Were Pterosaurs Really Rarer in the Southern Hemisphere? Biases in the Gondwanan Fossil Record
Pterosaur fossils have been discovered on every continent, including Antarctica. Yet much of the evolutionary history of pterosaurs familiar to us today has long been shaped by the comparatively rich fossil records of Europe, Asia, and North America. The picture becomes far more fragmentary when we turn to Gondwana, the great landmass that once dominated the Southern Hemisphere. Some regions have yielded nearly complete skeletons and even soft tissues, whereas others are represented by little more than an isolated tooth, a single vertebra, or a few difficult-to-identify limb bones. This extreme disparity makes it difficult to determine the true diversity and distribution of pterosaurs across the Southern Hemisphere.
The earliest definitive pterosaurs currently known date to the Norian stage of the Late Triassic. They diversified rapidly during the Jurassic and survived until the end of the Cretaceous. Pterosaur skeletons were delicate, and relatively few geological settings were capable of preserving them in a reasonably complete state. This problem is particularly pronounced in Gondwana: Triassic and Jurassic remains are scarce, while most of the known record comes from the Cretaceous. A review compiling pterosaur body fossils and trace fossils from the Gondwanan landmasses—including South America, Africa, the Arabian Peninsula, Madagascar, the Indo-Pakistani subcontinent, Australia, New Zealand, and Antarctica—showed that differences in the number of fossils known from each region do not necessarily reflect equivalent differences in the original distribution of pterosaurs. In many cases, the pattern is strongly influenced by differences in preservation, collecting effort, and the history of research.
The richest and most diverse Gondwanan pterosaur record currently known is concentrated in South America, particularly in Brazil's Araripe Basin. The Crato and Romualdo Lagerstätten have yielded large numbers of Cretaceous pterosaurs, including specimens preserving soft tissues such as cranial crests and wing membranes. These fossils provide information on anatomical structures that are difficult or impossible to reconstruct from bones alone. Numerous lineages are represented in South America, including Ornithocheiridae, Tapejaridae, Nyctosauridae, and Ctenochasmatidae.
The apparent richness of the South American record, however, cannot be taken as a direct measure of how many more pterosaurs actually lived there than elsewhere in Gondwana. This is the so-called Lagerstätten effect: when a small number of deposits preserve fossils exceptionally well, one region can accumulate far more recognizable taxa than areas with poorer preservation. At least part of the observed disparity in diversity therefore reflects uneven preservation and sampling.
If fossil abundance is set aside and attention is focused on the pterosaur groups that can actually be identified, the Gondwanan record is strikingly diverse. Late Triassic Argentina has yielded Yelaphomte praderioi and Pachagnathus benitoi, both assigned to Raeticodactylidae, demonstrating that pterosaurs were already present in Gondwana early in their evolutionary history. By the Jurassic, South America had produced the darwinopteran Allkaruen koi, Madagascar had yielded teeth assigned to Rhamphorhynchidae, and Antarctica had produced a Jurassic pterosaur humerus that is more conservatively regarded as belonging to an indeterminate rhamphorhynchoid. Although these fossils are few in number, they are enough to establish that early pterosaurs were not restricted to the Northern Hemisphere.
Some of these specimens also illustrate how unstable the taxonomy of early Gondwanan pterosaurs can be when the available material is fragmentary. The Brazilian Faxinalipterus minima was originally regarded as a pterosaur, but after further preparation and phylogenetic analysis, its holotype was placed within Lagerpetidae, a group closely related to pterosaurs but lying outside Pterosauria itself. A maxilla previously referred to Faxinalipterus was subsequently named Maehary bonapartei and interpreted as a member of the broader clade Pterosauromorpha. When a particular interval is represented by only a handful of incomplete specimens, the reassessment of a single fossil can substantially alter our understanding of the regional pterosaur record.
By the Late Jurassic and Early Cretaceous, clearer geographic patterns begin to emerge. Ctenochasmatids are known from both Africa and South America, and their temporal distribution broadly matches that seen in Laurasia. Gnathosaurinae, a subgroup of ctenochasmatids characterized by elongate jaws and numerous teeth, has also been recorded from the Late Jurassic of Uruguay. It has previously been proposed that ctenochasmatids may have dispersed from Eurasia into South America through a corridor linking northern and southern landmasses. The age and phylogenetic position of the Uruguayan material are consistent with that hypothesis, although the available specimens are not sufficient to reconstruct the actual dispersal route in detail.
Ornithocheirids show an even broader distribution during the Early Cretaceous. Fossils belonging to Ornithocheiridae have been reported from South America, Africa, Australia, and the Arabian Peninsula, and the stratigraphic range of Gondwanan ornithocheirids broadly matches that of their Laurasian counterparts. This provides direct evidence that ornithocheirids were not geographically restricted to a single continent and had already occupied widely separated landmasses during the Early Cretaceous.
Reliable tapejarid records, meanwhile, are concentrated in the Early Cretaceous in both Laurasia and Gondwana. South America has yielded particularly abundant material, including species of Tapejara, Tupandactylus, and Caiuajara. Some of these fossils preserve individuals at different growth stages. In the Goio-Erê Formation of Brazil, for example, Caiuajara dobruskii is represented by dozens of individuals spanning a wide ontogenetic range, with estimated wingspans from about 0.65 to 2.35 metres. Limb proportions remain relatively similar through growth, whereas the angle of the premaxillary crest becomes progressively steeper, approaching 90° in adults. A sample containing multiple growth stages of the same species makes it possible to follow changes in cranial-crest morphology through ontogeny directly.
Argentina's Pterodaustro guinazui provides another unusually informative record. The Lagarcito Formation has yielded nearly complete individuals, juveniles, and even unhatched pterosaur eggs, allowing researchers to examine aspects of development from hatching onward. In Chile, the Quebrada Monardes Formation has also produced thousands of pterosaur bones from an approximately two-metre-thick interval of sandstone and conglomerate. The concentration of these remains in deposits interpreted as having formed under flood-related conditions has been taken to represent the remains of one or more pterosaur aggregations.
The Gondwanan and Laurasian records are not identical, however, and some lineages show conspicuous differences. Pteranodontians are represented by multiple Late Cretaceous occurrences in Laurasia, whereas in Gondwana this lineage is represented in the review by Epapatelo otyikokolo from the Maastrichtian Mocuio Formation of Angola. The contrast is even greater for Chaoyangopteridae, which is best known from the Early Cretaceous of China. At present, the most secure Gondwanan representative is the African form Apatorhamphus gyrostega. Several other Gondwanan pterosaurs have at times been assigned to Chaoyangopteridae, but different phylogenetic analyses have produced conflicting placements, so their membership in the clade remains uncertain.
Azhdarchids show a different pattern. By the Late Cretaceous, Azhdarchidae had become one of the most widespread pterosaur clades represented in the Gondwanan record, with reported occurrences from South America, Africa, the Arabian Peninsula, Australia, New Zealand, and Madagascar. Together with the broad Early Cretaceous distribution of ornithocheirids, these fossils demonstrate that at different times during the Mesozoic, some pterosaur lineages occupied vast areas of Gondwana.
The greatest source of distortion in our present understanding lies in the enormous differences in the amount and quality of information available from different regions. The Araripe Basin can preserve complete skulls and skeletons, cranial crests, and wing membranes. Elsewhere, the record may consist of little more than an isolated tooth or a single limb bone. The former can sometimes be identified to genus or species and used to study growth and soft-tissue anatomy; the latter may establish only that a broad pterosaur group was present.
The most secure conclusion from the existing fossil record is therefore that Gondwana supported multiple pterosaur lineages from the early history of the group through to the end of the Cretaceous, and that several major clades were distributed across multiple Gondwanan landmasses. Determining exactly how many species lived in each region, or which part of Gondwana truly supported the greatest diversity, remains much more difficult because the completeness and quality of the fossil record vary so dramatically from one region to another.
Author: Shui-Ye You
Reference:
Pentland AH and Poropat SF. (2023). A review of the Jurassic and Cretaceous Gondwanan pterosaur record. Gondwana Research.
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