Re: Majungatholus and kin

["Mickey_Mortimer" <Mickey_Mortimer11@email.msn.com>]

Erik Omtvedt wrote-

> Which theropods is the Madagascar hunter Majungatholus more closely
related to?

Majungatholus is a member of the Abelisauridae.  Although abelisaurid
interrelationships have yet to be studied, Majungatholus shares several
synapomorphies with Carnotaurus sastrei.  Together, they form the
Carnotaurinae.  At least two other carnotaurines are known- one undescribed
species from the Rio Colorado Formation of Argentina (Coria et al., 2000)
and Indosaurus from the Lameta Formation of India.  The situation regarding
Indosaurus is rather complicated.  At least two abelisaurids are known from
the Lameta Formation, Indosaurus matleyi and Indosuchus raptorius.  These
are both based on skull roofs- Indosaurus is a carnotaurine, while
Indosuchus resembles the more primitive Abelisaurus.  Chatterjee (1978)
described some cranial elements (premaxillae, maxilla, dentary tip) from the
Lameta Formation as Indosuchus.  As these are very similar to Majungatholus,
they should probably be referred to Indosaurus.  New cranial and postcranial
material from the Lameta Formation has been discovered by Chatterjee (1996),
but it is uncertain whether this is from Indosaurus or Indosuchus.
Comparison between carnotaurine taxa is limited due to the undescribed
postcranium of Majungatholus, completely undescribed remains of the Rio
Colorado form and uncertain referral of elements to the Lameta form.
Majungatholus is more similar to Indosaurus than Carnotaurus in premaxillary
morphology (Sampson et al., 1996).  The former two share small, numerous
premaxillary foramina and fused interdental plates with vertical ridges.
Details regarding the premaxilla of the Rio Colorado species are unknown.
Majungatholus is also more similar to Indosaurus in maxillary morphology.
They share- posterodorsally oriented ascending process; more maxillary teeth
(14 and 16 versus 12); longer antorbital fenestra.  These are primitive
characteristics and suggest Indosaurus lacked the ultra-short snout of
Carnotaurus.  Incidentally, the Rio Colorado carnotaurine also lacks a short
snout.  It has a promaxillary fenestra however, unlike Carnotaurus,
Indosaurus or Majungatholus.  Indosaurus is said to have the bases of
Carnotaurus-like frontal horns preserved, while Majungatholus has a central
frontal dome and the Rio Colorado species has frontal "swells".  The lack of
frontal ornamentation in outgroups makes it difficult to say what state is
primitive.  Majungatholus is more like Carnotaurus than Indosaurus in skull
roof morphology, as it has a posteriorly tapering bridge of bone between the
supratemporal fossae and anteroposteriorly narrower supraoccipital crest
behind the supratemporal fossae.  Comparison with Abelisaurus suggests these
are primitive characters.  Finally, the Rio Colorado specimen is more
similar to Carnotaurus than Majungatholus in cervical morphology because it
has cranially projecting epipophyses.  These are also seen in the noasaurid
Noasaurus however, so their absence in Majungatholus could have been
secondarily derived.  It might be noted that the postcrania illustrated by
Chatterjee (1996) is more primitive than Carnotaurus, Majungatholus or the
Rio Colorado form (unexpanded coracoid; unshortened radius and ulna;
vertical anterior preacetabular edge; posterior postacetabular edge not
notched), which leads me to believe it is from Indosuchus, not Indosaurus.
I would say the premaxillary characters suggest Majungatholus is the sister
group to Indosaurus, while the skull roof characters shared by Indosaurus
and Carnotaurus, and Majungatholus and Carnotaurus are primitive features
lacking in Majungatholus and Indosaurus respectively.  The Rio Colorado
specimen is most primitive based on the elongate snout and promaxillary
fenestra.  Also keep in mind most abelisaurids are known only from
postcrania and could be carnotaurines potentially close to Majungatholus.

Mickey Mortimer