Fossils of the Van Buren Formation
and the Cambrian – Ordovician Boundary of Missouri
Bruce L. Stinchcomb

Boundaries of some portions of the geologic time scale such as the K/T boundary (where the fossil record changes abruptly) are definitive and thus generally clear as to exactly where the boundary is. At the K/T boundary major groups of organisms disappear (ammonites, coiled oysters, dinosaurs, belemnites etc.). Other major stratigraphic boundaries such as that of the Silurian-Devonian are not so clear and distinct--such is also the case with strata near the Cambrian-Ordovician boundary and its fossils

The Cambrian-Ordovician boundary in the Missouri Ozarks has traditionally been placed at the top of the Eminence Formation, a cherty dolomite where the chert sometimes carries what is distinctly an Upper Cambrian fauna including (most) of its trilobites. When detailed geologic work in the Ozarks after the Great War (WWI) began partially as a consequence of the availability of the motor car (Model T Ford), fossils were found in cherty strata similar to that of the Eminence Formation--but with some distinctly “new” additions--the most intriguing of which were small, peculiar cephalopods. It was realized that this sequence of Cambrian rocks in the Ozarks had some of the world’s oldest cephalopods and that specimens found in this strata, especially in an area around Potosi Missouri were the most distinct and abundant. (What were considered as cephalopods at the time, were also found in older, underlying, strata however these were found later to be monoplacophorans and not cephalopods). The zone or horizon which yielded these fossils was first referred to as the “Upper Eminence” due to the similar appearance of its cherts with those of the underlying Eminence Formation. Geologic work in the Current River region (now part of the Ozark National Scenic waterways), found the same horizon to occur around the Ozark town of Van Buren located on the Current River and the horizon was named the Van Buren Formation after that town. At this same time the Cambrian-Ordovician boundary was also placed at the boundary between the Eminence and Van Buren Formations. Earlier work in other parts of the Ozarks had placed this boundary near the top of the overlying Gasconade Formation from which strata delineating the Van Buren Formation had been derived. The validity of the Van Buren Formation being a distinct and recognizable formation (as is the case with most Phanerozoic strata), relied on its fossil fauna which included the before mentioned cephalopods.

Fossils of the Van Buren Formation represent a dilemma as to their belonging to either the Cambrian or the Ordovician period. There are the cephalopods, a faunal element which distinctly aligns the Van Buren with the Ordovician. Trilobites, which are usually an uncommon element in its fauna, are also considered to be Ordovician genera (trilobites are the fossils which many biostratigraphers “officially” use to determine where lower Paleozoic strata belong in the geologic time scale”. In addition to these, the author has found an early type of coral in the Van Buren and corals are also an Ordovician (and younger) organism. On the other hand mollusks other than cephalopods of the Van Buren have decidedly Cambrian aspects. Its gastropods are similar to those of the Eminence Formation as are its multiplated mollusks, the later, a problematic group which had its greatest diversity and abundance in strata just below and above the Van Buren Formation.

The Van Buren Formation “officially” came to an end when some members of the Missouri Geological Survey, in the late 1950’s decided that the formation was no longer a valid one and its strata was again relegated to the Gasconade Formation. The reason for this “demotion” being that Van Buren strata could not be recognized in water well cuttings derived from subsurface geology. The fact remained however, that its surface outcrops (especially cherts), yielded fossils which were distinctly different from both the underlying Eminence Formation and the overlying Gasconade. Regarding where the Cambrian-Ordovician boundary should be placed, the late Jim Stitt of the University of Missouri-Columbia, on the basis of trilobites, drew the boundary near the very top of the Eminence Formation. He found that some of the trilobites found at the very top of the Eminence had Ordovician affinities. More recently James W. Hagadorn formerly of Amherst College and now with the Denver Museum of Natural History, on the basis of fossil track ways found in the Gunter Sandstone (A sandstone which occurs at the base of the Van Buren, and which he considers because of its trace fossils to be Cambrian in age), draws the Cambrian-Ordovician boundary above this sandstone. Track ways in the Gunter include Climactichnites (the Cambrian motorcycle track way) and Protichnites, possibly an aglaspid track way--both of which are diagnostic Cambrian trace fossils. Considering this, the Van Buren Formation is very close to being Cambrian in age.

In contrast to these fossils however, an entirely new element first appears in the Van Buren---the cephalopod. Primitive cephalopods of the order Ellesmeroidea first appear in the Van Buren and cephalopods are a decidedly new element--a mollusk characteristic of the Ordovician Period but not of the Cambrian. Cephalopods also represent a direction in evolution leading toward a sophisticated nervous system (The octopus, a cephalopod, being a fairly intelligent animal*). These early and undoubted cephalopods of the Van Buren are, the author believes, to be the oldest (undoubted) cephalopods known. Cephalopods (or cephalopod-like fossils) have been found in the Llano Uplift of the Texas Hill country but these, like others in Texas and New Mexico may be the same age as the Van Buren or they may not be cephalopods. Much heralded Cambrian ellesmeroid cephalopods are also reported to occur on El Paso Mountain (Franklin Mts.) near El Paso Texas, however specimens seen by the author, look just like those of the Van Buren Formation of Missouri. The Cambrian age for the cephalopod bearing strata of El Paso Mountain is shaky--the zone yielding them most likely correlates with the Van Buren Formation and possibly represents the same faunal zone as found in Missouri as both regions are in the Cambrian Laurentian faunal province of North America.

The other earliest cephalopod occurrence is in China! Besides problems with intercontinental correlation and the fact that Cambrian fossils of China represent a different faunal province, there is question as to these small, cone shaped fossils really being cephalopods. It may well be that Missouri’s Van Buren cephalopods are the worlds earliest and first. It’s also noteworthy that these interesting fossils occur in what generally is considered to be the earliest Ordovician strata and that they are abundant--especially in some areas of Washington County Mo., some 65 miles south of St. Louis. Also noteworthy is the fact that they occur associated with stromatolites--a mollusk-stromatolite ecosystem which was a dominant one in the Cambrian and essentially disappears after the Lower Ordovician.

*Proponents of exobiology and supporters of the appearance of intelligence in “other worlds” take note--cephalopods represent an entirely separate (from the vertebrates) evolutionary pathway leading to a sophisticated nervous system and consequent intelligence. They are an ancient group of invertebrates. Their occurrence in the Van Buren is probably the world’s best and earliest occurrence of cephalopods which presumably evolved from monoplacophorans, a molluscan group also well represented in Cambrian strata of the Ozarks.

 Click on pictures to Magnify

Fig1

Group of ellesmeroid cephalopods Dakeoceras retrorsum from the Van Buren Formation at Sand Springs, southeast of Potosi, Washington Co., Mo.

Fig2

Group of the gastropods Sinuopea vera and a cephalopod (Dakeoceras sp.) in the middle all associated with a “core” which formed between digitate stromatolites.

 Fig3

Group of gastropods Sinuopea vera. These are some of the earliest undoubted gastropods (snails) in the fossil record. The Van Buren Formation which yields them is considered early-most Ordovician in age--older gastropods found in the underlying Eminence Formation of the Missouri Ozarks are the oldest of undoubted gastropods. Sand Springs locality SE of Potosi Mo.

Fig4

Ectenoceras pergracile. Elongate ellesmeroids (brevicones) from near the base of the Gasconade Formation (Euomphalopsis zone). These early cephalopods occur associated with large numbers of the gastropod Euomphalopsis where these mollusks are associated with siliceous sediments formed between stromatolite reefs. Gasconade Formation, 11 miles WSW of Potosi Mo.

 

Fig5

A monoplacophoran feeding on a stromatolite. Early mollusks like those shown here are generally associated with stromatolites upon which they probably fed. Ellesmeroid cephalopods are also believed to have evolved from monoplacophorans (monoplacs) as an almost continuous series of fossil forms occur between the two groups---often it is difficult to determine if a fossil is a monoplac or is a cephalopod. As a consequence of this, a lot of confusion exists in the literature of these two groups especially those near the Cambrian-Ordovician boundary. Artwork by Guy Darrough. The early Ordovician (lower Ordovician) was still a time of dominance of stromatolites in the fossil record. This stromatolite dominated ecosystem, a paleoecological throwback to deep into the Precambrian, became uncommon after the lower Ordovician.

Fig6

Reconstruction of two early living ellesmeroid cephalopods. Whether these animals had an eye is conjectural---however modern cephalopods have a highly sophisticated eye similar to that of the vertebrates. Artwork by Virginia M. Stinchcomb.

Fig7

Gastropods (Sinuopea) and small ellesmeroid cephalopod Burenoceras sp. (top) Van Buren Formation, Sand Springs locality SE of Potosi, Mo.

Fig8

Climactichnites sp. The “motorcycle track” fossil track way from the Gunter Member of the Van Buren Formation near Williamsville, Mo. Climactichnites is a puzzling and distinctively Cambrian fossil. It occurs in sandstone beds deposited in intertidal environments where it may have fed on moneran mats (cyanobacteria) which formed on the sand surfaces which were preserved as sandstone beds and which can exhibit “old elephant skin”, a pattern formed from these mats of cyanobacteria.

Fig9

Group of early mollusks:

1. A kirengellid monoplacophoran of the genus Kirengella sp. These are one of the elements of the Van Buren Formation which has Cambrian affinities. It is also a fossil which might suggest that the Van Buren Formation should be considered Cambrian in age. Note the ring of multiple muscle scars on this fossil---this is one of the distinctions which separate monoplacs from gastropods
.
2. A hypseloconid monoplacophoran. These elongate monoplacs are very close morphologically to primitive ellesmeroid cephalopods.

3. A Shelbyocerid monoplac. These fossil mollusks, like hypseloconids, are elongate however unlike hypseloconids they have an end (apex) which contains chambers like a cephalopod but with no siphuncle. In earlier literature they were confused with and often considered to be cephalopods---they are most characteristic of the latest Cambrian.

4. An early gastropod (Sinuopea). Gastropods are more typical of the Ordovician than they are of the Cambrian. Gastropod-like fossils occur throughout Cambrian rocks but all of these except for those of the very latest Cambrian are questioned as to their being actual gastropods---many paleontologists considering them to be representatives of extinct molluscan classes.

5. Dakeoceras retrorsum. This is a nice specimen (internal mold) of this early cephalopod showing the phragmocone (chambered region of the shell) and the living chamber (bottom). Notice the similarity of this to the monoplacs. Cephalopods are an element of the Van Buren Formation which give it a very Ordovician “flavor”. From the Van Buren Formation south of Potosi, Mo.

Fig10

Paleontological zones in the Lower Ordovician Van Buren and Gasconade formations of the Missouri Ozarks.

Selected references

Bridge, J., (1930) Geology of the Eminence and Cardereva Quadrangles. Missouri Bureau of Geology and Mines, Vol. 24. (A definitive work on Ozark Cambrian and Lower Ordovician fossils.)

Flower, Rousseau H., 1964. The Nautiloid Order Ellesmeroceratida (Cephalopoda). Memoir 12, New Mexico Bureau of Mines and Mineral Resources. (A definitive work on these early cephalopods--with respect to the Cambrian-Ordovician boundary see pages 150, 151.)

Getty, Patrick R. and James Whitey Hagadorn, 2008. Reinterpretation of Climactichnites Logan 1860 to include subsurface burrows, and erection of Musculopodus for resting traces of the trailmaker. Journal of Paleontology, 82:1161-1172.

Ulrich, E. O. and Foerste A. F. and Miller A. K., 1943. Ozarkian and Canadian Cephalopods. Pt II Brevicones. Geological Society of America, Special Papers, No. 49. (Another definitive work on these early cephalopods.)

Fossils of the Van Buren Formation and the Cambrian – Ordovician Boundary of Missouri
Bruce L. Stinchcomb
MAPS Digest, April 2010; Vol. 33, No. 1


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