Subfossil mammalian tracks (Flandrian) in the Severn Estuary, S. W. Britain: mechanics of formation, preservation and distribution

Abstract

Mammalian tracks and trackways are widely preserved at all stratigraphical levels in the Flandrian sediments of tidal mudflat and marsh origins which formed over the last 8000–9000 years on the marginal wetlands of the inner Bristol Channel and Severn Estuary. The fauna recorded in this way, however, is less diverse than that known from the few, small assemblages of skeletal remains, including some from archaeological sites, so far assessed. Missing or rarely represented in terms of tracks are the smaller of the large mammals. Humans are represented by tracks throughout the Flandrian deposits. In the earlier Flandrian, they were accompanied by deer and aurochs which gave way, in the later Flandrian, to domesticated cattle and sheep/goat; there are sporadic indications of the presence of horse. Wolf/dog, represented by just two records, is the only smaller large mammal so far recorded as a track. The tracks were made, modified, and eventually preserved under a wide variety of sedimentological conditions on the margins of the estuary. Those environmental conditions constrained the quality of the anatomical evidence preserved in the tracks but can be inferred from the character of the tracks. Field experiments suggest that the moisture content of the sediment at the time was crucial to the general nature of the tracks. The mudflat–marsh silts which received the tracks varied from semi–liquid to firm, depending on tidal and seasonal factors and on the elevation of the sedimentary surface relative to the tidal frame. Some tracks were made in marsh peats which offered little resistance. Deep tracks preserving little detail were produced in weak sediments of high moisture content; tracks formed in strong, firm muds retained full anatomical detail. Referring to mechanical theory, and to a series of laboratory experiments using plasticine, the act of making a track is shown to be similar in character and effect to the indentation of an ideal elastic–plastic material by a punch. The punch represents the descending limb of the animal, the face of the punch the sole of the animal's foot, and the elastic–plastic material the sediment which is pierced and deformed by the downward action of the limb. The character of the experimental tracks, and the range and relative size of the deformation structures they include, is qualitatively similar to what is recorded from the field. Many of the tracks recorded from the field were variously modified in a changeable and dynamic environment before final burial and preservation.

The ‘Goldilocks’ effect: preservation bias in vertebrate track assemblages

“…As substrate is deformed by a load, it is pushed down and out from beneath the indenter (figure 8c). The base of the actively deforming zone of substrate undulates against the rigid, non-moving zone [13]. A cross section through this area results in a subsurface track containing a ridge of non-deformed substrate.…”

“…This has wide-ranging implications for interpretations of palaeodiversity and palaeoecology based on vertebrate track assemblages preserved in lithified muds and silts. If a track is exposed in a layer corresponding to that marked, tracks may appear to contain an internal ridge running across the widest part of the track ((c) modified from [13]). Presence of small and large tracks indented to the same depth on a single track-bearing surface (assuming time-averaging/transmitted tracks can be accounted for) offer the highest possibility of presenting a true representation of faunal diversity in the area at the time of track formation.…”

“…These considerations are not trivial if data on fossil track occurrences and abundances are to be used in 'higher level' [12] palaeobiological and palaeoecological inferences. Allen [13] noted over a decade ago that a widespread understanding of track formation lagged behind knowledge of anatomical aspects and distributions of fossil tracks, and despite a number of rigorous experimental studies in the intervening years [14][15][16][17][18][19][20], this still remains the case.…”

“…Eight-node elements lack the numerical flexibility to deal with such a gradient, and so by increasing the number of nodes defining the element, a more accurate solution can be found. The volume of substrate modelled was equal to four times the foot length in all dimensions in order to avoid boundary effects [13].…”

The birth of a dinosaur footprint: Subsurface 3D motion reconstruction and discrete element simulation reveal track ontogeny

“…Experimental tracks have been created predominantly in horizontal layers of colored material that were later sectioned or split apart (15)(16)(17)(18)(19). Such destructive methods lack a temporal component, precluding direct association of individual track features with specific anatomical structures and explicit events in the step cycle.…”