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Re: beaks and teeth

At 10:46 AM 2/3/99 -0500, Patrick Norton wrote:
>A few questions about beaks and teeth:

An important question here: by "beak" do you mean "rhamphotheca" (a
cornified structure) or do you simply mean "a toothless part of the snout"?

>1) Except for dinosaurs, including some Tertiary birds,

No known Tertiary bird had teeth.  Some had "pseudo-teeth" (serrated edges
to their beaks), but all birds with actual teeth died out at or before the
K-T boundary.

>I am not aware of   
>any animal that had (or has) both a beak and teeth. Are there other   
>examples of this?

Dicynodonts and trilophosaurs have toothless premaxillae and anterior ends
to the dentary: some speculate that these critters had rhamphothecae.
Dicynodonts have a pair of teeth behind this; trilophosaurs have a whole
bunch behind.

>2) Is the presence of beaks on some dinosaurs inferred or confirmed by   
>fossil evidence? I think I recall reading somewhere of an Oviraptor skull   
>that was found with bits of a fossilized beak, but I may be wrong.

It was an ornithomimosaur, actually.  Also, some hadrosaurid specimens have
been discovered with possible rhamphothecal material preserved.

Inferred beaks?  Sure: just about any dinosaur which some part of the jaws
lacking teeth has been inferred to have beaks: oviraptorids, ornithomimids,
the predentary of ornithischians, the rostral of ceratopsians, etc.

>3) Beaks are found among several vertebrate and invertebrate taxa,   
>including dinosaurs, birds, turtles, fish and mollusks. Are the beaks of   
>these animals considered examples of convergence, homologous structures,   
>or what?

Okay, folks, let's work this one out together...

To be homologous structures, beaks would have to present in the common
ancestor of all of these taxa.  Let's take mollusks & vertebrates: actually,
let's just take mollusks.  Do all mollusks have beaks?  No.  Only
cephalopods do.  Mollusks basally don't have beaks (and the vast majority of
mollusks lack beaks, with no evidence they ever had one).

How many fish are beaked?  Relatively few, considering fish diversity.  More
importantly, do the most primitive known fish have beaks?  No.  Do the fish
closest to tetrapods have beaks?  No.

Turtles and dinosaurs: all proposed turtle relatives (pareiasaurs,
procolophonids, lepidosaurs, sauropterygians) are toothed and lack beaks.
Do basal dinosaurs have beaks?  No.

Do basal birds have beaks?  Good question: they certainly have teeth.  Some
have suggested they may have had cornified tissue around the snout.  Most of
the theropod lineages considered closest to birds (dromaeosaurids,
troodontids, etc.) are toothy: no evidence of big rhamphothecae.  Really
beaky theropods (ornithomimids, oviraptorosaurs) have toothy relatives
between them and birds.

So, convergence, convergence, convergence, convergence, etc.  Heck, even the
toothless snouts of _Confusciusornis_ and Neornithes are almost certainly
convergence, because many toothed taxa (enantiornithines, ichthyornithines,
hesperornithiforms) lie between _Confuciusornis_ and the modern toothless
bird clade.

>4) Among the vertebrates with beaks, are there any osteological features   
>on the  bones of the mouth region that indicate the presence of a beak? I   
>recall a talk at the last SVP meeting speculating on the presence of a   
>keratinous sheath covering the buccal region in some dinosaurs instead of   
>"cheeks", but the evidence presented seemed (to me) more to refute the   
>existence of muscular cheeks than confirm the presence of a sheath.

Part of the Witmer (and others) study is to try and find what (if any)
osteological features truly correlate with rhamphothecae, with muscular
cheeks, etc.  Results are not yet published.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:tholtz@geol.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661