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）中颅腔与脑组织的高保真对应关系，颅内模（endocranial casts）可用于还原已灭绝哺乳动物的整体脑形态。骆驼科（Camelids）包含现生骆驼、美洲驼与羊驼，现生类群具有高度发达的脑沟回结构与复杂脑形态。此前学界认为骆驼科大脑的演化历史存在独特的新皮层生长动力学特征，这或是其现生脑状态的成因。但此类推断仅基于约40百万年前（始新世）至11百万年前（中新世）的化石颅内模记录，该记录在近1000万年的时段中存在缺失。
本研究首次报道两件骆驼科颅内模标本，填补了骆驼科脑演化近期历史的空白：一件是约1.2百万年前的古骆马未定种（Palaeolama sp.）新标本，另一件是约4.4万至1.1万年前的巨型骆驼科物种北美巨驼（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）。
研究发现，新皮层复杂度在中新世至更新世期间发生了显著演化。早在约1.2百万年前，骆驼科脑的形态特征就已具备现生类群的典型形态，尤其在额叶与顶叶区域，此类特征或具有系统发育信息价值。新的化石数据表明，在更新世时期，骆驼科的脑动力学模式发生了变化：新皮层向矢状窦（sagittal sinus）内陷，而非始新世至中新世类群所观察到的向外突出现象。