The locomotor adaptation of Australopithecus has long been of interest to paleoanthropologists due to the taxon's morphologically mosaic skeleton and its functionally transitional nature as a habitual biped, bridging the earliest hominins and the genus Homo. This combination of largely derived traits in the lower limb indicative of bipedality (e.g., Lovejoy, 1988; McHenry and Coffing, 2000; Stern, 2000; Harcourt-Smith and Aiello, 2004; Richmond and Jungers, 2008; Daver et al., 2022) and the many apelike features of the upper limb (e.g., Ward, 2002; Tocheri et al., 2008; Larson, 2012, 2013; Young et al., 2015; Stamos and Alemseged, 2023) has led to an enduring debate regarding the nature and extent of arboreality in Australopithecus. This debate is largely driven by disagreements over the functional significance of the apelike morphology encountered in the upper body. Some view the persistence of the ‘apelike’ traits to suggest that arboreal behaviors continued to positively impact fitness (Jungers, 1982; Jungers and Stern, 1983; Stern and Susman, 1983; Stern, 2000; Green and Alemseged, 2012; Kappelman et al., 2016; Ruff et al., 2016). Others argue the same features are nonadaptive retentions occurring alongside morphological traits clearly indicative of selection for bipedality at the expense of arboreality (Lovejoy et al., 1973; Latimer et al., 1987; Lovejoy, 1988, 2007; Latimer and Lovejoy, 1989; Harcourt-Smith and Aiello, 2004).

Further complicating matters is the additional disagreement over how to identify the role of stabilizing selection in the fossil record. Ward (2002) contended that fossil evidence of an ‘apelike’ trait compromising a derived function would support hypotheses of persistence through stabilizing selection. However, Stamos and Alemseged (2023) recently challenged whether such evidence would be sufficient to prove stabilizing selection was acting on traits promoting arboreality. Notably, both sides agree that a promising way forward is to focus our attention on traits associated with arboreality that are ontogenetically plastic.

One aspect of morphology we know to (re)model during an individual's lifetime is the cortical cross-sectional geometry of long bones (Ruff et al., 2006). The quantification of long bone cross-sectional geometry has been used to identify osteological correlates of function and behavior in a number of taxa, including modern humans (Ruff, 2000, 2003a), other primates (Jungers and Minns, 1979; Ruff, 2002; Carlson, 2005; Ruff et al., 2013), and mustelids (Kilbourne and Hutchinson, 2019; Parsi-Pour and Kilbourne, 2020), to name a few. In the upper limb of modern humans, humeral and clavicular diaphyseal breadths exhibit greater asymmetry than bone length, with elements from the right side tending to have broader diaphyses—a pattern linked to differences in physical activity and upper limb loading (Auerbach and Raxter, 2008). Additionally, analyses of external morphological features and internal cortical geometry have highlighted the utility of an ontogenetic sample to identify plastic traits (Tardieu and Trinkaus, 1994; Sumner and Andriacchi, 1996; Tardieu and Preuschoft, 1996; Ruff, 2003a, 2003b; Mitteroecker et al., 2004; Cowgill, 2007; Green and Alemseged, 2012; Green, 2013; Ruff et al., 2013, 2018; Barros, 2014; Morimoto et al., 2018; Nalley et al., 2019, 2024; Stamos and Weaver, 2020; Cosnefroy et al., 2022; Farrell, 2024; Farrell and Alemseged, 2025). These relationships underscore the value of diaphyseal cross-sectional geometry, and an ontogenetic perspective, for investigating upper limb function.

Here, we introduce and describe the morphology of the DIK-1-1 clavicles for the first time using traditional approaches and three-dimensional geometric morphometrics. The DIK-1-1 clavicles are part of the Dikika Child skeleton (DIK-1-1), which represents a well-preserved female juvenile Australopithecus afarensis individual, aged 2.4 years that derives from sediments dated to 3.32 Ma (Alemseged et al., 2006; Wynn et al., 2006). In addition to an almost complete skull, the find includes an exquisitely preserved endocast (Gunz et al., 2020), all cervical and thoracic vertebrae (Ward et al., 2017; Nalley et al., 2019), both scapulae (Green and Alemseged, 2012), the clavicles, the hyoid bone, the sternae, many ribs, a distal humerus, and hand phalanges. The skeleton also preserves additional postcranial elements including substantial portions of both femora, tibiae, fibulae, and an almost complete left foot (DeSilva et al., 2018). At the time of discovery of DIK-1-1, only aspects of the face were exposed. Most of the skeletal elements from the upper body were found encased in a highly endured sandstone block. Therefore, carefully exposing and recovering both clavicles required extensive cleaning and preparation with an air scribe at the National Museum of Ethiopia, a process that took over a decade. Now, the two complete clavicles are separated from the rest of the skeleton, and we present their state of preservation and description below.

Furthermore, we compare the external shape of these elements with an ontogenetic sample of extant apes and KSD-VP-1/1f, an adult Au. afarensis from sediments dated to approximately 3.58 Ma (Haile-Selassie et al., 2010; Melillo, 2016). To assess morphological and functional patterns more comprehensively, we analyze internal cortical distribution using cross-sectional geometry across the same taxa to determine whether the same morphological affinities arise or if a different functional signal emerges. We have three main predictions: First, we expect limited ontogenetic plasticity in the external shape of the clavicle as studies of modern humans suggest that external morphology is relatively constrained (Corrigan, 1960), while internal cortical geometry exhibits greater responsiveness to mechanical loading (Auerbach and Raxter, 2008). Second, we anticipate that the external shape of the DIK-1-1 and KSD-VP-1/1f clavicles will not clearly align with either Pan or Homo due to significant morphological overlap between these taxa (Squyres and DeLeon, 2015; Melillo, 2016). Instead, the fossils will likely cluster among Homo, Pan, and Pongo (Melillo, 2016) rather than showing a clear affinity to a single extant genus. Third, we predict that ontogenetic plasticity in the internal cortical geometry of Au. afarensis will affect the ratio of the principal second moments of area (IMAX/IMIN) more than the ratio of the second moments of area about anatomical axes (IX/IY) as evidence from extant taxa suggests increasing load predictability with age drives greater eccentricity in cross-sectional shape without substantially altering load orientation (Farrell and Alemseged, 2025). Ultimately, by expanding the scope of our description and comparative analysis to include internal morphology, we aim to expand our understanding of the early hominin shoulder girdle and contribute to ongoing debates regarding its functional and evolutionary significance.