Supplementary Material for: Endocranial casts of Camelops hesternus and Palaeolama sp., new insights into the recent history of the camelid brain

Endocranial casts are capable of capturing the general brain form in extinct mammals due to the high fidelity of the endocranial cavity and the brain in this clade. Camelids, the clade including extant camels, llamas, and alpacas, today display high levels of gyrification and brain complexity. The evolutionary history of the camelid brain has been described as involving unique neocortical growth dynamics which may have led to its current state. However, these inferences are based on their fossil endocast record from approximately ~40 Mya (Eocene) to ~11 Mya (Miocene), with a gap in this record for the last ~10 million years. Here, we present the first descriptions of two camelid endocrania that document the recent history of the camelid brain: a new specimen of Palaeolama sp. from ~1.2 Mya, and the plaster endocast of Camelops hesternus, a giant camelid from ~44-11 Kya which possessed the largest brain (~990g) of all known camelids. We find that neocortical complexity evolved significantly between the Miocene and Pleistocene Epochs. Already ~1.2 Mya the camelid brain presented morphologies previously known only in extant taxa, especially in the frontal and parietal regions, which may also be phylogenetic informative. The new fossil data indicate that during the Pleistocene, camelid brain dynamics experienced neocortical invagination into the sagittal sinus rather than evagination out of it, as observed in Eocene to Miocene taxa.

由于该演化支（clade）的颅腔与脑组织具有高度保真度，颅内模（endocranial cast）能够还原已灭绝哺乳动物的整体脑形态。骆驼科（Camelidae）是涵盖现生骆驼、美洲驼与羊驼的演化支，现生类群均呈现出极高的脑回折叠（gyrification）程度与脑复杂度。此前学界认为，骆驼科脑部的演化历史伴随独特的新皮质（neocortex）生长动力学特征，这或是其现生脑形态的形成原因。然而此类推论的依据仅为距今约40 Mya（始新世，Eocene）至11 Mya（中新世，Miocene）的化石颅内模记录，该记录在最近约1000万年存在断层。本研究首次报道两件骆驼科颅内模标本，以此填补骆驼科脑部演化的近期历史记录空白：一件是距今约1.2 Mya的古骆马属（Palaeolama sp.）新标本，另一件是距今44~11 Kya的巨型骆驼——西方驼驼（Camelops hesternus）的石膏颅内模，该物种拥有已知骆驼科中最大的脑容量（约990克）。本研究发现，新皮质复杂度在中新世至更新世（Pleistocene）期间发生了显著演化。早在距今约1.2 Mya时，骆驼科脑部的形态特征（尤其是额叶与顶叶区域）就已与现生类群一致，此类特征或许也具备系统发育信息价值。新的化石数据显示，在更新世时期，骆驼科脑部的生长动力学发生了转变：新皮质向矢状窦（sagittal sinus）内陷入，而非始新世至中新世类群中观察到的向外凸起。