The early Cretaceous had a great diversity of pterosaurs. All four major groups of pterodactyloid – Archaeopterodactyloidea, Dsungaripteridae, Pteranodontoidea, and Azhdarchoidea – are present, and very diverse (Barrett et al., 2008). Even a few anurognathids were still present until at least the Aptian. By the end of the Cretaceous, this diversity had been reduced to only two clades: Pteranodontia and Azhdarchidae. What happened?
I looked through the lit and did a census of Cretaceous pterosaur taxa to get a sense of how diversity changed over time. I adopted a few arbitrary criteria for selecting what made the list, so things didn’t get too out of hand: the remains have to be diagnostic to one of the arbitrary taxonomic categories I chose; the remains have to be differentiable from other taxa in the same formation (for example, Santanadactylus was not counted as it cannot be satisfactorily distinguished from other contemporary anhanguerians like Anhanguera and Maaradactylus due to lack of overlap); the remains have to be dated narrower than Early/Late Cretaceous. Classifications follow my phylogenetic analysis (so e.g. Argentinadraco and Montanazhdarcho are considered azhdarchids), as well as those of Longrich et al. (2018) and Pêgas et al. (2019). I used this data to generate diversity curves for each stage of the Cretaceous.
The absolute diversity curves spike in the Aptian-Albian before crashing in the Cenomanian to the Turonian, then gradually increase to the end of the Maastrichtian. I would not read anything into this. The Aptian-Albian was when several major pterosaur lagerstatten were deposited, so the peak in diversity is certainly an artifact of preservational bias.
Scaling the diversity curves to show relative diversity of each taxonomic category tells a different story, however. Throughout the Early Cretaceous, the majority of pterosaur diversity is toothed pterosaurs, such as ctenochasmatoids, dsungaripterids, and lanceodontians. Toothless pterosaurs appear at the very beginning of the Cretaceous, and slowly increase in diversity up to the Cenomanian. Then, between the Cenomanian and the Coniacian, toothed pterosaurs (as well as non-azhdarchid azhdarchoids) disappear entirely. Nyctosaurids and pteranodontids appear in the Turonian-Coniacian. Meanwhile, azhdarchids steadily increase in diversity from their first appearance in the Aptian-Albian up to the end of the Cretaceous.
This turnover may be even sharper than it looks. No toothed pterosaurs in the dataset are dated strictly to the Turonian, only more broadly to the Cenomanian-Turonian (from the Chalk and Winton Formations). It’s possible, therefore, that these lineages went extinct either before or very shortly after the Cenomanian-Turonian boundary (Pentland et al., 2019). If this were the case, then the only pterosaur lineages that would make it to the Turonian are Azhdarchidae, Pteranodontia, and whatever Cretornis is. Why did so many pterosaurs go extinct at or shortly after the Cenomanian-Turonian boundary?
The Cenomanian-Turonian boundary is associated with an anoxic event. Preceded by changes in oceanic ecosystems through the Cenomanian, it ended up as the second worst extinction of the Cretaceous (Fischer et al., 2016). It heavily affected aquatic fauna, but had little to no impact on terrestrial animals (Eaton et al., 1997). Although the greatest effect seems to have been on small things, it’s likely that the extinctions at the base of the marine food chain had a ripple effect, impacting marine reptiles as well. The last ichthyosaurs disappear at the boundary (Bardet, 1992). Pliosaurs hung on for a little while after, but disappeared before the Coniacian (Albright et al., 2007). They may have been a dead clade walking: pliosaurs survived the event but could not recover, and the impact the event had led to their eventual extinction. The extinction of ichthyosaurs and pliosaurs left the seas open for mosasaurs to become apex predators; in fact it’s possible that competition from mosasaurs contributed to the final extinction of pliosaurs (Schumacher, 2011). I’ll also note that spinosaurids, who had been becoming steadily more aquatic over time (Ibrahim et al., 2020), also seem to disappear at the end of the Cenomanian.
I think a similar thing may have happened with pterosaurs. The toothed pterosaur groups that went extinct around the Cenomanian-Turonian boundary likely fed on aquatic prey such as fish and crustaceans: Anhangueria, Lonchodectidae, Mimodactylidae, and Targaryendraconia. Although they lived on the wing, they would have occupied a comparable tier on the food chain to ichthyosaurs, pliosaurs, and Spinosaurus. So these pterosaurs may have been impacted by the aquatic dieoffs in a similar way, and went extinct either at the beginning of the Turonian or not long afterward. Pteranodontids and nyctosaurids quickly replaced them, taking over fishing niches from the Turonian onward.
The disappearance of non-azhdarchid azhdarchoids is a more interesting question. They’re unlikely to have had as aquatic diets as the toothed pterosaurs of the time. Could the Cenomanian-Turonian event have had a greater effect on terrestrial fauna than we recognize, did these pterosaurs have greater aquatic components of their diet than we currently think, or is it just coincidence? The dating of the Goio-Ere Formation is of crucial importance here; it’s been dated anywhere between the Aptian and the Campanian, and since it contains Caiuajara and Keresdrakon it could potentially be the youngest definite record of non-azhdarchid azhdarchoids. For the purposes of the dataset I adopted an Aptian-Albian date after Batezelli (2015). Whether more ambiguous taxa like Argentinadraco and Montanazhdarcho are azhdarchids or non-azhdarchid azhdarchoids is also highly important here.
Of course, there are still many questions that remain unanswered. The youngest chaoyangopterid is Microtuban, which unlike every other chaoyangopterid was found in marine sediments. Might it have fed on marine food, and thus been directly impacted by the event? The sudden appearance of pteranodontians in the Turonian-Coniacian produces a ghost lineage going all the way back to the beginning of the Cretaceous, and we have no idea what “branch-pteranodonts” were like or how they survived the event. Ctenochasmatoids also fed aquatically, but there are no confirmed Cenomanian specimens to my knowledge. Did they go extinct before the Cenomanian-Turonian boundary, or have we not found them yet? And given the nature of pterosaur preservation, how much of this apparent pattern is affected by preservational bias is unknown.
Before you mention Piksi: if that thing even is a pterosaur, I consider it not diagnostic enough to any of the taxonomic categories, so it was not included. Same for Navajodactylus.
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