**4. Discussion**

We used a retrodeformation protocol to produce a restoration of the Steinheim cranium. The application seemingly restored object symmetry to the specimen [3], despite the poor starting conditions of this incomplete and severely deformed fossil. We relied on the preserved portions, mainly on the left side of the cranium, to drive the reshaping of its counterpart. Even though some directions of the taphonomic deformations were not addressed, our application minimises their effect. The reconstruction (Figure 7), hence, allows us to better contextualise Steinheim among the coeval—or at least chronologically close—Middle Pleistocene *Homo* specimens. The neurocranial shape in the posterior view appears intermediate in morphology between the populations of Sima de los Huesos and early Neanderthals (e.g., Saccopastore 1), in keeping with the slight lateral expansion of the parietal walls after the reconstruction as compared with the more vertical and 'compressed' profile in *Ori*. In this respect, the neurocranial morphology of *R.D.* seems to approach the morphology of the early Neanderthal from Altamura [60]. In the posterior view, the original 'roofed' appearance (as described by Schwartz and Tattersall [36]) weakens in *R.D.* neurocranium, appearing close in morphology to penecontemporaneous individuals such as Skull SH5 from Atapuerca [40,54,61], except for the further laterolateral enlargement of the parietals. This trait, difficult to discern before restoring symmetry, places the maximum width of the skull in a slightly lower position relative to that of the original specimen, and roughly at the level of the temporal squama, similar to the typical Neanderthal condition [62] (Figure 4a,b). It is also possible to see a change in the relative position of the two mastoid processes, which, although partly damaged, after retrodeformation show reduced development compared with those of SH5. Their slight rotation can be interpreted as a trait anticipating the Neanderthal condition of tapering [44,45,62], although high variability in this feature among the Middle Pleistocene humans has been observed [63].

**Figure 7.** The retrodeformed model of Steinheim.

An almost symmetrical pattern of contraction and expansion is visible at the level of the glenoid fossae (Figure 5), associated with a change in the relative size of the postorbital portion of the neurocranium. This contributes, in turn, to the slight shortening and laterolateral enlargement of the neurocranium. On the other hand, the 'strip' recorded along the midsagittal axis (Figure 5) corresponds to an almost continuous area of contraction, which is a clear indication of the taphonomic deformation that occurred along this axis

(see Figure 2c). This 'strip' can probably be traced back to a local expansion along the midsagittal line due to the two 'halves' moving in opposite directions.

As can be seen from Figure 5, the retrodeformation was not able to address the anteroposterior vectors of deformation. This is because such vectors acted in a single straight line, rather than bilaterally. Thus, it is not possible to reach evidence-based assumptions on whether the flexion of the basicranium reflects the original condition or it is the result of taphonomic deformation. Nevertheless, the restored midsagittal profile of the cranium suggests that the anteroposteriorly elongated profile of the neurocranium could possibly be associated with a less flexed basicranium. As we proposed in Figure 2b, the present flexion could be related to a deformation operating along the sagittal axis on the upper midface.

Unfortunately, the almost completely missing left portion of the face made it difficult to correct for some local modifications in this area. Nonetheless, it is still possible to carefully evaluate whether some features are due to taphonomic deformation. As mentioned above, the retrodeformation resulted in a 'proper' midsagittal profile (Figure 4d) by undoing the rotational deformation caused by the anterolateral crushing (Figure 2d). By examining the lateral view of the reconstruction, it is more evident how the 'plica' obliterating the frontonasal suture—which is not found in any other hominin from Middle to Late Pleistocene—is consistent with an anterior crushing of the upper part of the nasal portion (Figure 2a). This, in turn, can be associated with the 'notch' found along the lowerright orbital rim, corresponding to a point of weakness represented by the zygomaxillary suture. We sugges<sup>t</sup> that the peculiar facial morphology of Steinheim is mostly a result of the crushing that occurred in the upper portion of the midface (Figure 2b). In our opinion, the reconstruction showed that the infraorbital plate was in origin possibly less flexed than *Ori* suggests.

As evidenced by the PCA (Figure 6), Steinheim is distinguished from the rest of the sample, and this 'uniqueness' can be traced back to its complex pattern of taphonomic deformation. Nonetheless, when a part of this is corrected by retrodeformation, it is possible to see how the new model approaches the fossil human subsample, towards the Neanderthal cluster. We hypothesize that since some of the deformation vectors—namely, those operating on the anteroposterior axis—cannot be intercepted by the retrodeformation, Steinheim still presents itself with a 'unique' morphology, distinguished from other Middle Pleistocene specimens.
