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?
