Brocklehurst et al. 2021 review extinct and extant archosaur lungs

Brocklehurst et al. 2021 employed extant phylogenetic bracketing
to look at lung and rib design in extant taxa to understand extinct taxa, specifically archosaurs.

Unfortunately,
Brocklehurst et al. cherry-picked their phylogenetic bracket.

In other words
they added some taxa that should not be there and omitted some that should be there. In particular, the authors perpetuated the myth that pterosaurs are archosaurs. They haven’t been archosaurs for twenty one years (Peters 2000).

Furthermore
the authors consider the dinosaur-mimic Silesaurus an archosaur in-group taxon. In the large reptile tree (LRT, 1794+ taxa) Silesaurus nests just outside the Archosauria, in the Poposauria, Since basal poposaurs, like Poposaurus (Fig. 1), were also bipeds, this taxon should have been considered. The authors nest Euparkeria as the outgroup for the Archosauria. In the LRT Euparkeria is several nodes removed.

Figure 1. Poposaurus skeleton and skull. Proportions indicate bipedal configuration.

Figure 1. Poposaurus skeleton and skull. Proportions indicate bipedal configuration.

The authors note,
“Out-group comparisons show that heterogeneously partitioned lungs and a PPS are present in both archosaurs and turtles [29], and unidirectional flow is a basal diapsid character present in archosaurs, turtles and lepidosaurs [27].” 

In the LRT two unrelated clades
convergently developed a diapsid skull architecture, 1) lepidosaurs and 2) basal diapsid (Petrolacosaurus and kin). The last common ancestor of these otherwise unrelated ‘diapsids’ was Silvanerpeton, an amphibian-like reptile (= amniote) in the Viséan. Turtles and their ancestors are still not diapsids.  This yet another myth perpetuated by the Brocklehurst steam dispelled here.

Figure 1. The origin of dinosaurs in the LRT to scale. Gray arrows show the direction of evolution. This image includes Decuriasuchus, Turfanosuchus, Gracilisuchus, Lewisuchus, Pseudhesperosuchus, Trialestes, Herrerasaurus, Tawa and Eoraptor.  Note the phylogenetic miniaturization at the origin of Archosauria (Crocs + Dinos).

Figure 2. The origin of dinosaurs in the LRT to scale. Gray arrows show the direction of evolution. This image includes Decuriasuchus, Turfanosuchus, Gracilisuchus, Lewisuchus, Pseudhesperosuchus, Trialestes, Herrerasaurus, Tawa and Eoraptor.  Note the phylogenetic miniaturization at the origin of Archosauria (Crocs + Dinos).

If you’re going to study archosaur lungs
they must be seen in a valid phylogenetic context, somewhat lacking here. For instance, the swinging pubis of extant crocs is a novel trait not seen in basal bipedal crocs like Trialestes.

The authors state, 
“The crocodilian lung is thought to be a better representation of the ancestral lung morphology of archosaurs.”

Since only crocs and dinos comprise the Archosauria in the LRT, that is a reasonable consideration. However, basal bipedal crocs are sisters to basal bipedal dinosaurs (Fig. 2), so that puts ann overlooked crinkle in this hypothesis. Large, aquatic extant crocs are much more lethargic than their small, gracile, long-legged, active, bipedal ancestors were. In other words, look out for possible reversals here.

Figue 1. A new reconstruction of the basal bipedal croc, Pseudhesperosuchus based on fossil tracings. Some original drawings pepper this image. Note the interclavicle, missing in dinosaurs and the very small ilium, only wide enough for two sacrals. The posterior dorsals are deeper than the anterior ones.

Figue 1. A new reconstruction of the basal bipedal croc, Pseudhesperosuchus based on fossil tracings. Some original drawings pepper this image. Note the interclavicle, missing in dinosaurs and the very small ilium, only wide enough for two sacrals. The posterior dorsals are deeper than the anterior ones.

Don’t overlook the hallmark of flapping forelimbs,
the locked down stem-like coracoid, is present in the basal bipedal crocodylomorph, Pseudhesperosuchus (Fig. 1). Yes, they were THAT active, but that trait never really took off.  : – )

Co-author Emma Schacher talked about dinosaur lungs
in this 2019 Tedx Talk on YouTube:

References
Brocklehurst RJ, Schachner ER, Codd JR and Sellers WI 2020
Respiratory evolution in archosaurs. Philosphical Transactions of the Royal Society B 375: 20190140. http://dx.doi.org/10.1098/rstb.2019.0140
Peters D 2000. A Redescription of Four Prolacertiform Genera and Implications for Pterosaur Phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106 (3): 293–336.

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