Sunday, December 13, 2020

[Paleontology • 2020] Cretaceous Origins of the Vibrotactile Bill-tip Organ in Birds

in du Toit, Chinsamy & Cunningham, 2020. 


Some probe-foraging birds locate their buried prey by detecting mechanical vibrations in the substrate using a specialized tactile bill-tip organ comprising mechanoreceptors embedded in densely clustered pits in the bone at the tip of their beak. This remarkable sensory modality is known as ‘remote touch’, and the associated bill-tip organ is found in probe-foraging taxa belonging to both the palaeognathous (in kiwi) and neognathous (in ibises and shorebirds) clades of modern birds. Intriguingly, a structurally similar bill-tip organ is also present in the beaks of extant, non-probing palaeognathous birds (e.g. emu and ostriches) that do not use remote touch. By comparison with our comprehensive sample representing all orders of extant modern birds (Neornithes), we provide evidence that the lithornithids (the most basal known palaeognathous birds which evolved in the Cretaceous period) had the ability to use remote touch. This finding suggests that the occurrence of the vestigial bony bill-tip organ in all modern non-probing palaeognathous birds represents a plesiomorphic condition. Furthermore, our results show that remote-touch probe foraging evolved very early among the Neornithes and it may even have predated the palaeognathous–neognathous divergence. We postulate that the tactile bony bill-tip organ in Neornithes may have originated from other snout tactile specializations of their non-avian theropod ancestors.

Keywords: evolution of novel sensory systems, links between soft tissue and bone morphology, vestigial sensory organs, avian palaeoecology, ecomorphology, plesiomorphy in palaeognathous birds

(a–d) Pitting pattern on the surface of the beak bones and sagittal sections showing the soft tissue histology of the bill-tip for four extant species of birds, illustrating three different types of bony bill-tip organs. 
(1) No bony bill-tip organ (most birds): (a) kelp gull (Larus dominicus). 
(2) Remote-touch probing billtip organ, in neognathous birds: (b) hadeda ibis (Bostrychia hagedash); and in palaeognathous birds: (c) northern brown kiwi (Apteryx mantelli). 
(3) Non-probing palaeognathous bill-tip organ: (d) elegant crested tinamou (Eudromia elegans). 

 (e,f) Cranial fossils of two species of lithornithids, showing high degree of pitting on the surfaces of their beaks, similar to all extant palaeognathous birds, potentially indicative of a bony bill-tip organ. 
(e) Lithornis promiscuus: (i) skull and attached maxilla (USNM 391983) showing the shape of the beak relative to the skull; (ii) distal portions of maxilla and mandible (USNM 336535).
 (f ) Paracathartes howardae: maxilla (USNM 404758) and distal portion of mandible (USNM 361437).

A modern species of remote-touch probing bird: a Hadeda ibis Bostrychia hagedash probes for food using remotetouch on a beach in South Africa, much in the same way the lithornithids would have done millions of years ago 
(photo: Peter Ryan)

C. J. du Toit, A. Chinsamy and S. J. Cunningham. 2020. Cretaceous Origins of the Vibrotactile Bill-tip Organ in Birds. Proc. R. Soc. B.  DOI: 10.1098/rspb.2020.2322
Sensing good vibrations: a remarkable sensory organ in the beaks of ancient fossil birds