Shale, and one that later turned out to be a conodont-animal _eater_, but none of these satisfied the requirement of a match with the number, types, and geometry of the already-observed bedding-plane assemblages. The affinities of conodonts were essentially a mystery, because there just were not enough clues. Figuring out the affinities would also help understand their function. By the 1970s and early 1980s, people were starting to favour a Chordate affinity (i.e. the group we, fishes, and other vertebrates are in, but also including "soft-bodied" creatures like tunicates, larvellaceans, and cephalocordates (remember _Amphioxus_?)), based on the phosphatic composition and details of the structure (which look rather "bony") - but the evidence was still not conclusive. Clearly the "holy grail" of conodont workers was to find a genuine "conodont animal" preserved with its soft tissue structure. Finally, in 1983, Briggs et al. published the first conclusive example of a conodont animal with soft tissue. It fit the expected characteristics perfectly - it had the expected geometry, types, and numbers of elements in a conodont assemblage at one end. An interesting feature of the discovery, from the Carboniferous "Granton shrimp band" near Edinburgh, Scotland, is that the specimens lay in a museum drawer for decades before their significance was recognized. The Granton conodont animal specimens (there are about 9 specimens now known) are an elongated, eel-looking organism, about 40mm long, with an asymmetric tail fringe at one end (posterior), and a conodont assemblage at the other end - the anterior end. The "plate-like" elements are at the back, while the "blade" and "spiny" ones are forward. There are also two dark "blobs" on either side of the assemblage. Some people have speculated they might be eyes. The most interesting feature is the V-shaped impressions along the sides of the specimens. They look very similar to the V-shaped bundles of muscles, known as "myotomes" found in non-vertebrate chordates, like _Amphioxus_. In fact, the conodont animal in general bears a surprising similarity to _Amphioxus_ in overall structure and size, although there are some great differences too (the presence of conodonts being the most glaring :-)). Anyway, those myotome-like structures, and the phosphatic and "bone-like" internal structure of conodonts seem to point the same way -> Chordate affinities. Not only that, but conodonts could represent the first "bone" material found in chordates. There is also some possibility of Hemichordate affinities (arrow-worms have been suggested), but current thinking is that conodonts represent something akin to cephalochordates (e.g., _Amphioxus_) and/or jawless craniates (e.g., hagfish and lamprey), but not the same. They probably represent some other, unique kind of early chordate, and people have begun to classify them as such, and refer to the sybphyllum "Conodonta" within the phylum Chordata. If correct, you can think of conodonts as an extremely successful group of mostly Paleozoic, almost completely soft-bodied, perhaps even "fish-like" chordates, with a unique type of food-gathering and processing equipment (in function and growth). Of course, despite the progress on affinities, there is still quite a bit of controversy about the function of conodont elements. Although it is clear they are involved with feeding, how exactly did they work, and what about that weird "soft-tissues all around" deposition of phosphatic material, and the differentiation of the elements? These are hot areas of study right now. In some sense, conodonts are sort of what you would expect between a completely "soft-bodied" form like the cephalochordates or tunicates, and later "fish" with cartilagenous and bony skeletons, but they are probably an offshoot, rather than on a direct line (my opinion). Even if not on the direct lineage, they certainly provide some clues to early chordate design and lifestyles. The earliest "true" conodonts are late Cambrian, but phosphatic, "simple cone" morphologies with slightly different growth patterns from later conodonts - the "paraconodonts" - occur down into the late Precambrian, and if ancestors of later conodonts, as suspected, that could extend the record of "fish-like" chordates back to just after the Ediacaran faunas. Incidentally, I would love to see a young-Earth flood-geology creationist who subscribes to the "hydrodynamic sorting" hypothesis try to explain how conodonts (which are basically delicate "sand to silt" size fossils) are sorted into such a fine stratigraphic sequence, despite being found in lithologies as distinctive as limestone, shale, siltstone, and every shade in between; and are found exclusively in marine sediments. Selected references: Aldridge, R.J., Briggs, D.E.G., Clarkson, E.N.K., and Smith, M.P., 1986. The affinities of conodonts - new evidence from the Carboniferous of Edinburgh, Scotland. Lethaia, v.19, p.279-291. [Followup to the 1983 paper.] Briggs, D.E.G., Clarkson, E.N.K., and Aldridge, R.J., 1983. The conodont animal. Lethaia, v.16, p.1-14. [Probably will go down in history as the most important paper on conodonts since their discovery.] Du Bois, E.P., 1943. Evidence on the nature of conodonts. Journal of Paleontology, v.17, no.2, p.155-159. [One of the earlier papers on conodont bedding-plane assemblages.] Purnell, M.A. and von Bitter, P.H., 1992 (15 Oct.). Blade-shaped conodont elements functioned as cutting teeth. Nature, v.359, p.629-630. [Evidence for a "slicing" action for some of the elements.] Rhodes, F.H.T., 1952. A classification of Pennsylvanian conodont assemblages. Journal of Paleontology, v.26, no.6, p.886-901. [Proposes an "assemblage-based" classification of conodonts, based on bedding-plane assemblages.] Sansom, I.J., Smith, M.P., Armstrong, H.A., Smith, M.M., 1992 (29 May). Presence of the earliest vertebrate hard tissues in conodonts. Science, v.256, p.1308-1311. [Describes the fine internal structure of conodonts, and compares them to vertebrate bone.] -Andrew macrae@pandora.geo.ucalgary.ca or: macrae@geo.ucalgary.ca

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