Evolution of avian-like rictus

[Vili Koskinen]

Hello everyone,

My name is Vili Koskinen

I am a museum exhibition technician/palaeontological sculptor from Finland.

I'm interested in the scientific consensus regarding the evolution of avian-specific mouth corners/cheeks (rictus, the soft tissue structures around the beak, distinct from mammalian cheeks).

Specifically, at what evolutionary stage or in which groups did these distinct avian soft tissue structures likely emerge? Is there an expectation to find such specialized mouth structures in the earliest representatives of the Paraves group, particularly those that still possessed teeth?

Thank you for your insights!

Regards, Vili Koskinen

[Jaime Headden]

The avian rictus is (probably) a remnant of the sauropsid rictus that exists still in crocodilians, in lizards, snakes, and even turtles. This corresponds to a fleshy, fatty region without muscles innervated by the rictal branches of the trigeminal nerve (V). The rictus has not been well studied in non-avian sauropsids, but in mammals, it's much better studied, but only in comparison to the buccal muscles/nerves, which are hardly correspondent structures (given what mammals do to their facial muscles during ontogeny.

There's not much to go on here because, unfortunately, there's not much study done to it: its innervation, dermal structure, keratinization in some taxa, how small and large it can be, etc. At some point down the road, some PhD will assign the task to a student and we'll get some work on the tissue organization for the region and its comparative anatomy. Casey Holliday had something to say about it years ago due to a rise in discussion on whether the rictus was a comparable structure to dinosaur "cheeks" (this being before Ali Nabavidadeh's work on the region, which still leaves open this enormous gap between the upper and lower jaws, without filling it with knee-jerk "well, something has to fill it" cheek analogues).

Cheers,

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Jaime A. Headden

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[Gregory Paul]

Note that condors have exceptionally well developed cheeks, as I illustrate on p 29 of the Princeton Dinosaur Guide 3rd edition. 

GSPaul

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[Jaime Headden]

Note that "rictus" (the Mundplatte) is distinct from the cere, which in turkeys, condors, etc. can enveloped the rictus. I would be cautious about conflating the two structures.

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[Vili Koskinen]

Thank you Jaime and Gregory for your comments!

This subject is fascinating, yet frustrating!

The following is a quote from artist R.J Palmer. What do you think?

My personal take is when you have lips you have the expose jaw meat, when you have a beak or no lips, you have the bird cheek. Bird cheek is used to help form a mouth seal with the inflexible beaks. When ya got lips, its not needed.

Is the function of rictus or variety of rictuses in birds how well studied? Is that comment valid?

Cheers!

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[Jaime Headden]

There's been a lot of talk about the so-called oral seal and its importance in retaining moisture, preventing desiccation, decay and aerial exposure of teeth, etc., without a lot of hard data. Some conference abstracts and work in progress have been summarily reported, but nothing concrete regarding the nature and necessity of specific tissues in specific arrangements. There's a few things we do know regarding that region of the avian face: There's a rictus, it's amuscular, it can be extensively and sensorily innervated, and it may perform a function in oral intake in some birds, such as flamingos, in (get this) forming a negative pressure zone in the mouth during feeding. But it's hardly required: phalaropes and other probing birds can do this while opening their extremely long beaks. For birds like hummingbirds, it's the only way for them to drink.

The necessity of an "oral seal," when it comes to extensive oral integument as with "lips," is a different argument, and doesn't really touch on the rictus (or the cheek region) at all. Separating the three modules here (lips, dental desiccation and moisture loss in the mouth, and the development of short or extensive "false cheek") will be important going forward. "Lips" in snakes and lizards are innervated by the same nerves that innervate the beak in birds and the external facial skeleton of crocodilians, but not the rictus. So that, too, becomes an important quality to consider.

Now, if we were to talk about the "oral seal" and persons' opinions on it, we can do that. Without further evidence for its necessity, I would caution bandying the term around, as it's become something of a bugbear in the paleo periphery. It sounds  too reasonable, with a lot of "that sounds technical, so it must be plausible" sense to it, and without a lot of data to back it up. The actual evidence will be a lot more complicated and likely involve a number of caveats, as with anything we don't really understand quite yet. The recent work on the novel muscle "exoparia" by Sharpe et al. is an example of what levels and amount of evidence is required to infer what seems a logical and plausible conclusion. The same goes for Nabavizadeh's work on the anterior branch of the mAME complex, which intrudes into the "buccal muscle" territory that edges its way into the discussion on rictuses (and which supplants the "cheek muscle" to some degree). Arguments for a tissues necessity without evidence for it will leave a lot of people grasping for evidence to their conclusions.

[1] https://onlinelibrary.wiley.com/doi/full/10.1111/joa.14242

[2] https://anatomypubs.onlinelibrary.wiley.com/doi/10.1002/ar.23988

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[Vili Koskinen]

Thank you again Jaime!

Your comments are appreciated! They are thoughful, informative and clears things what is known and what is not about these oral tissues.

Yes, those studies made by Sharpe et al. and Nabavizadehin are good examples and we should expect the same approach when on is studying, well...anythig!

Two more questions:

1. About keeping teeth moist with lips- argument. What about ichtyornis-like toothed birds? They seem to have well developed beak, but also true teeth. This animal goes against that tight-seal -in -the- mouth- argument

2.So, is lizard/snake-like "lips" that extends all the way back to the jaw joint the conservative but safe choice, if I would reconstruct, say a microraptor or deinonychus?

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[Jaime Headden]

The questions being asked are themselves tricky, falling under the umbrella of what was discussed before. Indeed, the entire subject of what was around the mouths of extinct sauropsids and even basal therapsids is an open question, involving various conditions of the Extant Phylogenetic Bracket, the "null" hypothesis and how we reconstruct that, degrees of inference based on half-understood conditions of bone texture, microstructure, Sharpey's fibres, and natural preservation and what is found/inferred from that. And of course, the "common sense" that gave rise to arguments such as the necessity of an oral seal, and so forth.

But there are some ... speculations. I'd rather not invest too much time in them here. I already talked about Ichthyornis (in connection to Hesperornis) here, and the reasons why I reconstruct it with a half-beak/half-lip, but that's really just scraping the tip of the research which is further referenced throughout the body of work being talked about in this thread (not anything to do with me, mind you).

Of particular note here, is the nature of inference (a logical argument, not a scientific one; although they should align they don't always) and the null. The null is the basic argument from which all things spring, the natural assumption based on various preferred or observed conditions. "All birds have beaks" is a null, because there is no contradiction in extant taxa. To infer that things such as the spikes on pelagornithid beaks are "teeth" requires extraordinary evidence which has been lacking, but was discussed at one time or another; so we revert to the null, and assume, from the beginning, that the argument is "if the spikes are part of the beak, then..." We also get into a bit of begging, but that's how these things go.

As a result, we can only really speculate from what we see: fish, amphibians, various of them, mammals, squamates, turtles, crocs, and of course, birds. All of these tetrapod and outlier groups give us the various constraints upon which we depend, and from them (or because of them) we are required to use them as the models for our extinct taxa, some far removed in morphology from their origin. Classically, it's "if crocs don't have lips, and birds don't have lips, do dinosaurs?" Or, "if turtles don't have lips, but snakes and lizards have lips, then do ichthyosaurs?" Or, "if teeth desiccate while not inundated with water (we're begging here, pardon me) then do pterosaurs have some method of preventing this when their teeth are always exposed?" Consider long-tooth flying foragers such as many anhanguerids and rhamphorhynchids. They would spend most of their time out of the water, so the aerial exposure to their teeth must be great. So... do they desiccate?

These we want to know before we can reasonably answer questions like "do dinosaurs really have lips?" with such certainty that all of paleontology follows along. We're getting there, and the concensus is, it seems to me, falling that way among opinions. But facts? Slightly behind.

Anyways, I hope this helps.

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[Vili Koskinen]

Thank you for the response! it certainly gave me plenty to think about! I do appreciate the effort! 

All the best!

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