(Credit, again, to Neil from Microecos). And I'm afraid that may just be the most excitement that we get in this post. While some fossils are problematic because they're so strange that they can't be easily compared to living animals, others are problematic simply because they're rubbish.
In 1840, the palaeontologist Georg Graf zu Münster ('Graf' being a German title that generally gets translated as 'Count') published his Beiträge zur Petrefakten-Kunde, in which he described a number of fossils held in his collection. This book included a section on fossils from the Ordovician Orthoceratite Limestone of the Fichtel Mountains in Bavaria. Which, close to the end, included this little tidbit:
Unter mehreren Bruchstücken einiger mir noch unbekannten Versteinerungen kommen auch einige röhrenformige Korper vor, welche ich anfänglich für den von Murchison aus der 27sten Tafel abgebildeten Myrianites hielt, allein genaue Untersuchung zeigte, dass diese Korper formliche Schalen hatten und daher vielleicht zu den Serpuliten gehört hatten, daher ich sie vorläufig Serpularia genannt habe. Aus der Taf. IX. Fig. 14 und 15 sind zwei Arten von dergleichen Bruchstücken abgebildet; Fig. II. Serpularia crenata; glatt gebogene Röhre, aus dem Rücken crenulirt. Fig. 15. Serpularia bicrenata; glatte etwas zusammengedrückte ganz grade Röhrchen, die an beiden Seiten crenulirt sind.
Translated with the help of Google Translate, I think this means: "Among several fragments of fossils unknown to me occured a tube-like body, which I initially took for Myrianites as figured by Murchison in the 27th plate, until close examination showed that this body had distinct signs of segmentation and was therefore perhaps one of the Serpulidae. Therefore, I have provisionally called it Serpularia. On Plate IX Figs 14 and 15 are shown two types of the like fragments; Fig. 14, Serpularia crenata: smooth curved tube crenulated from the back. Fig. 15, Serpularia bicrenata: smooth, slightly compressed, quite straight tubes that are crenulated on both sides".
As perfunctory as it was, that seems to be all there was to say on the matter. The good Graf's Serpularia has pretty much never been mentioned again*, beyond being cited to cause a name change in a later homonymous gastropod genus, and a brief listing in Howell's (1962) coverage of worm fossils for the Treatise on Invertebrate Paleontology that adds nothing to the original description.
*Though if it were to be mentioned again, it would probably have to be under a different name. The name 'Serpularia' had earlier been used by Fries in 1829 for a genus of slime moulds. At the time, slime moulds were treated as fungi, and hence fell under the purview of botanical rather than zoological names, but with the recognition that they are amoebozoans an increasing number of authors would move them into the field of the Zoological Code.
Münster believed that his fossils belonged to the Serpulidae, a family of annelid worms. Annelids, being mostly soft and squishy things that do not stand up well to decay, have a pretty deplorable fossil record, but serpulids are a bit of an exception. These are sessile worms that secrete a calcareous tube in which they live their lives (modern serpulids appeared on this site in this post). Unfortunately, while these tubes are eminently fossilisable, they are also a bit nondescript, and have little to mark them as uniquely serpulid.
Because of the dominance of annelids among modern worms, there has been a definite tendency in the past to assume that any given worm-like fossil represents an annelid. Howell's (1962) aforementioned list of annelids includes the Ediacaran Spriggina (identity still under debate, but probably not an annelid) and the Cambrian Pikaia (now generally regarded as an early chordate). Similarly, any worm-like tube has been assumed a serpulid. But even among annelids, serpulids are not the only tube-bearing worms. At least two other families, the Sabellidae and the Cirratulidae, include species producing calcareous tubes. There are also other groups of non-annelid worms that, though relatively uncommon or unprepossessing today, may have been more prominent in the past. After all, we are talking here about a period of hundreds of millions of years. We know that vertebrates have gone through a great deal of evolutionary change over that period; why should we assume that worms have not?
So while fossils have been assigned to the serpulids going back as far as the Cambrian (if not beyond), there is little reason to take those assignations at face value. When so-called Palaeozoic serpulids have been examined critically in recent years, they have so far proven to lack features that would definitely confirm their identification (Vinn & Mutvei 2009). Weedon (1994) found that Palaeozoic fossils that had been assigned not only to the Serpulidae, but to the modern genus Spirorbis, had a shell microstructure that suggested a relationship to bryozoans or brachiozoans rather than to annelids. Without a similar close analysis, we could not assume a priori that Münster's Serpularia were not serpulids, but odds would currently be against it.
REFERENCES
Howell, B. F. 1962. Worms. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt W. Miscellanea: Conodonts, Conoidal Shells of Uncertain Affinities, Worms, Trace Fossils and Problematica pp. W144–W177. Geological Society of America and University of Kansas Press.
Münster, G. 1840. Beiträge zur Petrefacten-Kunde von Herm. v. Meyer und Georg Graf zu Münster vol. 3. In Commission der Buchner'schen Buchhandlung: Bayreuth.
Vinn, O., & H. Mutvei. 2009. Calcareous tubeworms of the Phanerozoic. Estonian Journal of Earth Sciences 58 (4): 286–296.
Weedon, M. J. 1994. Tube microstructure of Recent and Jurassic serpulid polychaets and the question of the Palaeozoic 'spirorbids'. Acta Palaeontologica Polonica 39 (1): 1–15.
If you intend to make a series on Palaeozoic problematica, I nominate Typhloesus wellsi for an entry.
ReplyDeleteThe drawings immediately bring to mind mollusks for me, especially scaphopods or caecid gastropods. Is there any chance "Serpularia" could be molluscan in nature?
ReplyDeleteI wouldn't think there would be any inherent reason that it couldn't be a mollusc, though again that would require close examination (though I'm guessing that Münster's figures would represent a piece of each fossil type rather than an entire specimen). Münster seems to have ruled out molluscs because he thought the furrows on the Serpularia fossils indicated a segmented animal but I wouldn't think that the only possible interpretation. They may be simple ornamentation, for instance, or they may represent growth lines. Also, I don't see any a priori reason to assume that the two 'species' necessarily come from the same sort of animal as each other.
ReplyDeleteMünster's Serpularia would be probably be too early to be specifically either a scaphopod or caecid, though. Caecids only have a fossil record going back to the Eocene. Scaphopods probably appeared in the Devonian or Carboniferous, depending on how one interprets some very early fossils.
I should also note that molluscs have inspired comparable biases to what I described in the post about annelids. Because most living shelly animals are molluscs, there has been a bit of a tendency to assume that any shell comes from a mollusc. This has resulted in mollusc affinities being assumed for Palaeozoic groups such as tentaculitids that recent authors suspect probably sit elsewhere in the animal family tree.