Microscopic Treasures - Scolecodonts from the Type Cincinnatian
Mats Eriksson (Department of Earth and Ecosystem Sciences, Division of Geology, Lund University, Lund, Sweden)
Dry Dredgers meeting (Cincinnati, Ohio, USA)
16 November 2001
Polychaete annelid worms are a very abundant & successful animal group. They are diverse in feeding habit, morphology, preferences for depth & substrate, and geographically widespread. They have a scanty fossil record, due to their soft-bodied nature. They occur from the Middle Cambrian onward. Scolecodonts range from the earliest Ordovician (latest Cambrian?) to Recent.
Many modern marine polychaetes are colorful.
The oldest polychaete fossils are Middle Cambrian.
Types of polychaete fossils:
1) jaws (scolecodonts); 2) traces; 3) dwelling tubes; and 4) body imprints.
One animal could give rise to all these fossils.
The taxonomy of these different fossil categories has not been assimilated (different systematics for traces, tubes, imprints, scolecodonts).
scolex - worm
odous - tooth
Polychaete annelid jaw apparatuses consist of paired, commonly asymmetrical elements. Scolecodonts are acid-resistant microfossils that are commonly 0.1-2 mm long. The jaw apparatus disaggregates quickly - one finds isolated scolecodonts in the rocks.
Maxillae are numbered from back to front - I, II, III, IV, V. There are right-hand side & left-hand side elements. Carriers and basal plates are complementary elements to the maxillae.
There is a wide variety of jaw apparatus architecture and a wide variety in jaw form.
Multielement apparatuses. Not all polychaete annelids have multielement jaw apparatuses, though.
One modern polychaete annelid has 4 almost identical conical jaw elements.
Another modern form has 1 pair only of nearly identical jaw elements.
Some polychaete annelids lack jaws altogether.
Retrieving scolecodonts from rocks involves acid processing and kerosene/paint thinner disaggregation of rock - the 2 standard microfossil extraction methods.
Why the general lack of interest in scolecodonts? Confused taxonomy and their claimed uselessness in biostratigraphy, which is not justified and not supported by the data. Some scolecodonts are restricted to certain types of rocks (facies restricted) and some forms are long-ranging, though. Some, however, have short ranges and are widespread in terms of facies preferences.
Parataxonomy - the plague of scolecodontology. Rapid post-mortem disarticulation results in each element being systematically treated/described, traditionally. This violates naming rules and causes systematic chaos.
Research history of scolecodonts
1) Phase I - single-element taxonomy (1854-~1946): 1854-1856 - Pander, Massalongo & Eichwald; 1864 - Angelin suggests scolecodonts are polychaete jaws; 1879-1896 - Hinde has 4 important papers; 1933 - Croneis & Scott - coined the term “scolecodont”; 1933-1946 - Stauffer, Potter, Gries, Eller
2) Phase II - apparatus-based taxonomy (1947-~1978): 1947 - Lange; 1956-1969 - Polish school, with Kozlowski, Kielan-Jaworowska, Kozur, Taugourdeau, Sylvester, Schwab; 1970-1978 - Szaniawski, Wrona, Mierzeyweski, Zawidska. The phase II folks didn’t integrate the old single-element taxonomy into the new apparatus-based taxonomy.
3) Phase III - multi-element taxonomy (1979-2001): Bergman, Colbath, Eriksson, Hints
SEM as a standard tool has allowed for better comparisons of published finds (compared with potential artistic enhancement of line drawings in older literature).
Morphology and statistical methods are used in reconstructions while doing multi-element taxonomy.
Cincinnatian scolecodonts - 65 samples from 9 sections from the Upper Ordovician of the Cincinnati, Ohio area. Sample of ~100,000 scolecodonts (Edenian Stage to latest Richmondian Stage). This includes ~dozen articulated apparatuses. The Cincinnatian has abundant scolecodonts - up to a couple 1000 specimens per kilogram of rock. This study has involved sorting out the older confused taxonomy. There is noticeable asymmetry in apparatuses - corresponding elements on both sides don’t match in morphology/size.
Eight different families are common in the Cincinnatian Series. One family dominates - the Polychaetaspidae, which are sometimes >95% of the fauna, but are rare in the trilobites shales of the Waynesville Formation.
The diversity trend of scolecodonts in the Cincinnatian Series - diversity increases from the Kope Formation to the Richmondian Stage (a deep-water to shallow-water trend). So, can see a higher diversity of scolecodonts in shallow water deposits.
Cincinnatian associations - 5 tentative ones identified based on different frequencies of occurrences of different scolecodonts.
I - Kope Fm. association
II - Miamitown Shale association
III - Arnheim Fm.-Waynesville Fm. association
IV - trilobite shale association
V - Liberty Fm.-Whitewater Fm. association.
Most families and genera are shared between Laurentia to Baltica (in coeval deposits). Some rare forms in Laurentia are common in Baltica & some common forms in Laurentia are absent in Baltica.
The Cincinnatian has 40 to 50 species in 12 families, dominated by the Family Polychaetaspidae. None of these scolecodont families are still alive today. There are 5 faunal associations within the ~10 m.y. interval of the Cincinnatian (implying biostratigraphic utility). There is an apparent biotic response in the scolecodont fossil record at Holland's (1993) C3-C4 sequence boundary (= base of the Waynesville Formation).
Can see an increase in taxonomic diversity in shallow water deposits. Most families & genera have an intercontinental distribution. Faunal composition differs between Laurentia and Baltica (“endemic” species vs. non-endemic species).
There's a new genus & species in the Silurian of Gotland that has >5 mm long scolecodont elements, which is huge by scolecodont standards.
There's not much correlation between the size of scolecodonts and the size of the animal. There are some living decimeter-sized worms with ~1 cm sized elements.
Feeding ecology - need to be careful about interpreting feeding ecology based on fossils. These horrific-looking jaws suggests predation, but many modern forms with such jaws aren’t predators.
Why the bilateral body and yet asymmetry of jaw apparatuses? Don’t know.
Drop in diversity index - corresponds with the trilobite shale of the Waynesville Formation (abundance is high, but diversity is very low). Is this change taphonomic or due to the temporary deeper water setting?