By the Early Cretaceous, both birds and pterosaurs had achieved a cosmopolitan distribution, thanks to their abilty to fly. A lot has been written on the diversity of both groups up to the end of the Mesozoic, but it is widely agreed that even in the Maastrichtian pterosaurs were found in all continents including Oceania (Witton 2008) and Antarctica (Kellner 2019). And the same applied for birds, with both northern hemisphere formations like Hell Creek and southern hemisphere ones like the Lecho Formation offering insights to the diverse avifauna of this epoch.
In spite of this, however, there are two places in the world in which either group is completely and conspicuously absent. Both bird and pterosaur fossils are highly susceptible to preservation bias and rare in non-Lagerstätte sites (Silverstone 2016 among several others), but these particular sites are noted for their high degree of preservation and presence of microfauna from small amphibians to insects.
The first is the Kem Kem Group, from the Cenomanian of Morocco. This ecosystem, representing several estuarine stages from more inland freshwater habitats to coastoal waters, preserves a very rich vertebrate and invertebrate fauna, with dinosaurs like Spinosaurus being the best known residents. Among the finds is a very rich pterosaurian fauna dominated by azhdarchoids and pteranodontians: this included the tapejarid Afrotapejara, an undescribed azhdarchid, the chaoyangopterid Apatorhamphus, the incertae sedis azhdarchoids Leptostomia, Xericeps and Alanqa (the latter two have variously been interpreted as either azhdarchids or thalassodromines; see Longrich 2018 and Campos 2021 for instance) and severl ornithocheirids/anhanguerids such as Anhanguera, Coloborhynchus, Siroccopteryx and Nicorhynchus. Yet there is not a single avian fossil in sight, in spite of the fact that almost all contemporary formations have rich avifaunas, including contemporary African formations such as the Zebbag Formation (Contessi 2012). While an attempt in the paper is used to explain the absence of mammals (namely, competition with notosuchians), it only notes the strangeness of the lack of avian fossils. In general pterosaurs seem to occupy a very wide range of trophic niches in the Kem Kem Group, from the small probing Leptostomia to the frugivorous Afrotapejara to the durophagous Alanqa to the piscivorous ornithocheirids/anhanguerids to the raptorial Apatorhamphus and Xericeps, so one might be tempted to explain the absence of birds as particularly aggressive competitive exclusion on the part of pterosaurs. Still, this pterosaurian diversity is not too far off from those seen in other formations with birds, so the Kem Kem environment must have truly been strange to favor pterosaurs alone.
The other are the various Maastrichtian formations of India and Madagascar, which then were either still together as an island or just recently seperated. A diverse avifauna is known from the Maevarano Formation that includes Enantiornithes (both pengornithids such as Falcatakely as well as Vorona, now interpreted as an enantiornithean rather than as an ornithurine as previously believed; see Hartman 2019 and Pei 2020), flying dromaeosaurs such as Rahonavis and even possible relatives of Sapeornis (O’Connor 2010). Indian formations meanwhile preserve neither bird nor pterosaur fossils, but they don’t approach the sheer level of preservation detail as the Maevarano, which also includes postcranial remains of allotherian mammals, frogs and other fragile specimens. The absence of pterosaurs from Indo-Madagascar is just as puzzling as the absence of birds from the Kem Kem Group; pterosaurs in the Maastrichtian essentially surrounded this landmass, with South America, Antarctica, Australia and New Zealand each having specimens and Africa in particular preserving a rich pterosaurian fauna in the Ouled Abdoun Basin (Longrich 2018). Furthermore, unlike the Kem Kem Group there doesn’t seem to be anything close to a competitive displacement: none of the Maevarano birds are larger than average for Mesozoic birds, and with the exception of possibly Vorona and Rahonavis nearly all seem to occupy primarily arboreal niches. True, the toucan billed Falcatakely, the flying dromaeosaur Rahonavis and omnivoropterygids are all unusual elements for a Maastrichtian avifauna, but none seem to push the boundaries of the Late Cretaceous avian morphospace.
Ultimately, both cases are mysteries, but perhaps this is indicative that even cosmopolitan groups can have sufficiently large gaps in their range.
Mark Witton and Darren Naish, A Reappraisal of Azhdarchid Pterosaur Functional Morphology and Paleoecology, PLoS One. 2008; 3(5): e2271. Published online 2008 May 28. doi: 10.1371/journal.pone.0002271
Alexander Kellner, Taissa Rodrigues, Fabiana R. Costa, Luiz C. Weinschütz, Rodrigo G. Figueiredo, Geovana A. de Souza, Arthur S. Brum, Lúcia H.S. Eleutério, Carsten W. Mueller, Juliana M. Sayão, Pterodactyloid pterosaur bones from Cretaceous deposits of the Antarctic Peninsula, Earth Sciences • An. Acad. Bras. Ciênc. 91 (suppl 2) • 2019 • https://doi.org/10.1590/0001-3765201920191300
O’Connor and Forster, 2010. A Late Cretaceous (Maastrichtian) avifauna from the Maevarano Formation, Madagascar. Journal of Vertebrate Paleontology. 30(4), 1178-1201.
Nizar Ibrahim, Paul C. Sereno, David J. Varricchio, David M. Martill, Didier B. Dutheil, David M. Unwin, Lahssen Baidder, Hans C. E. Larsson, Samir Zouhri, Abdelhadi Kaoukaya, Geology and paleontology of the Upper Cretaceous Kem Kem Group of eastern Morocco, https://doi.org/10.3897/zookeys.928.47517 (21 April 2020)
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Contessi M, Fanti F (2012) First record of bird tracks in the Late Cretaceous (Cenomanian) of Tunisia. Palaios 27: 455–464. https://doi.org/10.2110/palo.2011.p11-114r
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