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Fossil Friday: The Temporal Paradox of Early Birds

Photo: Xiaotingia, by Bruce McAdam, CC BY-SA 2.0 , via Wikimedia Commons.

Xiaotingia is a feathered dinosaur from the Jurassic of China, which is about 5-10 million years older than the famous Archaeopteryx. Its description (Xu et al. 2011) attracted wide media coverage, because the included phylogenetic analysis dethroned the iconic “missing link” Archaeopteryx as the oldest and most basal bird, and instead attributed it to the dromaeosaurid dinosaurs, better known as raptors. Soon after, another study found Archaeopteryx to be closer to birds (Avialae) and Xiaotingia to be a relative of Anchiornis within the troodontid dinosaurs (Lee & Worthy 2011). An updated analysis by Senter et al. (2012) also found Archaeopteryx closer to birds but Xiaotingia as a dromaeosaurid raptor rather than related to Anchiornis in Troodontidae.

A new and very comprehensive analysis of 1,500 anatomical characters by Godefroit et al. (2013) again repatriated Archaeopteryx back into the bird lineage, but placed Xiaotingia one branch closer to modern birds than Archaeopteryx. A year later came a study of 853 characters by Brusatte et al. (2014), which confirmed the avian position of Archaeopteryx, but placed Xiaotingia outside of birds and again within troodontid dinosaurs. Finally, Cau (2018) placed Xiaotingia as sister group to the Scansoriopterygidae, which are of highly controversial phylogenetic position. Every new study with a different character set reshuffles the tree and proposes new relationships, only to be overturned soon after. Even though I support common descent, I have to admit that such stuff hardly qualifies as the rock-solid evidence that fossils are claimed to provide for Darwin’s tree of life.

An Even Bigger Controversy

Just two years later an even bigger controversy emerged about another feathered dino-bird from China. A wonderfully preserved bird-like fossil, named the dawn bird, Aurornis xiu, was described by Godefroit et al. (2013) from the Jurassic (165-153 mya) of China. The authors considered Aurornis as more basal and somewhat older than Archaeopteryx. But an embarrassing little piece of information was hidden in the online supplementary information of this sensational Nature publication: the Aurornis fossil had not been found during the team’s excavations in China, but had been acquired from a fossil trader. The authors even acknowledged that the fossil might also come from the Cretaceous Yixian locality and could be up to 35 million years younger (Balter 2013), which would of course invalidate its status as oldest bird.

Luis Chiappe, one of the world’s leading experts on fossil birds, commented that the dubious origin from a trader and exquisite preservation of the fossil should raise eyebrows. His colleague Stephen Brusatte argued that “in the case of things like Aurornis, CT scanning is essential. Without it, there will always be lingering doubt that the specimen is genuine” (Balter 2013). After all, paleontologists should be a bit more suspicious after the big scandal around the forged Archaeoraptor fossil in 2000 (Rowe et al. 2001). As Aurornis has not been CT-scanned until today, the jury is still out. That aside, the above mentioned new phylogenetic study by Brusatte et al. (2014) recovered Aurornis from birds and placed it within troodontid dinosaurs. That agrees quite well with its wings which appear to be too short for active flight. So, even if it should be a genuine fossil, Aurornis could be younger than Archaeopteryx and/or not a bird at all. The most recent study by Pei et al. (2017) suggested that Aurornis xui is most likely not a valid taxon and indeed just a synonym of Anchiornis huxleyi. But at least The Guardian had its cute headline, “Early bird beat Archaeopteryx to worm by 10m years” (Sample 2013).

The “Temporal Paradox”

A well-known problem of the avian fossil record is the fact that the earliest birds are older than all the fossils of their assumed ancestors and closer stem group representatives. This “Temporal Paradox” was described by bird paleontologist Alan Feduccia (1994, 1996). A good example is the case of the theropod raptors (Dromaeosauridae), which are considered to be close dinosaurian relatives of birds. However, their bones were only known as skeleton fossils from the Cretaceous and thus are at least 30 millions years younger than the oldest known bird, Archaeopteryx, from the Upper Jurassic. 

Therefore, some paleontologists have claimed that the temporal paradox has been solved by the discovery of Jurassic dromaeosaurids. A closer look reveals that these are all very incomplete fossils of dubious taxonomic affinity. The alleged Jurassic dromaeosaurids are only represented by isolated teeth from the Middle Jurassic of England (Metcalf et al. 1992), Scotland (Brusatte & Clark 2015), Madagascar (Maganuco et al. 2005), and China (Han et al. 2011), as well as the Upper Jurassic of Utah (Koparion, which is a troodontid and contemporary with Archaeopteryx; Chure 1994), and the Late Jurassic of Portugal and Germany (e.g., Lubbe et al. 2009). All these descriptions at best mention vague similarities with dromaeosaurid teeth without specifying any clear shared derived character states (putative synapomorphies) that would be necessary for a rigorous phylogenetic attribution to this taxon. This problem is complicated by the fact that theropod teeth are highly variable and often not diagnostic, so that “in general it can be hard to tell with confidence exactly which groups a given tooth might (or might not) belong to” (Hone 2011).

What about some other fossil finds that might help to solve the temporal paradox? There is indeed a whole range of feathered dinosaurs that have been described from the Middle and early Late Jurassic of China in past years. These include the genera Anchiornis (incl. Aurornis), Aorun, Eosinopteryx, Guanlong, Haplocheirus, Kileskus, Koparion, Ornitholestes, Pedopenna, Serikornis, Shishugounykus, Xiaotingia, and Zuolong, as well as four or five genera of the strange family Scansoriopterygidae, which possessed bat-like wings and may even have developed flight independently from birds (Wang et al. 2019). These taxa may well contribute to a minor amelioration of the temporal paradox, but they certainly have not solved it yet.

The biggest remaining problem is the phylogenetic uncertainty in most of these taxa, which some authors consider to be bird-like or more bird-like than Archaeopteryx (e.g., Anchiornithidae and Scansoriopterygidae), while other consider the same taxa as less bird-like than even Troodontidae and Dromaeosauridae. For example, Aorun is a coelurosaurian dinosaur that is 11.6 million years older than Archaeopteryx. However, it does not belong to the dinosaur-bird clade with pennaceous feathers (Pennaraptora). Instead, according to Xu et al. (2018) it belongs to the more primitive group Alvarezsauroidea. Kileskus and Guanlong from the Middle Jurassic of China and Proceratosaurus from the Middle Jurassic of England have been attributed to Tyrannosauroidea, but this is of course also a very basal theropod clade outside the feathered Pennaraptora. Juravenator from the Upper Jurassic Sonhofen limestone of Bavaria may have been a member of Maniraptora (Butler & Upchurch 2006) but this is still controversial and Juravenator was not older than but contemporary with Archaeopteryx. The systematics of all these taxa are still a big mess. But if Anchiornithidae (incl. Aurornis, Eosinopteryx, PedopennaSerikornis, and possibly Xiaotingia) and/or Scansoriopterygidae should indeed be Avialae (“birds”) like Archaeopteryx, as some specialists say (see above and Senter 2007), then the whole temporal paradox would be back big time, because none of the other and more basal coelurosaurids are older than those. This is another reason, why the problem cannot be claimed to be solved yet.

Alive and Kicking

So, for the moment being the temporal paradox of early birds is still very much alive and kicking, and it is supplemented by the problem of the Big Bang of Tertiary birds (Feduccia 19952003). A comprehensive phylogenetic study more recently confirmed such “a major radiation of crown birds in the wake of the Cretaceous–Palaeogene (K–Pg) mass extinction” (Prum et al. 2015). Wherever you look in the fossil record you stumble upon problems for the Darwinian story and evidence that is better explained by intelligent design. That’s why I as a professional academic paleontologist came to doubt neo-Darwinism and embrace the theory of intelligent design.


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