Birdwatchers know that the best place to see many different species of birds is in the Southern Hemisphere. So why is there more bird species diversity here than in other regions such as Europe? In this paper, Daniel Field and Allison Hsiang studied a 52-million-year-old fossil bird and published their new findings in BMC Evolutionary Biology.
Extant birds
International Association of Crested Apocyns
Many birdwatchers know that the best way to see an astonishing number of bird species is to head to the Southern Hemisphere. Brave birdwatcher Noah Strycker spent a full three months in South America (often referred to as the Bird Continent) during his journey to observe a record 6,042 species of birds in 2015, while only 12 days in Europe.
This is not to say that birds found at more southerly latitudes are more valuable than those found in Europe; it simply reflects the extremely uneven global distribution of the 11,000 species of birds that exist in the world.
Not only are there more species in the Southern Hemisphere, but many of the major groups of birds live only in South America, Africa and Australia – these are the remnants of the ancient southern hemisphere supercontinent Gondwana.
But how did this uneven geographical distribution of existing bird diversity come about? Does it merely reflect the fact that most of the populations of living birds appeared on the southern continent and failed to disperse elsewhere? Or are there more complex models that control the evolutionary history of the geographical distribution of birds through geological age?
White-crowned banana crane, wilderness, Western Cape, South Africa
Daniel J. Field
The only way to definitively answer this question is to flesh out the fossil record of modern birds. In their latest work, just published in BMC Evolutionary Biology, Daniel Field and Allison Hsiang studied a well-preserved 52 million-year-old fossil bird called Foro panarium. It was first discovered in western North America in the 1980s and named in the 1990s. F. panarium presents a mysterious combination of anatomical features that hinders a robust assessment of its class-like edges.
By re-examining its anatomical features in the latest evolutionary environment, Field and Hsiang's work supports the earliest known relative of a living bird called the crested banana crane (or "banana-eating bird"). The crested banana is an extremely beautiful group of birds, including 24 medium-sized fruit-fed birds, several of which contain unique pigments that produce bright green and magenta hues. In addition to these interesting aesthetic properties, the modern crested banana is also known for its presence only in sub-Saharan Africa.
But if the fossil Foro panarium came from North America, what does it tell about the ancient geographical distribution of this group that led to the modern crested apocynacea? And how did the modern crested banana crane become unique to Africa today?
It turns out that The Foro panarium is by no means the first fossil bird to be found outside the modern distribution of its closest extant relatives. Many other flocks of birds that appear today only on the southern continent, such as Seriemas in South America or ratbirds in Africa, have fossil specimens from all over the world.
Zebra, Nairobi National Park, Kenya
Filmed by Daniel J. Field
Field and Hsiang's paper reaffirms the complexity of this observation, which can be used to analyze the original geographical distribution of modern birds. Although Field and Hsiang's analysis of only surveying modern birds confidently infers that the most recent common ancestor of all extant birds appeared in the southern hemisphere, their analysis combined with fossils yielded an apparently less certain result.
These analyses suggest that modern birds did not originate in the southern hemisphere, or that if they had originated in the southern hemisphere, the subsequent changes in the geographical distribution of modern birds would have been far more complex than one could have known without a fossil record.
Future work will try to more clearly identify the main factors revealed by the fossil record that are driving a noticeable shift in geographic distribution. Is the spread of these conjectures related to a period of climate change in Earth's history? Or could they be related to periodic extinction events throughout the geological period?
In addition to understanding the possible historical implications of the geographic history of the lineage including the Crested Bulbophyllum, Foro panarium provides additional information for the chronology of the Crested Bulbophyllum lineage itself. Fossil age makes F. Panarium is not only representative of the oldest known lineage of crested apocynacea, but also the oldest representative of major groups including crested cuckoo, cuckoo and bustard (large ground-dwelling birds).
Thus, at least 52 million years ago, the crested apocalypse deviated from its closest kind, which may indicate that this group was part of rapid bird radiation after the late Cretaceous extinction event (the famous dinosaur extinction) 66 million years ago.
Foro panarium also provides an interesting insight into the biological evolution of the modern crested apocynacea. Many fully arboreal extant birds have relatively short hind limbs, and specialization lowers their center of gravity to enhance habitat stability.
The extant crowned banana cuckoo is arboreal and has relatively short hind limbs. However, F. panarium's hind limbs are quite long, similar to those of extant ground-dwelling birds. This observation may indicate that the ancestors of the crested bulbul acquired arboreality relatively late — consistent with the idea that multiple arboreal lifestyles appeared independently in extant birds after the end-Cretaceous extinction event.
The multifaceted insights gained by Foro panarium illustrate the great importance of the fossil record in accurately understanding the evolutionary history of modern birds. Clearly, a better understanding of the evolutionary kinship of modern bird fossils has the potential to fundamentally improve our understanding of how and when modern birds and their typical geographic distribution evolved. As each new bird fossil is unearthed, our depiction of the evolutionary history of birds will continue to become clearer. It's like adjusting the focal length through binoculars and focusing on the distant crested banana.