How do you distinguish spinosaur tooths from plesiosaur tooths?

[LordWampa]

Just what the title says. What are the characterictics you search for in a tooth to do the ID?

[Troodon]

Spinosaurid teeth from the Kem Kem have strong mesial and distal carinae with a straight crown typicallythe entire length of crown..  Occasionally the carinae will be beaded.  Flutes(ridges) can be present on one, both or none of the crown and they run the full length of the crown.  One cannot differentiate teeth between the different species of Spinosaurids in the Kem Kem

 

Leptocleididae is the plesiosaur genus present in the Kem Kem.

 The papers description of the teeth:

Dentition. Crowns are long and slender and taper to a sharp point, Basally, the crown’s cross section is  subcircular to slightly ovoid, with maximum diameter slightly more than minimum diameter; the crown becomes more labiolingually flattened and ovoid toward the tip. Crowns have the sigmoidal curvature typical of plesiosaurs. Crowns range from weakly recurved to hooked.  Variation occurs with recurved teeth near the front of the jaws, and straighter teeth near the middle of the jaws. Thin enamel ridges form the carinae. The anterior carina projects to form a cutting edge near the apex, but a cutting edge is absent basally; neither does the posterior carina contribute to a cutting edge. The enamel has a smooth, glossy surface, but bears a series of sinuous enamel ridges on the mesial, lingual, and distal surfaces; the labial surface has fewer ridges.  There are 15-20 ridges per tooth, with more ridges basally than apically. Many short ridges near the base disappear or converge with other ridges as they extend apically; a few short ridges also arise near the tip.

Paper:

Bunker, Georgina; Martill, David M.; Smith, Roy E.; Zouhri, Samir; Longrich, Nick (1 December 2022). "Plesiosaurs from the fluvial Kem Kem Group (mid-Cretaceous) of eastern Morocco and a review of non-marine plesiosaurs

[LordWampa]

Thank you so much! So if I understand it properly, if I buy a tooth from Kem Kem it may be a S. Aegyptiacus or a S. Marocannus right? So the maximum presicion you can have is Spinosaurus sp.

[Troodon]

 

Identification is best as Spinosaurid indet.  Thats because you are looking at:

 

Sigilmassasaurus brevicollis

 

and

 

Spinosaurus aegyptiacus but many paleontologists do not agree with that and identify it as Spinosaurus sp.

So lots of debate what the species are and how many exist.