Morphology and paleoenvironmental implications of

adhesive meniscate burrows (AMB), Paleogene Willwood Formation and other continental deposits

Introduction

Adhesive meniscate burrows (AMB) were first reported from alluvial paleosols in the Paleogene Willwood Formation (Bown and Kraus, 1983), though similar burrows are recognized in continental deposits of the Permian Council Grove Group (Counts and Hasiotis, 2009),  Triassic Chinle Formation (Hasiotis and Dubiel, 1994), Jurassic Morrison Formation (Hasiotis and Demko, 1994), and Cretaceous Tuscaloosa Formation (Savrda et al., 2000).

Systematic ichnotaxonomy of such burrows has complicated by a perceived lack of distinct morphologic characters.  In addition, backfilled burrows have historically been interpreted as fodinichnia produced by deposit-feeding organisms mostly under subaqueous or saturated-soil conditions.  Close examination of hundreds of new specimens from the Willwood Formation and observations of modern backfilling soil biota, however, indicate that there are morphologic features that distinguish AMB from other backfilled ichnogenera and that these same features are produced by extant burrowing organisms, primarily insects, in predominantly well-drained soils or in wet soils during episodes of better drainage.

Straight to sinuous, unlined, non-branching burrows backfilled showing a nested series of ellipsoidal packets, such that each packet appears to overlap on one side.  The packets are composed of very thin and tightly spaced meniscate laminae.  Burrows are adhesive (hence the name), meaning they do not weather differentially from the surrounding matrix and cannot be easily removed. Most are natural cross-sections and burrow surfaces are extremely rare.  Packets and menisci are commonly accentuated by alternating zones of oxidized and unoxidized Fe compounds.

AMBs occur in groups of tens to thousands of individuals in Willwood paleosols.  They are most abundant in strongly developed paleosols with rhizoliths, often to the exclusion of other trace fossils, and less common in deposits showing only insipient paleopedogenesis.  Well-developed paleosols are characterized by red, yellow-brown, purple, and gray mottles, abundant carbonate rhizoliths and pedogenic nodules,  Fe-oxide nodules, and clay slickensides (Kraus, 1997). Modern soils with these features experience saturated conditions for several months of the year followed by longer periods of better drainage and low-water tables (Torrent et al., 1980).  Weakly developed paleosols are characterized by gray to green-gray matrix colors, diffuse yellow-brown or purple mottles and contain fewer rhizoliths, burrows, and nodules than well-developed paleosols (Kraus, 1996).  These features suggest poor drainage conditions and high rates of sedimentation.

Ethology is the study of animal behavior.  Trace fossils records of animal (and sometimes plant) behavior preserved in a media, lithified soil in the case of AMB.  The following drawings (modified from Smith et al. 2008) illustrate the behaviors interpreted for the different morphologies that comprise AMB based on the observations of many fossil specimens and of traces produced by extant burrowing taxa. 

Compound trace fossils composed of packets, interpreted as remnants of air-filled cells excavated and inhabited by the tracemaker for a short time period (domichnion), and meniscate laminae interpreted as backfilled sediment during locomotion (repichnion). Short burrow sections containing menisci unbound by packets represent locomotion as the primary behavior.

Right: Photographs of two experiments using cicada nymphs.  Nymphs were allowed to burrow in containers filled with colored sand to help accentuate the packet and laminae produced during excavation and forward locomotion.  See Smith and Hasiotis (2008) for more examples and a thorough explanation of burrowing behaviors responsible.

One or more soil-dwelling insect taxa  likely constructed AMB, by excavating and occupying a moving cell that is backfilled as they burrow through the soil.  Possible tracemakers include burrower bugs (Hemiptera: Cydnidae) and cicada nymphs (Hemiptera: Cicadidae), and less likely adults and larvae of burrowing ground beetles (Coleoptera: Carabidae) and scarab beetles (Coleoptera: Scarabaeidae).