Homo antecessor skull fragments and dinosaur stomach stones make anatomy and function not directly preserved in fossils visible through two different physical traces.
Science··Morning
Three layers of the skull
Three adult Homo antecessor parietal bones from Gran Dolina make it possible to compare this species' skull layer by layer. Fragments ATD6-115, ATD6-128 and ATD6-401 place adult material beside an earlier picture that had rested on one juvenile bone in 2017. In the Journal of Human Evolution study, the diploe, the spongy middle layer, represents between 41 percent and 50 percent of total cranial thickness. Total thickness falls within the ranges observed in modern humans and Neanderthals, and clearly below the values reported for Homo erectus and other early Pleistocene hominins. This comparison does not make Homo antecessor wholly modern; it places only total parietal thickness and the distribution of its layers beside selected hominin groups. The concrete advance is that three adult fragments can now be examined quantitatively within the same framework, extending a comparison previously limited to juvenile material.[1]
What does wear on stones reveal?
Dinosaur gastroliths preserve a different structure: mechanical wear produced inside the stomach. Ryuji Takasaki of Okayama University of Science and colleagues compared the shapes and wear of fossil stomach stones with examples from living birds and crocodilians. The Paleobiology paper reports that stones became smoother as they were abraded in the stomach, with the trend clearest among plant-eating groups. The authors infer two digestive arrangements from those traces. A muscular stomach capable of grinding plant material may have developed in the theropod line. Ornithischians and sauropods exceeding 20 metres may instead have combined developed teeth and jaws with prolonged fermentation in an elongated digestive tract. A Tarbosaurus skeleton with a gastrolith-bearing region and Psittacosaurus material appear on the fossil side of the comparison. The boundary of the inference is part of the result: wear on stones is a physical trace of past digestion, but the process inside a living dinosaur was not observed directly.[2]
Two traces, two questions
The two reports do not extract the same kind of information from fossils. The Homo antecessor fragments compare measurable layers of preserved bone and establish a broader adult basis for describing cranial structure. The gastrolith work uses the shape of preserved stones as an indicator for a digestive function that was not itself preserved. The distance between evidence and interpretation therefore differs. In the skull research, the measured quantity belongs to the bone itself, and cross-species similarity follows from comparing those measurements. In the stomach-stone research, abrasion is first related to living birds and crocodilians and then carried into possible digestive arrangements across dinosaur lineages. Read together, the developments show that fossils do not supply a complete biology on their own; different preserved parts answer different questions. Bone layers provide a more direct window onto anatomy, while stone shape provides a more indirect window onto function.[1], [2]
