Anurognathid eyes: the evidence for a small sclerotic ring

Ever since Bennett 2007, pterosaur workers have been following like sheep to a shepherd and putting a giant sclerotic ring in the antorbital fenestra. That’s a problem. Andres et al. (2010) even reported the confluence of the naris and antorbital fenestra (but to his eye, by placing the sclerotic ring in the antorbital fenestra, there was only one opening ahead of it, the naris). And this he used as a character to unite pterodactyloids with anurognathids in phylogenetic analysis. (I’m not sure it can get any worse than this.)

Figure 1. The flathead anurognathid in visible light (above) and UV (below). Sclerotic rings n pink. Maxillae in blue.

Figure 1. Click to enlarge. The flathead anurognathid in visible light (above) and UV (below). Sclerotic rings n pink. Maxillae in blue. Nasals highlighted in upper left image.

Sclerotic rings are bones formed in the scleral portion of the eyeball. They form a disc around the iris. Then everything rots, they usually fossilize as a ring or the scattered remnants thereof. I have never seen such a ring fossilized standing on edge in a crushed fossil. But this is what you have to believe you get if you follow the Bennett 2007 hypothesis in the flathead anurognathid (Fig. 1, SMNS 81928a & b). Witton, Andres and others bought into this.

When I took a look at that fossil, I found two small sclerotic rings in the back half of the skull (Fig. 1), where they are in all other pterosaurs and anurognathids. What Bennett took to be a giant sclerotic ring on the left is actually the maxilla, convex ventrally. It is difficult to make out the bones of the skull in that fossil, but I have provided a guide here.  Putting the bones back together in a reconstruction seals the deal (Fig. 6). Everything fits and there’s only gradual, not radical, change between sister taxa.

Figure 2. Click to enlarge. Batrachognathus with sclerotic ring impressions highlighted.

Figure 2. Click to enlarge. Batrachognathus with sclerotic ring impressions highlighted. This specimen has the largest eyes known among anurognathids.

In Batrachognathus (click this link to see all bones identified) some of the bones are red/brown. Others are represented by impressions. Such is the case with the sclerotic rings. Here they are larger and more owl-like than in other anurognathids. Nevertheless, a skull reconstruction (Fig. 6) is fairly standard in most aspects.

Figure 3. Click to enlarge. The GLGMV 0002 specimen attributed to Dendrorhynchoides. Sclerotic rings highlighted.

Figure 3. Click to enlarge. The GLGMV 0002 specimen attributed to Dendrorhynchoides. Sclerotic rings highlighted.

The GLGMV 0002 specimen (Fig. 3, Hone and Lü 2010) has two small sclerotic rings at the back of its skull. See the reconstruction (Fig. 6) to make sense of this roadkill.

Figure 4. Click to enlarge. Sclerotic rings on Jeholopterus.

Figure 4. Click to enlarge. Sclerotic rings on Jeholopterus.

Jeholopterus, the vampire pterosaur, also has two small sclerotic rings at the back of its skull. Nothing big filling the front half here.

Figure 5. Click to enlarge. Anurognathus holotype with bones identified. Here the sclerotic ring is not so clear, but the jugal is. And the jugal carries the sclerotic ring.

Figure 5. Click to enlarge. Anurognathus holotype with bones identified. Here the sclerotic ring is not so clear, but the jugal is. And the jugal carries the sclerotic ring. Orange = nasal. Yellow = pmx. Pink = lacrimal. Green = maxilla. Blue = jugal. Digital graphic segregation helps locate and identify elements here. That mass of green is wing membrane.

Ironically, Anurognathus has a very small sclerotic ring over its small jugal. While there is a large antorbital fenestra here, which Bennett and Andres would call the orbit, there is no sclerotic ring in there. It should be very easy to see if present. Trouble is, it’s not present. Observations should be repeatable. Bennett called the flathead pterosaur a juvenile Anurognathus. If they are conspecific, they should have the same bones. So, where is the giant sclerotic ring?

Figure 1. Anurognathid skulls in phylogenetic order.

Figure 6. Anurognathid skulls in phylogenetic order.

The variety in anurognathid skulls is a wonder to behold, but the little monster created by Bennett (20078) stands out as an outlier in this group of skull reconstructions (Fig. 6). Clearly something is wrong here (not duplicated in any other anurognathid).

References
Andres B, Clark JM and Xing X 2010. A new rhamphorhynchid pterosaur from the Upper Jurassic of Xinjiang, China, and the phylogenetic relationships of basal pterosaurs, Journal of Vertebrate Paleontology 30: (1) 163-187.
Bennett SC 2007. A second specimen of the pterosaur Anurognathus ammoni. Paläontologische Zeitschrift 81(4):376-398.
Hone DWE and Lü J-C 2010. A New Specimen of Dendrorhynchoides (Pterosauria: Anurognathidae) with a Long Tail and the Evolution of the Pterosaurian Tail. Acta Geoscientica Sinica 31 (Supp. 1): 29-30.

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