Presbyornithidae - Significance in Avian Evolution

Significance in Avian Evolution

The implication of the plethora of this and other, ecologically similar Neornithes (e.g., the wastebasket taxon "Graculavidae") from the Late Cretaceous and early Palaeogene is that shore habitats offered most resources for ancestors of modern birds. The reasons seem to have been that arboreal niches were where the main radiation of the Enantiornithes had taken place some time earlier, and later on because the Cretaceous–Paleogene mass extinction affected both aquatic and terrestrial habitats extensively, leading to the almost total collapse of their trophic webs. In marine habitats, the climatic changes associated with the mass extinction's cause(s) caused a wholesale die-off of oceanic phytoplankton and thus their food webs were destroyed from the bottom up. In terrestrial habitats on the other hand, apart from the loss of the primary production capacity, the keystone species, which were in almost all cases dinosaurs, disappeared, leading the trophic webs on dry land to collapse also from the inside out.

Specialized taxa of the older bird radiations that were very well adapted to their particular ecological niche and dependent on the intactness of the trophic webs had generally no chance to survive such mass extinctions. It is now apparent that at least the main evolutionary lineages of modern bird families already existed at the end of the Cretaceous, albeit they were somewhat marginal compared to the dominant, earlier groups of birds such as Enantiornithes and Confuciornithidae. This serves to show that in evolution the possession of derived or "modern" characters can actually be a disadvantage when a species needs to compete against well-established but more "primitive" lineages, especially as it must be understood that "primitive" refers only to descendence from a lineage that had been established a longer time ago, not that these species were any more generalist or less well-adapted than "modern" forms. In fact, that there were "no" (probably rather: very few) arboreal Neornithes by the end of the Cretaceous is today believed to be because the "primitive" Enantiornithes had had more time to develop adaptations to an arboreal lifestyle and were actually able to outcompete the "modern" arboreal forms, leaving vacant only a few possibilities for early Neornithes to evolve an arboreal lifestyle.

At any rate and their evolutionary relationships nonwithstanding, most bird taxa that survived the mass extinction seem to have been living in environments where they could utilize both terrestrial as well as marine or limnic food resources (the ancestors of the Galliformes probably being the one noteworthy exception). Until the trophic webs had diversified and become complex enough again, such generalist forms were at a competitive advantage. When specialization became a feasible evolutionary strategy again, however, they were outcompeted by more advanced taxa. Note that here, too, "generalist" does not imply that these birds were competitively inferior in their entire ecological niche, only that whenever some form evolves specialization for living in part of this niche, the generalis is at a competitive disadvantage in that particular part of its niche. As time progresses and consequently opportunities for specialization accumulate, it may happen that the generalis forms are either forced to specialize themselves to maintain a competitive edge, or disappear, their niche being in effect divided up by specialist forms.

Principles in evolution as demonstrated by the Presbyornithidae

• Generalist forms which have more "fall-back" potential if part of an ecological niche gets destroyed are better adapted to survive mass extinctions than specialized forms which occupy a narrow ecological niche.
• Primitiveness in descent does not translate into "primitiveness" in morphology or adaptation. The former is an absolute value defined by when the lineage in question separated from relatives which later underwent additional radiation. The latter is dependent on when the last mass extinction created opportunities for the survivors to embark on a new adaptive journey.
• After a lineage has been able to evolve uninterrupted for a considerable amount of time, there is a trend for generalists to be competitively excluded from more and more of their niche(s) by specialist forms, both related and unrelated, that by and by adapt to particular parts of the generalist's niche.
• With continuing uninterrupted evolution of a lineage, remaining plesiomorphies are usually non-adaptive: Presbyornis was a bird which, although primitive by descent and generalist by ecology, was uniquely and highly adapted to its particular mode of life, in a way that is not found anymore in modern waterfowl, because the ecological niche to which it was most well adapted was later partitioned away for the most part, with other opportunities for generalist forms arising in the process.