Logan A. Wiest

THE WACO MAMMOTH NATIONAL MONUMENT MAY REPRESENT A DIMINISHED WATERING-HOLE SCENARIO BASED ON PRELIMINARY EVIDENCE OF POST-MORTEM SCAVENGING

Abstract: The Waco Mammoth National Monument (WMNM) potentially represents the only recovered Columbian mammoth (Mammuthus columbi) herd to date, but the “herd” interpretation is dependent on the demographics of the accumulation as well as a catastrophic kill mechanism. The demographics are consistent with an extant elephant herd that is lacking only infants, and the generally accepted cause of death is a catastrophic flood and rapid burial based primarily on fossil articulation and associations with an ancient river channel and aquatic fauna. Herein we present new ichnological evidence of post-mortem biogenic bone modification contrary to a flood scenario with rapid burial. Traces on bone include branching furrows (Corrosichnia type), paired grooves (Machichnus regularis and M. bohemicus), arcuate grooves that penetrate the cortical material (Brutalichnus brutalis), roughly triangular punctures with jagged margins (Nihilichnus nihilicus), and hemispherical borings (Cubiculum isp.). The branching furrows are interpreted as root-dissolution features, whereas the remaining suite of traces demonstrate scavenging of the mammoths by rodents, carnivores, and hide beetles during a period of dry-decay and prolonged subaerial exposure. We propose that a drought scenario is a more plausible kill mechanism for this particular assemblage because: (1) a diminishing watering hole concentrating the local fauna explains the high taxonomic diversity; (2) migration to a distant water source explains the absence of M. columbi calves; and (3) a drought provides a parsimonious explanation for the site history in light of new observations regarding vertebrate and invertebrate scavenging. Under this scenario, the mammoths of WMNM represent at least one social group that perished during an anomalously dry season.

ICHNOLOGICAL EVIDENCE FOR ENDOBENTHIC RESPONSE TO THE K–PG EVENT, NEW JERSEY, U.S.A.

Abstract Degree of bioturbation, Thalassinoides isp. morphology, and diameters were compared across the Cretaceous–Paleogene (K–Pg) boundary interval at three localities along the New Jersey coastal plain. Within this regionally extensive ichnoassemblage, mean burrow diameters decrease abruptly by 26–29% (n = 1767) at the base of the Main Fossiliferous Layer (MFL) or laterally equivalent horizons. The base of the MFL has been previously interpreted as the K–Pg boundary based on the last occurrence of Cretaceous marine reptiles, birds, and ammonites, as well as iridium anomalies and associated shocked quartz. Along with the mean, the maximum and minimum burrow diameters exhibit a negative shift, which indicates that the changes are the result of a directional reduction in diameter, rather than an artifact of decreased variance. As a proxy for the size of the tracemaker, a change in burrow diameter indicates a decrease in thalassinid crustacean body size. We interpret this shift as dwarfing within the endobenthic community as detrital food sources became scarce following the mass extinction. Despite the difference in size, there is no change in framework geometry. Ichnofabric indices generally increase up-section at each site across the K–Pg chronostratigraphic boundary, indicating a regional reduction in sedimentation rate, which is supported by a gradual increase in glauconite maturity. Overall, the ichnological evidence at these localities suggests that a prolonged period of negative feedback followed a short-term positive endobenthic response across the K–Pg boundary.

Contesting early archaeology in California

The peopling of the Americas is a topic of ongoing scientific interest and rigorous debate1,2. Holen et al.3 add to these discussions with their recent report of a 130,000-year-old archaeological site in southern California, USA: the Cerutti Mastodon (CM) site, which includes the fragmentary remains of a single mastodon (Mammut americanum), spatially associated stone cobbles, and associated lithic debris that they claim indicates prehistoric hominin activity. In sharp contrast, we contend that the CM record is more parsimoniously explained as the result of common geological and taphonomic processes, and does not indicate prehistoric hominin involvement. Whereas further investigations may yet identify unambiguous evidence of hominins in California around 130,000 years ago, we urge caution in interpreting the current record. There is a Reply to this Comment by Holen, S. R. et al. Nature 554, http://dx.doi.org/10.1038/nature25166 (2018). Holen et al.3 claim prehistoric hominin involvement at the CM site based primarily on four lines of evidence: a reliable radiometric age; the presence of stone artefacts; clear evidence of tool-imparted percussion damage to the remains of the mastodon; and an undisturbed geological context. We take no issue with the published age for the site, but we believe that the latter three claims warrant further examination. The CM site stone artefacts are an assortment of cobbles and fractured cobble fragments excavated from a sandy silt matrix. There is no evidence of formal stone tool forms or diagnostic lithic microor macro-debitage. Instead, the CM site artefacts are identified by their proximity to the remains of the mastodon and their large size relative to the enveloping sediment. Additionally, surface features such as the presence of pitting and scratching on cobble surfaces, the presence of several cobble fragments with fracture morphologies reminiscent of hammerstone and/or anvil usage, and the presence of several refitting cobble fragments are interpreted as evidence of hominin percussive activities on-site. The lack of discarded formal tools and diagnostic lithic debitage is noteworthy, and is unusual relative to most archaeological assemblages that purport hominin processing of proboscidean remains (although see Haynes4). We also note that upslope of the site there are numerous alluvial fans, with clastic material a common occurrence. The cobbles and pebbles at the CM site can be derived from modest alluvial fan input with fines subsequently winnowed with lower energy fluvial erosion. Crucially, none of the criteria that Holen et al.3 use to define stone artefacts either requires prehistoric hominin involvement or meets the accepted criteria for falsifying natural ‘geofacts’5. The range of possible geological interpretations for the lithic assemblage highlights the critical issue of equifinality, in which an end product such as a shattered cobble may be generated via many and potentially unrelated means. It is a principle that applies to the bone record as well. We contend that Holen et al.3 presented equivocal evidence in support of tool-imparted percussion damage to the remains of the mastodon. The critical observations are of spiral fractures, cone flakes, impact flakes, bulbs of percussion, impact notches, negative flake scars, anvil-polished specimens, percussion-fractured specimens, and refitting specimens. These bone damage features are inferred to implicate sole agency by prehistoric hominins. As with the stone artefact record, issues of equifinality must first be addressed, including the question of whether other processes could produce such bone damage. Haynes6 presents compelling evidence of non-cultural and/or non-prehistoric processes producing comparable damage at the 24,000-year-old Inglewood Mammoth Site (IMS), Maryland, USA. As at the CM site, the IMS contains the remains of a single juvenile proboscidean recovered in situ from a sealed deposit of sandy clays with pebbles and cobbles6. Haynes6 provides descriptions and images of curvilinear and spiral ‘green-bone’ fractures on cranial, axial and appendicular specimens. Some of these fractures are recent in origin, probably related to heavy earthmoving equipment working on-site6. Other damage may reflect perimortem injuries sustained by the mammoth. No evidence of prehistoric hominin activities are noted or suspected for the site. Post-burial bone notches, impact points and impact scratches occurred on a number of specimens. We report a similar record of fractured proboscidean bones at the Waco Mammoth National Monument (WMNM), Waco, Texas, USA. The site contains the remains of at least 26 mammoths buried in fluvial sands, silts and clays, and dates from 66,800 to 51,300 years ago7. The WMNM was initially investigated as a potential archaeological site, although no evidence of prehistoric human activities was discovered. Figure 1 shows post-burial damage to WMNM mammoth long bones morphologically consistent with observations from the IMS and CM sites. This includes damage that resembles spiral fractures with associated sedimentary abrasion, hammerstone pseudo-notches8, negative flake scars, partially detached flakes and incipient notches, and bulbs of percussion. Such damage, including spiral fractures, is well known in the fossil record from as early as the Triassic period9 and can occur post-burial6. They neither require nor solely indicate prehistoric human agency4,6,8. Other proboscidean assemblages share a similar taphonomic signature with the WMNM, IMS and CM sites. Holen and others report various combinations of spiral fractures, impact points, bulbs of percussion and bone flakes at numerous other late Pleistocene mammoth death sites in the Americas10,11. As with the CM site, these latter assemblages uniformly lack unambiguous stone tools, cut marks, or any other unquestionable evidence of hominin activities, and most predate well-vetted geochronological and palaeogenomic evidence of the initial peopling of the Americas around 15.5 thousand years ago1,2,12–14. Moreover, it is not just what is present at the CM site, but also what is missing. Specifically, hammerstone striae and/or pits (HSSP)15 are noticeably absent despite reasonable cortical bone preservation, several hundred bone fragments, purported hammerstones, and purported anvil abrasions on both the bones and the cobbles. Experimental studies show that hammerstone-percussed proboscidean limb bone fragments should bear HSSP on greater than 30% of specimens created when using a hafted hammerstone and anvil12. The absence of HSSP at the CM site—a proposed percussed bone assemblage—cannot be explained using current experimental models and contradicts the assumption of hammerstone-wielding hominin involvement in bone breakage. Lastly, we question the assertion of an “undisturbed geologic context” at the CM site. Although the distance between some refitted finds necessarily suggests pre-burial breakage and scattering of some items (for example the molar fragments), other features of the record plausibly reflect subsequent forces modifying the assemblage over the last 130,000 years. As fluvial deposits slowly covered the remains, the bones of the mastodon would have remained semi-pliable for years6. Proboscideans or other large mammals subsequently using the muddy watercourse could potentially trample, displace, fracture, abrade and reorient (for example the semi-vertical tusk) the interred materials4,6. Later sediment compaction by metres of overburden and then eventual

Near-Surface Imaging (GPR) of Biogenic Structures in Siliciclastic, Carbonate, and Gypsum Dunes

High-resolution geophysical methods, such as ground-penetrating radar (GPR) imaging, are increasingly applied to ichnological research. Large vertebrate and invertebrate burrows and tracks can be detected and resolved using center frequencies of > 400 MHz. Geophysical images of bioturbation structures in siliciclastic, carbonate, and evaporite (gypsum) dunes exhibit characteristic electromagnetic signal returns, which are associated with active burrow openings (ground–wave gap), filled burrows (hyperbolic diffraction and “pull up”), and large tracks (concave up patterns). The noninvasive imaging can be used for pseudo-3D visualization (closely spaced survey lines) and monitoring of biogenic activity (repeated surveys). Because biogenic structures induce distinct anomalies in geophysical records collected at frequencies typical of many geological investigations, caution must be taken to avoid misinterpreting them as primary sedimentary structures.

REPLY: THE WACO MAMMOTH NATIONAL MONUMENT MAY REPRESENT A DIMINISHED WATERING-HOLE SCENARIO BASED ON PRELIMINARY EVIDENCE OF POST-MORTEM SCAVENGING

We sincerely welcome this discussion from Smith (2017) in response to our article regarding the Waco Mammoth National Monument (WMNM) that indicates the Columbian mammoth herd was subject to subaerial exposure and scavenging prior to burial, rather than being killed and buried in a single event (Wiest et al. 2016). Smiths comments do not dispute the methods or results in Wiest et al. (2016) but do highlight the inconsistencies between our conclusions and the previous understanding of the causative agents surrounding the fossil assemblage (Fox et al. 1992). We appreciate this opportunity to clarify and highlight the empirical basis for the interpretation in Wiest et al. (2016). The major finding from our study is the preponderance of diverse bioerosive traces and weathering features that suggest dry-decay, …