| Popis: |
Many paleontological studies conclude that all orders of modern birds had developed from a dinosaurian phylogenetic ancestry by the mid-Cretaceous. Recently, however, an alternative hypothesis states that the only modern-bird-like Cretaceous bird was a Stone-curlew-like shorebird; this bird passed the KT-boundary, where the dinosaurs died out, while all neognath orders were assumed to develop from this taxon (FEDUCCIA, 1995). This hypothesis implicitly assumes that the ancestral feeding system transformed into the specialized types of probing that are typical for many wading birds. That hypothesis of transition is tested by gradually improving the model of the ancestral mechanism for the functional requirements that are measured experimentally in sandpipers. The pecking mechanism is shown to be the ancestral trophic system. Five major points of transition, bifurcation and branching are deduced. Among these, in particular the strong improvements for economizing the probing type of feeding are decisive. After lengthening and slenderizing the beaks, and modifying the construction of the jaw apparatus to carry penetration forces onto the braincase, not only branching followed for different compromises in beak curvature, slenderness, etc., but also a bifurcation did for further improvement. Either horizontally, or vertically flattened beaks are required for penetration improvement; only the latter type can be more improved by two subsequent innovations. First de-coupling of the symphyseal and maxillary motion (change from pro- into distal rhynchokinesis), and the symphyseal and mandibulary motion must occur ; this de-coupling economizes the grasping and improves holding prey strongly. Second, development of the capacity of 'remote' touch, replacing the ancestral 'direct' touch capacity, reduces the number of probes required to locate prey drastically. Models for these two innovations are calculated and tested. It is shown that sensorial and mechanical requirements for improved probing are mutually exclusive and forces to compromises in construction of the beak tips. The morphospace formulated by this deduction serves as a basis for evolutionary interpretation in a next paper (ZWEERS & VANDEN BERGE, 1997). |
