Danita S. Brandt

Ecydsial efficiency and evolutionary efficacy among marine arthropods: implications for trilobite survivorship

A comparison of ecdysial patterns between trilobites and other macrobenthic marine arthropods (crabs, shrimp, lobsters, horseshoe crabs) reveals differences that may have evolutionary consequences. Limulus and many malacostracans apparently have a signature ecdysial style; conversely, a range of moult configurations characterized trilobite ecdysis, and this variation is evident even within individual species. A canalised ecdysial habit may be safer or metabolically more efficient and therefore, summed over the history of the class, evolutionarily advantageous. Some trilobite clades show evolutionary trends toward morphologies that would have facilitated ecdysis (e.g. reduction in the number of thoracic segments, reduction in the number and prominence of spines), but co-adaptation or multiple-use effects complicate the evolutionary signal of ecdysial selection. Survivorship analysis supports a possible link between ecdysial habit and evolutionary success: genera with fewer thoracic segments (= easier ecydsis) are longer-lived. The increased predation pressure on trilobites through the Palaeozoic would have amplified the evolutionary impact of an inefficient moult habit. The cumulative effects of a less-than-optimal ecdysial habit, and a physiology that apparently required reconstituting a calcitic exoskeleton de novo with each moult, are compelling biotic factors to consider in examining functional interpretations, life histories, evolutionary trends, and ultimate disposition of the Trilobita.

Differential preservation of brachiopod valves; taphonomic bias in Platystrophia ponderosa

Pedicle valves greatly outnumber brachial valves in disarticulated samples of the Upper Ordovician brachiopod Platystrophia ponderosa. Some articulated specimens are compressed (shortened), probably by burial and compaction. The shortening is accommodated by fracturing, which is largely confined to the brachial valve. This preferential fracturing of the brachial valve can account for the observed paucity of whole brachial valves of P. ponderosa in the fossil record, and suggests that mechanisms other than differential current transport contribute to valve disproportionation. Discriminating between autochthonous and transported assemblages on the basis of valve ratios may lead to erroneous paleoecological interpretations, because brachial valves could be destroyed preferentially, without significant transport to the burial site.

Multiple-Rusophycus (Arthropod Ichnofossil) Assemblages and Their Significance

Rusophycus is an ichnogenus comprising shallow burrows generally attributed to trilobites and other arthropods. The paleoethological interpretation of these structures is not conclusively known; workers variously have attributed the ichnogenus to feeding, resting, hiding/escape, hunting or nesting behaviors. Rusophycus morphology varies from unornamented, bilobed forms to forms that preserve details of ventral anatomy of the trace maker. Rusophycus occurs as single or multiple impressions. Some examples of multiple-Rusophycus assemblages are clearly the result of the activity of a single animal (e.g., serially arranged impressions of the same size), but others represent the activity of several individuals. Associated traces representing multiple individuals are especially interesting, for they may give evidence of complex behavior, for example, interactions between the trace makers. Some of the multiple-trace assemblages show alignment (congruent anterior-posterior orientation of the individuals) suggesting rheotactic behavior; other assemblages comprise randomly oriented traces. The difference between the aligned and randomly oriented assemblages may reflect differences in the current energy and amount of available food, and may also suggest the following feeding modes for these benthic-feeding arthropods: (1) alignment with head into the current in resource-poor environments, (2) orientation with head at an oblique angle to the current in high-energy, resource-poor environments, and (3) random orientation in low-energy, resource-rich environments.

Ecdysis in Flexicalymene meeki (Trilobita)

The ecdysial style of Flexicalymene meeki (Foerste) from the Upper Ordovician of the Cincinnati arch (Ohio) is best characterized by a spectrum of configurations. Recurring patterns of displaced tergites and points of disarticulation indicate differential susceptibility of the exoskeleton to disarticulation during ecdysis. Specimens identified as exuviae are characterized by one or more of the following: thoracic segments displaced (unevenly spaced, disarticulated or telescoped); pygidia and/or cephala rotated away from the sagittal line; cephalon sharply bent ventrally at first thoracic segment; librigenae separated or folded ventrally along facial sutures; “extra” tergites (exuvia) superposed on the new exoskeleton; and missing elements, especially librigenae and hypostomae. The range of exuvial configurations suggests a variety of ecdysial behavior in this trilobite.

Scorpion taphonomy: criteria for distinguishing fossil scorpion molts and carcasses

Abstract The ability to distinguish fossil arthropod carcasses from their molts is necessary for a more complete understanding of the arthropod fossil record and for more accurately assessing the role of fossil arthropods in paleoecosystems. Taphonomic characteristics, e.g., recurrent patterns of disarticulation of exoskeletal elements, are the primary data that have been used to differentiate fossil exuvia and fossil carcasses among arthropods. This study documents recurrent taphonomic patterns in modern scorpion carcasses and molts and extends these patterns to the fossil record to define criteria by which fossil scorpion molts might be distinguished from fossil scorpion carcasses. The three most useful and statistically significant characters in making the scorpion carcass/molt distinction are: position of the chelicerae (drawn in or extended); position of walking legs (folded or splayed); and body line (straight or curved). Two other characteristics, the position of pedipalps and presence or absence of telescoped segments, approach statistical significance and are also potentially useful. Disarticulation data are not as useful for distinguishing fossil scorpion molts and carcasses, because there are no statistically significant differences in length of time to total disarticulation or in the sequence of disarticulation between scorpion molts and carcasses. Among extant arthropods, scorpions possess the body plan most similar to that of the extinct eurypterids. Therefore, the taphonomic criteria developed for distinguishing fossil scorpion molts and carcasses may have implications for understanding molting among eurypterids.

A new ichnospecies of arthrophycus from the late carboniferous (Pennsylvanian) of Michigan, USA

A unique ichnofossil assemblage from Pennsylvanian-age sandstones near Eaton Rapids, Michigan, USA, comprises straight-to-curved traces preserved in convex hyporelief, with the transverse ridges and median grove associated with the ichnogenus Arthrophycus. The Michigan traces show some branching or pseudo-branching (also known from other Arthrophycus specimens) and are among the smallest structures (millimeters in diameter) attributed to this ichnogenus. The orientation of the Michigan Arthrophycus burrows is distinct from other ichnospecies of this taxon in the pronounced co-planar alignment of the burrows, as opposed to the multiplanar, interweaving, “bundled” nature typical of the ichnogenus. On this basis we assign the Michigan specimens to a new ichnospecies of Arthrophycus. The “paralleling behavior” of the new taxon may reflect a strategy of the tracemaker to avoid previous burrows and reflect differences in resource availability or current energy. This new taxon supports previous records of the occurrence of this ichnogenus in Upper Carboniferous strata.

A Curious Rusophycus (Arthropod Ichnofossil) Assemblage from the Upper Ordovician of Ontario, Canada

The ichnogenus Rusophycus includes a wide range of short bilobate excavations generally attributed to variable feeding behaviors of arthropods, especially trilobites. An unusual Rusophycus assemblage from Upper Ordovician Georgian Bay Formation in Ontario departs radically from previously described examples and presents new challenges for understanding the behavior represented by these traces. This specimen is unique in the arrangement of multiple Rusophycus burrows in a circular, lens-shaped array (as opposed to a linear or random arrangement typical of other Rusophycus assemblages). The size and shape of the individual Rusophycus components are consistent with traces attributed to the coeval trilobite Flexicalymene. Multiple Rusophycus assemblages likely reflect aggregations of trilobites in response to a local concentration of food. The topology of this particular Rusophycus assemblage suggests that the trilobites opportunistically exploited a rich and narrowly restricted food source, perhaps the decaying remains of a buried organism.

In Search of the Arthrophycus Parallelus Tracemaker

Abstract Arthrophycus is an iconic ichnogenus known from lower Paleozoic clastic strata worldwide, yet its origin remains controversial. A medial groove imparts a bilobed symmetry in some forms, implicating arthropods as the tracemaker. Other forms have regular annulae that evoke an annelid body plan. Transverse ridges in some bilobed forms of Arthrophycus, however, were deemed by some as too blunt to have been made by arthropods, and the annulation is unlikely to have anything but a superficial resemblance to “worms.” Recent work has converged on a nontrilobite, long-bodied arthropod as the likely Arthrophycus tracemaker. Given the breadth of morphological variation and range in size included in various ichnospecies of Arthrophycus, there are likely multiple tracemakers for this ichnogenus, and the tracemakers may belong to more than one phylum. Some ichnospecies of Arthrophycus may even represent the only physical record of an unknown or poorly fossilized group of organisms. Observations of the physiology and neoichnology of modern Arthrophycus tracemaker analogs support the conclusion that the maker of the Carboniferous ichnospecies Arthrophycus parallelus is neither an annelid nor a previously described arthropod but a yet unknown member of the Ecdysozoa.

Modern Analogs for the Study of Eurypterid Paleobiology

Eurypterids are extinct, chelicerate arthropods whose life habits might be elucidated through comparison with living analogs. There are at least two potential eurypterid analogs, xiphosurans and arachnids (specifically, scorpions). Eurypterids and scorpions share striking morphologic and structural similarities despite their different habitats (aquatic vs. terrestrial); eurypterids and xiphosurans share numerous morphological characters and an aquatic habit. Despite the physiological differences inherent between aquatic and terrestrial chelicerates, the similarities in the basic body plan suggest that eurypterids and scorpions faced similar functional challenges during ecdysis, but eurypterid feeding was probably more similar to that of xiphosurans. For studies on the mechanical strength and functional morphology of the eurypterid exoskeleton, Limulus is the closer analog. The choice of modern analog for other aspects of eurypterid paleobiology, including reproduction and whether eurypterids were active predators, is a matter of discussion. The lack of a single, clear eurypterid analog from among extant chelicerates may reflect that eurypterids occupied an ecological niche intermediate between xiphosurans and arachnids. The search for a modern analog for eurypterids, then, is not likely to yield a single model organism.

Defining the Ichnogenus Arthrophycus Using Numerical Taxonomic Methods

ABSTRACT A century and a half of imprecise or overly brief descriptions and ambiguous or unclear illustrations have rendered the ichnogenus Arthrophycus ripe for reexamination. An abundance of well-preserved, accessible Arthrophycus specimens from museum collections made it possible to use numerical taxonomic methods to distinguish among ichnospecies of Arthrophycus and refine the diagnosis of the ichnogenus using measurable, reproducible characters. We defined 16 characters for the numerical analyses. Cluster analysis sorts the ichnospecies of Arthrophycus into two groups: a cluster of five ichnospecies, comprising A. alleghaniensis, A. brongniartii, A. lateralis, A. minimus, and A. parallelus, which conform to a coherent concept of Arthrophycus, and a second cluster comprising 14 ichnotaxa questionably assigned to Arthrophycus. Principal coordinates analysis supports the separation of these two groups. The numerical analyses provide a rationale for taxonomic revision of the ichnogenus and the ichnospecies that have been assigned to it.