I thought I had sent this message earlier, but it hasn't showed up on list, so my apology if the earlier transmission now shows up, in addition to this:
Heinz Peter Bredow said, "Regarding the so called "three-meter gap" [beneath the K/T iridium layer]. This gap shrinks if additional footprint data of dinosaurs is available. Footprints seems to be more acid resistant than fossil bones....Martin Lockley described such a case for a site in southeastern Colorado in "Tracking [Dinosaurs, 1991, page 202]".
That brings to mind an interesting consideration: In the instance Lockley describes, the ornithopod footprints were only 37 cm below the iridium layer, and, as we know, to date, no dinosaur bones have been found equally near the K/T (iridium) layer. It is HPB's statement that, "Footprints seems to be more acid resistant than fossil bones", that really 'hits home' with me.
Why? Other than Early Cretaceous natural casts of bones that have disintegrated, most all of the bones and teeth found in this area of Maryland are those preserved in the very protective (from acid ground waters in this quite swampy area) Arundel clay.
That makes me wonder if the paucity of dinosaurian (and other) bones immediately beneath the K/T iridium layer might be, in large part, due to the intensely acid rain that would undoubtedly have resulted from the impact event, soaking down into bone-bearing layers and dissolving such materials.
Point of this story? Aside from watching for footprints, I suggest paleontologists keep a more careful outlook for natural casts of dinosaur (and other) bones (and teeth) within reasonable distances below the iridium layer. Natural casts of bones and teeth are much more difficult to notice and identify than are silicified natural bones and teeth, and they may be the only evidence that is left of these items in many immediately sub-K/T areas. If such casts are there, finding them could shed considerable light on immediately pre-impact vertebrate diversity, etc.
"You know my method. It is founded upon the observance of trifles." -- Sherlock Holmes in The Boscombe Valley Mystery