Trilobite Lagerstatten

Lagerstatten (derived from the German mining term translated loosely as "mother lodes") are extraordinary fossil assemblages. Trilobite Lagerstatten include obrution deposits, reflecting a rapid smothering of benthic faunas by sediment, yielding fully articulated remains and Konservat-Lagerstatten, in which even soft parts are preserved by a combination of rapid burial, anaerobic decay, and early diagenetic mineralization (Seilacher et al. 1985). In this section we describe examples of trilobite Lagerstatten from New York State to illustrate the general taphonomic concepts.

A famous trilobite site is Beecher's Trilobite Bed in the Upper Ordovician Frankfort Formation just north of Rome, New York. A 5-mm-thick, light-gray layer within a dark-gray mudstone contains an unusual collection of fossil material (Cisne 1973). The Triarthrus eatoni specimens within this layer have their appendages and some internal organs replaced by pyrite. Beecher (1893, 1894, 1895) reported the discovery by Valiant and the preparation of specimens. Cisne (1975, 1981) prepared very-high-resolution radiographs of Beecher's specimens and was able to report on internal structures never seen before. Whittington and Almond (1987) also examined specimens from the beds and suggested that certain structural elements of the appendages were utilized in food transport, among other things. Replacement of organic tissue in such high resolution by pyrite is very unusual. Briggs, Bottrell, and Raiswell (1991) examined these trilobites and concluded that this type of soft tissue replacement was due to bacterial decay in anoxic conditions resulting in sulfide formation. The rocks represent deep-water turbidites, and anything buried probably would have been subject to anoxic conditions.

The concentration of organic matter in the sediment was relatively low, and the concentration of iron in the pore-water relatively high. In this situation the sulfide produced at local decay sites was precipitated as an iron monosulfide (a precursor to pyrite) at the site where it was formed, resulting in the nearly perfect replacement of soft tissue by pyrite (Briggs and Edgecombe 1993). Beecher's Trilobite Bed remains the most productive source of preserved trilobite soft tissue known.

Trilobites found on limestone bedding planes typically are compressed for the same reasons that trilobites are compressed on shale partings. Their history is similar in that they were buried by a calcareous mudflow with little transport. Some of the best trilobites, however, from the physical preservation standpoint, are found within limestones. In some instances, these trilobites were apparently caught up in a calcareous sediment flow, killed, probably transported some distance, and entombed in the settled sediment. In such limestones the trilobites typically are randomly oriented, with the bodies flexed in unusual postures, and retain much of their original three-dimensional character. Large numbers of IsoteJus gigas have been taken from the limestones of the Middle Ordovician Trenton Group in central New York. The specimens from bedding planes are flattened and commonly upside down, while those from within the limestone retain their three-dimensional character.

In the Walcott-Rust Quarry within the Trenton Group, several layers of the thinly bedded limestone have yielded excellent trilo-bites (Brett et al. 1997,1999). One thin micritic limestone yielded large numbers of articulated C. pleurexanthemus, FJexicaJymene senaria, and MeadowtowncJJa trentonensis (but no I. gigas). More than 9 8% of the trilobites on the base and those within the layer were upside down while 60% of those on the upper surface, including many that were still partially within the limestone, were right side up. The thinner bed probably records a large population of trilobites caught in a current and transported far enough to be mixed within the small amount of sediment involved. Most of the trilobites were killed in the process. The larger more robust ones may have managed to struggle to the surface and die there or were covered and killed by a subsequent event. The observation that no /. gigas were involved suggests that they were large and strong enough to escape. Two other, thicker beds had no trilobites on the base or top but well-preserved specimens internal to the limestone. These internal trilobites, which included /. gigas, were randomly oriented.

Within the thin bed, coiled and semicoiled C. pleurexanthemus and F. senaria were discovered by C. D. Walcott (who also discovered the Burgess Shale), and there was evidence of appendages preserved as calcite infillings (Walcott 1876, 1877b). This unique mode of preservation is believed to result from anoxic bacterially induced calcite precipitation within the appendage, followed by calcite in-filling as the appendage material decayed (Brett et al. 1997, 1999). The Ceraurus bed is a thin, 15 to 50-mm bed that normally would be expected to be heavily bioturbated. A possible explanation is that the subsequent thicker beds were deposited a short time after this one, essentially sealing it and making any decay anaerobic. The low level of iron in the sediment precluded significant pyrite formation.

This form of appendage preservation is unique and was only found when Walcott made sections of the trilobites and observed them by transmitted light. The very fine calcite in-filling in a carbonate matrix is very difficult to see and evaluate by reflected light. This difficulty raises the question as to whether this mode of preservation is actually more common but generally unobserved.

The Lower Silurian Rochester Shale contains several horizons yielding numerous trilobites. Specimens of the large lichid genus Arctinurus, when articulated, are usually found right side up but flattened. DaJmanites JimuJurus, from several layers, is almost always inverted and often occurs in narrow, elongate "windrows" that may show some evidence of a preferred orientation. The living Arctinurus animals probably were buried in place by a heavy blanket of sediment, which resulted in their death. The D. JimuJurus, however, were transported somewhat before the burial process, aggregated, and current aligned in windrows. The fact that most are upside down is less easily explained. Convex surface down may be a preferred orientation during transport in a current, or the living animal may have been tumbled by the current and the upside-down trilobites may not have been able to right themselves. Again, alignment probably indicates slight transport of dead individuals by a current. Alternatively, it is possible that these trilobites swam upside down to escape the sediment and were buried in this position. A possible model for this latter behavior comes from the living horseshoe crab, LimuJus, which normally crawls around the bottom but more rarely swims upside down.

The siltstones, mudstones, shales, and limestones of the Middle Devonian Hamilton Group have yielded enormous numbers of articulated specimens of trilobites. One well-studied bed, the Browns Creek Bed, a lime mudstone in the lower Cen-terfield Limestone, yields superbly preserved EJdredgeops rana, MonodccheneJJa macrocephaJa, and PseudodecheneJJa rowi. This bed can be traced from Centerfield, Ontario County, to East Bethany, Genesee County, a distance of over 49km (31 miles). The trilobites are found in random orientation, indicating they were swept up, tumbled, and transported some distance from their original position in the sudden event forming this bed.

Horizons within the lower Wanakah and the Windom members of the Hamilton Group are especially productive of E. rana. Especially notable are the "Grabau Trilobite Beds" (lower Wanakah Shale) along the Lake Erie shore near Eighteenmile Creek and the Smoke Creek Beds in the former Penn-Dixie Quarry at Hamburg, Erie County. These trilobite beds yield three-dimensional clusters of E. rana, which Speyer and Brett (1985) divided into molt and body clusters. The clusters are nearly intact, which suggests little transport. These beds are the result of very rapid burial and death, with very little turbulence during the burial event.

The E. rana specimens from the lower Wanakah Murder Creek Beds are found tightly coiled, fully outstretched, and in various semicoiled configurations in between these two extremes. Typically, in the semicoiled specimens, the pygidium appears to be missing. On close examination, however, the pygidium can always be found inside the body cavity of the trilobite. It is suggested that the pygidium was displaced when the tightly coiled trilobite, upon decay, partially opened and viscous mud filled the now-empty cavity.

The trilobites in the former Penn-Dixie Quarry are mostly in the Smoke Creek Beds of the Windom Member named for their outcrop on Smoke Creek, Erie County.1 Well-preserved individual trilobites are common in these beds, and occasional clusters of 5 to 20 individuals are known. The general preservation is similar to that of the Murder Creek Beds.

In summary, taphonomic analysis is a major factor in understanding trilobite deposits, as well as fossil deposits in general. The understanding that comes from such analyses not only is intellectually rewarding but also provides a foundation for interpreting the geological history of fossil sites. There is also a predictive factor that cannot be ignored. For example, storm deposits over stable sea bottoms, below storm wave-base, often bury a living fauna, resulting in well-preserved articulated fossils such as trilobites and crinoids. Fossil preservation provides important clues in the interpretation of fossil deposits and ancient environments. Taphonomy and its concepts are powerful tools for general fossil collectors as well as professional paleontologists.

1 The Penn-Dixie Quarry is a public fossil site operated by the Hamburg Natural History Society (P.O. Box 772, Hamburg, NY 14075; 716-627-4560).

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    What did fossil lagerstatten reveal about trilobites?
    6 months ago

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