Silvia Gonzalez

Early Hominin Foot Morphology Based on 1.5-Million-Year-Old Footprints from Ileret, Kenya

Hominin footprints offer evidence about gait and foot shape, but their scarcity, combined with an inadequate hominin fossil record, hampers research on the evolution of the human gait. Here, we report hominin footprints in two sedimentary layers dated at 1.51 to 1.53 million years ago (Ma) at Ileret, Kenya, providing the oldest evidence of an essentially modern human–like foot anatomy, with a relatively adducted hallux, medial longitudinal arch, and medial weight transfer before push-off. The size of the Ileret footprints is consistent with stature and body mass estimates for Homo ergaster/erectus, and these prints are also morphologically distinct from the 3.75-million-year-old footprints at Laetoli, Tanzania. The Ileret prints show that by 1.5 Ma, hominins had evolved an essentially modern human foot function and style of bipedal locomotion.

Intertidal Holocene footprints and their archaeological significance

The Holocene mud-flats of Formby Point, at the mouth of the Mersey estuary in northwest England, have long provided information about their palaeoenvironment. Now they yield a more direct evidence — in the form of preserved footprints — of the people and animals that frequented the foreshore.

Survival of the Irish elk into the Holocene

The giant deer Megaloceros giganteus was a celebrated victim of the Late Pleistocene megafaunal extinction, the timing and causes of which are hotly debated. Until now, it was believed that the giant deers demise occurred during the Late Glacial (about 10,600 years ago), before the Pleistocene–Holocene boundary. Here we report new radiocarbon dates from two specimens in stratified contexts, which indicate that a giant deer population still existed around the northern Irish Sea Basin in the early Holocene — 1,400 years after their supposed extinction.

Subglacial deformation at sub-freezing temperatures? Evidence from Hagafellsjökull-Eystri, Iceland

Abstract We report evidence of deformation at sub-freezing temperatures beneath Hagafellsjokull-Eystri, an Icelandic surge-type glacier. The bed of a piedmont lobe that advanced during the 1999 surge comprises deformed blocks of glacier ice set within frozen sediment. This material has also been injected through overlying ice to form a network of crevasse-squeeze ridges. This layer contains evidence for two phases of deformation under contrasting rheological conditions: (1) deformation under relatively warm conditions, when the blocks of glacier ice acted as competent clasts within an unfrozen deforming matrix and (2) subsequent deformation at sub-freezing temperatures when the ice blocks were attenuated into the surrounding frozen matrix along fractures and planar shears enriched with excess ice. This suggests that the basal thermal regime of the advancing ice margin changed from warm-based to cold-based during the surge event. The persistence and potential prevalence of subglacial sediment deformation at sub-freezing temperatures has fundamental implications for our understanding of the dynamic behaviour, sediment flux and geomorphic ability of cold-based glaciers.

Holocene human and animal footprints and their relationships with coastal environmental change, Formby Point, NW England

Abstract Human and animal footprints and tracks exposed in the intertidal zone on the Formby coast are described. Their distribution, age and stratigraphic and palaeoecological setting in relation to Holocene coastal environmental change is discussed. The regional significance of these prints and their relationship to human and animal bones found in the intertidal zone is discussed and their relationship to the local archaeological record is summarised.

Valsequillo Pleistocene archaeology and dating: ongoing controversy in Central Mexico

Abstract A review of the Valsequillo archaeological finds from the last century is given, such as the cranial remains (Dorenburg and Ostrander skulls), the engraved bone fragments, butchering marks, green bone fractures and flint points and scrapers. However, most of these finds are now missing. Their original dating is reviewed, along with the controversial Uranium Series and Fission Track dates from Hueyatlaco. Further relative dating techniques, such as the use of diatoms and bone assemblages, are discussed. Recently human and animal footprints from the Xalnene Ash at Toluquilla quarry have been described, mapped and dated by optically stimulated luminescence, and there has been new dating of the Valsequillo Gravels by AMS radiocarbon dating of molluscs and electron spin resonance of mammoth bone. The Xalnene Ash dates have proved controversial and the dating issues are reviewed. A suggested dating framework for the Valsequillo sequence in which the archaeological artefacts and footprints are found is given and all the stratigraphy with archaeological remains is considered to be Late Pleistocene in age.

Dating of the Valsequillo volcanic deposits: Resolution of an ongoing archaeological controversy in Central Mexico

The timing and origin of the earliest human colonization of the Americas has been the subject of great debate over the last 100 years and is still a matter of heated discussion today (Renne et al., 2005; Gonzalez et al., 2006a). It is widely accepted that the Clovis culture was the first to migrate into the New World at 13.1 ka (Waters and Stafford, 2007). However, archaeological evidence, in the form of stone tools, linguistics, craniometrics and genetics all suggest that the first Americans were ethnically diverse, and a few sites dated to 15e16 ka challenge the ‘Clovis First’ model (Goebel et al., 2008). Perhaps the most spectacular challenge to the ‘Clovis First’ model was the reported presence of human footprints within a basaltic ash (Xalnene Ash, Valsequillo Basin, Central Mexico), dated to 38.04 8.57 ka using optically stimulated luminescence (OSL; Gonzalez et al., 2006a). However, Renne et al. (2005) challenged the validity of the footprints by dating lapilli from the Xalnene Ash using 40Ar/39Ar and reported an age of 1.30 0.03 Ma (2s). Renne et al. (2005) also reported a reversed palaeomagnetic polarity for the ash, consistent with deposition during chron C1r.2r (Cande and Kent, 1995). Such antiquity casts considerable doubt on the interpretation of the impressions as human footprints. Gonzalez et al. (2006b) questioned the 40Ar/39Ar age and highlighted the heterogeneous nature of the lapilli as a potential problem for 40Ar/39Ar geochronology. The lapilli contain phenocrysts and include xenocrysts. Olivine phenocrysts can be contaminated with excess Ar (ArE; McDougall et al., 1969) and hence the dating of

Analysis and Preservation of Pleistocene Human and Animal Footprints: An Example from Toluquilla, Valsequillo Basin (Central Mexico)

Human and animal footprints found in the Valsequillo Basin were formed on the upper bedding planes surfaces of a volcanic ash (Xalnene Ash) deposited by a subaqueous volcano along the shores of a Pleistocene lake. The footprints were made on lake shorelines and the exposed lake floor during low stands associated, either with water displacement during the volcanic eruption, or due to climatically-driven fluctuations in the water balance. The Xalnene Ash has been dated to at least 40 K BP and consequently the human footprints provide evidence for much earlier colonization of the Americas than is often accepted. The methodology used to record, analyze and conserve these footprints used three-dimensional laser scanning, with sub-millimeter precision. This data were used to reproduce polymer models of individual footprints using the application of rapid-prototyping technology. This technology has wide significance for the study of ichnofacies in general. The characteristics of the human footprints and some problems associated with the volcanic ash as a molding medium for the clarity of these characteristics are given. These human footprints and their dating resolve the controversy related to the age of the archaeological and associated megafaunal remains that were reported in the Valsequillo Gravels in the 1960s and 1970s.

Timing of the prehistoric eruption of Xitle Volcano and the abandonment of Cuicuilco Pyramid, Southern Basin of Mexico

Abstract The Cuicuilco pyramid was one of the first true urban centres in the Basin of Mexico. Its construction started a few centuries BC, during the Late Preclassic period. The pyramid is partially covered by a basaltic lava flow produced by the Xitle monogenetic volcano. New stratigraphic work around the pyramid and the volcano together with new radiocarbon dates indicate that the pyramid and nearby settlements were abandoned as a direct consequence of the volcanic activity of Xitle. The new dates, obtained from material which clearly is contemporaneous with the volcanic activity, suggest that the eruption took place around 1670 years BP, some 300 years later than previously thought.

Terrestrial glacial sedimentation on the eastern margin of the Irish Sea basin: Thurstaston, Wirral

This paper provides the first comprehensive description and interpretation of Pleistocene glacigenic deposits exposed in a cliff section at Thurstaston on the Wirral Peninsula, NW England. The section occupies a strategic position where the Irish Sea ice sheet impinged on the English Midlands and North Wales coast. The section comprises six lithofacies: diamicton, gravel, sand, laminites, mud and cobble pavements. The diamicton lithofacies can be divided into an upper, clast-poor sandy diamicton and multiple units of a lower, clast-rich sandy diamicton. Both diamictons exhibit cobble pavements. The two diamicton lithofacies are distinguished on the basis of textural composition, clast lithology, clast shape, clast surface features and clast macrofabrics. Between the diamicton lithofacies are interbeds of sands and gravels of variable thickness. Minor mud and laminites also occur in close association with the sands and gravels. Both the diamictons are interpreted as basal, deformation tills with the interbeds of the sand, gravel and mud lithofacies as indicators of subglacial meltwater flow and ponding. The cobble pavements are interpreted as the result of clasts sinking within, or to the base of, the deforming layer. The sedimentary succession at Thurstaston is best explained by the advance and subsequent recession of a single terrestrially based ice sheet during the Late Devensian. There is no evidence at Thurstaston to suggest a glaciomarine origin for the Late Devensian deglaciation sediments on this margin of the Irish Sea basin. The evidence at Thurstaston points to constructional deformation with the net accretion of till and interbeds caused by the upward migration of the deforming layer base.