Aurélie 



I have never seen anything like the marks you illustrate. They are a challenge.



Christian suggests the possibility of chemical corrosion. My problem is that I can't see what corrosive agent could produce marks, in cortical bone, that were so consistent in shape and size, and separated from each other.



So my bet is borings by some invertebrate organism. To that extent I agree with Adam's post suggesting dermestsid beetles.



However your marks are virtually all roughly circular, and so do not mimic the elongated pupal chambers of dermestids.



Interestingly, Hasiotis (Sedimentary Geology 167 (2004) 177–268) discusses borings on dinosaur bones that seem to closely resemble those you have. He describes them thus:





START QUOTE

(C) Small hemispherical borings on the surface of a femur of D4.18. Type 18—circular to elliptical borings in dinosaur bone, Fig. 13C–H



Description: Predominantly circular to slightly elliptical in plan-view, the borings are preserved as molds and casts within the bone and are shallow hemispheres typically 0.01–4.0 mm deep. Some elliptical pits appear to be incomplete borings. The borings range from 0.5–1, 2.5–3, and 4–5.0 mm in diameter. Clusters of borings are random with no particular distribution between borings. Some skeletal elements contain both small and large borings, but one size always dominates the bone surface. Borings from different quarries have similar diameters, shapes, and distributions across bone surfaces. None of the dinosaur bones examined contain deep or fully penetrating holes or trails.



Occurrence: Dinosaur bone borings were observed in quarries in the Brushy Basin Member in Colorado, Utah, and Wyoming.



Tracemaker: The morphologies suggest that these borings were most likely produced by the larvae of carrion beetles (Coleoptera: Dermestidae) and are very similar to the traces of modern dermestids(Fig. 13G–H)." 

END QUOTE





In case you can't get to Hasiotis's original article, there is an image of Fig. 13C at  https://app.box.com/s/f1r7ece5menr1rucboe8kk50x7b7ez0v



A final question. Among the pits, are there any deeper borings that turn into tunnels that are perpendicular to the surface of the cortical bone? In the image with three separate bones, I wondered whether this was the case for some of the marks on the bone on the right.



Please let us know if you solve this intriguing problem.



Richard Wright