Send in the nautiloids: Early Ordovician fossils of the National Park Service

Previously we looked at the National Park Service records of the late Cambrian and Late Ordovician. What comes between the late Cambrian and Late Ordovician? That's right, the Early Ordovician. (The Middle Ordovician also does, but at the moment I've got Early on my mind. You know, in North America it might actually be more practical to divide the period into Early and Late, based on the lowstand that separates the Sauk Sequence from the Tippecanoe Sequence; or, maybe not. Who knows? You start to think about things like that when contemplating the divisions of the geologic time scale; where would the lines have been placed if it had been initially developed somewhere other than northwestern Europe?)

The Early Ordovician is kind of a transitional episode; in some ways, it's like a tag to the end of the Cambrian, considering it's the wind-down of the Sauk Sequence. I get the feeling that this epoch was not necessarily the most pleasant 15 or so million years in North America, at least in some areas. For example, in the Upper Midwest the Early Ordovician is represented by our old friend the Prairie du Chien Group. The PdC was certainly not bereft of life, but a whole lot of that life was stromatolite-building cyanobacteria, and a general rule of thumb is that any non-Precambrian geologic unit that features intervals of wall-to-wall stromatolites was probably not that hospitable while being deposited. Otherwise, the groups most commonly reported from Lower Ordovician rocks in NPS areas are brachiopods, nautiloids, gastropods, trilobites, graptolites, and conodonts. Apart from the nautiloids, you might think you were back in the late Cambrian. The evolutionary flowering that is evident in Middle and especially Upper Ordovician rocks was just kind of simmering: mollusks and echinoderms were chugging (although a lot of those early echinoderms didn't catch on), but corals and bryozoans were in low gear.

By my count, as many as 15 NPS units and affiliated areas have fossiliferous Lower Ordovician rocks, although the records at several of these units are not known to be particularly impressive at this time. Four of the parks are represented by the good old PdC, with fossils at Effigy Mounds NM, MNRRA, Saint Croix National Scenic Riverway, and potentially one of the units of NPS-affiliated Ice Age National Scientific Reserve. The distribution is a decent match for Early Ordovician finds in the United States as a whole (compare to the Early Ordovician records in the Paleobiology Database, for example). The major holes are New Mexico/Oklahoma/Texas, the Hudson River Valley, and Utah; none of Utah's NPS units includes a classic Ibexian sequence, but Great Basin NP next door in Nevada is Ibexian-adjacent and no slouch itself. It hosts one of the more interesting Early Ordovician fossil records in the NPS, along with Chesapeake and Ohio Canal National Historical Park, Death Valley NP, and Pictured Rocks National Lakeshore.

As usual, click to embiggen. Our Early Ordovician parks are 1) Death Valley NP; 2) Great Basin NP; 3) Kobuk Valley NP; 4) Denali NP & Preserve; 5) Yukon-Charley Rivers NPres; 6) Wind Cave NP; 7) Mississippi National River and Recreation Area; 8) Saint Croix National Scenic Riverway; 9) Picture Rocks National Lakeshore; 10) Ice Age National Scientific Reserve (affiliated area); 11) Effigy Mounds NM; 12) Ozark National Scenic River; 13) Buffalo National Scenic River; 14) Great Smoky Mountains NP; 15) Chesapeake and Ohio Canal National Historical Park.

Chesapeake and Ohio Canal NHP preserves good examples of the standard Early Ordovician assemblage in the Stonehenge Limestone and overlying Rockdale Run Formation. The Early Ordovician of both Death Valley NP and Great Basin NP is represented by rocks of the Pogonip Group, although different formations are present at the two parks. (When considering Paleozoic paleontology, if Death Valley or Yukon-Charley Rivers don't have something, it might not be present in the NPS. In this case Death Valley has the better record.) Great Basin is distinguished for examples of some of the wacky echinoderms that proliferated during the Ordovician. Pictured Rocks National Lakeshore is noted for the oldest NPS vertebrate material (I reserve the right to be cagy about the exact placement of conodonts), consisting of armor attributed to Anatolepis (Miller et al. 2006; a pteraspidomorph, or ostracoderm in more old-timey lingo). Pictured Rocks, incidentally, has an interesting unresolved stratigraphic issue: the above specimens came from the Au Train Formation and were associated with other microfossils of Early Ordovician age (Miller et al. 2006). However, a dissertation (Oetking 1952) documented Au Train macrofossils from the lakeshore area that appear to be correlative to Platteville fossils (i.e., Late Ordovician). This all suggests to me that either the identifications of Oetking's fossils are overly generous, or that the rocks identified as the Au Train Formation are more complex than currently suspected (a cryptic unconformity, etc.).

References

Miller, J. F., R. L. Ethington, and R. Rosé. 2006. Stratigraphic implications of Lower Ordovician conodonts from the Munising and Au Train Formations at Pictured Rocks National Lakeshore, Upper Peninsula of Michigan. Palaios 21:227–237.

Oetking, P. F. 1952. The relation of the Lower Paleozoic to the older rocks in the northern peninsula of Michigan. Dissertation. University of Wisconsin, Madison, Wisconsin.