Two Late Pleistocene human femora from Trinil, Indonesia: Implications for body size and behavior in Southeast Asia

Hominin postcranial remains from the Late Pleistocene are relatively rare in Southeast Asia (Santa Luca, 1980; Baba et al., 1990; Brown et al., 2004; Détroit et al., 2004, 2019; Shackelford and Demeter, 2012; Curnoe et al., 2019). Recent discoveries have increased the range of variation in both body size (Brown et al., 2004; Détroit et al., 2013) and potentially locomotor behavior (Jungers et al., 2009a; Détroit et al., 2019) among Late Pleistocene populations from this region. The significance of archaic vs. modern postcranial morphology, in terms of both behavior and ancestry, has also been a topic of ongoing discussion for Late Pleistocene Southeast Asia and East Asia as a whole (Tocheri et al., 2007; Jungers et al., 2009a, Jungers et al., 2009b; Shang and Trinkaus, 2010; Curnoe et al., 2015, 2019; Xing et al., 2018; Wei et al., 2020, 2021).

Here we describe and place into temporal and regional context two Late Pleistocene partial femora, designated Trinil 9 and 10, from the site of Trinil on Java, Indonesia. The specimens were discovered by T. Jacob in 1978 and were included in some later compendia of Indonesian fossil hominin specimens (de Lumley, 1993; Indriati, 2004). However, aside from a few metrics included in a brief comparative analysis (Grimaud-Hervé et al., 1994), they have remained undescribed and largely unknown. The indicated discovery location (Widiasmoro, 1991) was recently identified as a Late Pleistocene fluvial terrace, provisionally—through stratigraphic correlation—assigned an age of 31 ± 6 ka (Berghuis et al., 2021). This would make them part of only a very small number of adult femora known from Late Pleistocene Southeast Asia, the others being the Deep Skull femur from Niah Cave, Malaysia (Curnoe et al., 2019, 2021), fragmentary remains from Wajak, Indonesia (Storm, 1995), and the Terminal Pleistocene sample from Tam Hang, Laos (Shackelford and Demeter, 2012).

Dubois' early discoveries at Trinil played a pivotal role in the development of hominin paleontology (Theunissen, 1989). In addition to the famous calotte and Femur I, which formed the basis for defining the taxon Pithecanthropus erectus (Dubois, 1894), four other femora (II–V) from Trinil were later identified by him among museum collections (Dubois, 1932, 1934; see Table 1). Based on both morphological comparisons (Ruff et al., 2015b) and new stratigraphic and dating analyses (Hilgen et al., in prep.; Pop et al., in prep.), it now appears that Femur I is significantly younger than the calotte and Femora II–V. Femora II–V and the calotte derive from Early or Middle Pleistocene fossiliferous beds, whereas Femur I derives from younger terrace deposits, probably dating to around the Middle to Late Pleistocene boundary (Hilgen et al., in prep.; Pop et al., in prep.).

The inclusion of the two new femora described herein, Trinil 9 and 10, expands the temporal range of specimens recovered from the Trinil site and allows further investigation of temporal trends in morphology in this region. We also include broader comparisons of femoral structural properties and reconstructed body size with a large sample of other Pleistocene and more recent Homo from East Asia, Africa, and Europe/West Asia. We aim to assess long-term temporal trends as well as regional differences in these characteristics, and address their implications for the evolution of postcranial morphological variation within Homo.

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