Palatodonta

Palatodonta Temporal range:Middle Triassic,
Skull diagram
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Superorder:	†Sauropterygia
Clade:	†Placodontiformes
Genus:	†Palatodonta Neenanet al.,2013
Type species
†Palatodonta bleekeri Neenanet al.,2013

Palatodontais anextinctgenusofneodiapsidreptile known from the earlyMiddle Triassic(earlyAnisianstage) of theNetherlands.It was initially described in 2013 as abasalplacodontiformclosely related to a group ofmarine reptilescalledplacodonts,characterized by their crushing teeth and shell-likebody armor.Under this interpretation,Palatodontais transitional between placodonts and less specialized reptiles. Like placodonts, it has a row of large teeth on itspalate,but while these teeth are thick and blunt in placodonts,Palatodontahas palatal teeth that are thin and pointed (like the teeth that line the jaws of most other reptiles).A 2023 study instead classified it as asauropterygomorphand the sister taxon toEusaurosphargis.In other words, it is close to, but not within,Sauropterygia(the group containing placodonts and other marine reptiles such asnothosaursandplesiosaurs).

Discovery

Palatodontawas first named by James M. Neenan, Nicole Klein and Torsten M. Scheyer in2013and thetype speciesisPalatodonta bleekeri.The generic name refers to the row of teeth on thepalatine boneof the palate. Thespecific namehonours Remco Bleeker, an amateur paleontologist, who discovered the fossil in the summer of 2010 at theSilbecoquarry nearWinterswijk.Palatodontais known from theholotypeTW480000470, a well-preservedskullof a juvenile individual.

Description

The skull is tall and has a short, blunt snout. Thepremaxillaehave a long and narrow upper branch extending between thenares(nostril holes), which are large and elliptical. Each premaxilla has four large and chisel-shaped teeth which are slightly procumbent (leaning forwards), similar to many placodonts. The teeth are strongly attached via long roots.The small size of the premaxilla and the rather anterior position of the nares are more similar toEusarosphargisand terrestrial reptiles rather than most sauropterygians.Themaxillaeare only partially preserved, forming much of the rear edge of the nares. Maxillary teeth are sharp, narrow, and slightly curved, unlike the plate-like teeth of placodonts. Six teeth were present in the right maxilla as originally preserved, though more were likely present in life. Thenasal bonesare incomplete, but they appear to have bordered the rear and inner portion of the nares. Theorbit(eye socket) was very large and circular. Its front edge was formed by a largeprefrontal,andPalatodontaappears to lack alacrimal bone.Thefrontalsform the front half of the orbit's upper edge. The lower edge of the orbit was bordered by a slender two-prongedjugal.This "boomerang-shaped" jugal is similar in shape to that ofParaplacodus;unlikeParaplacodus,it is large enough to exclude the maxilla from the rim of the orbit.

The temporal region (behind the orbit) has two large openings: anupper temporal fenestrawhich is encircled by bone, and alower temporal excavationwhich is open from below. This form of modifieddiapsidskull is common in early sauropterygians (includingParaplacodus) as well as modernlizards.The upper rear part of the skull roof is composed of theparietals,which enclose a largeparietal foramenand each possess a convex bulge in their outer rear portion. Two large curved bones separate the orbit from the upper temporal fenestra: thepostfrontalandpostorbital.The postorbital sends back a rounded plate; this plate makes up part of the postorbital bridge, a bony bar which separates the two temporal openings. The postorbital bridge is set relatively low on the skull, so that both temporal openings are well-exposed from the side. The jugal also contributes to the postorbital bridge, as well as the front edge of the lower temporal opening. It extends back far enough to contact the large and complexsquamosal,thus excluding the postorbital bone from the lower temporal opening. The tendency of the jugal to contact both the prefrontal and squamosal is a trait only shared withPlacodusamong placodonts. The rear edge of the lower temporal opening is formed by a tall and narrow bone. This bone was originally identified as aquadrate,which would have articulated with the lower jaw.An alternative interpretation considers it to be a reducedquadratojugal,which was previously reported as absent inPalatodontaand basal placodonts.

Micro-CT scanningof the specimen reveals more details of the jaw and internal structures of the skull. Themandiblewas much more slender than in placodonts, though the rear part is not fully preserved. Thedentaryhas at least 14 pointed teeth which are sharp and narrow, though they become larger and more robust further back in the jaw. The rear of the jaw includes a lowcoronoid,a thinsurangular,and an elongatedangularwhich likely contains the jaw joint. Thepalatine bone,which lies along thepalate(roof of the mouth) parallel to the maxilla, hosted a single row of large teeth similar to those of the maxilla. This is a specialization relative to most other reptiles, which lack palatine teeth or have multiple rows of tiny denticles. Placodonts are another exception with a single row of large teeth on their palatine, though their teeth are broad and plate-like. Other fragments of palate bones, such as thevomerandpterygoid,lack teeth or denticles. The only preserved portion of thebraincaseis abasisphenoid,the bone which connects the base of the braincase to the palate. The basisphenoid has two holes for eachinternal carotid arteryas well as stout vertical prongs. Various disarticulatedphalanges(finger bones) are mixed up with the skull.