[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

New theropod


Jonathas de Souza Bittencourt
Alexander Wilhelm Armin Kellner

Theropod remains are rare in Brazil. Most come from the Albian Romualdo
Member (Santana Formation, Araripe Basin), from where three species have
been described: the spinosaurids Irritator challengeri and Angaturama
limai (both based on cranial material), and the tyrannoraptoran
Santanaraptor placidus (posterior portion of a skeleton). Other
specimens are an unnamed sacrum with fragments of a right ilium (SMNS
58203), an incomplete posterior region of a small coelurosaurian
skeleton (SMNK 2349 PAL), and a large pelvis with elements of the
vertebral column and limbs (MN 4802-V). To those, we add a sequence of
sacral and caudal vertebrae (MN 4743-V), which are described here.

MN 4743-V consists of three tightly articulated (but unfused) posterior
sacral vertebrae and six anterior caudals. Three anterior chevrons are
also preserved, one of them complete. All centra are amphycoelous and
spool-shaped. No evidence of pleurocoels or ventral groove were
observed. The last two sacral vertebrae bear well preserved ribs, which
are fused with the transverse processes forming broad lateral processes.
These latter display the articulation area for ilium. The caudal centra
are taller than high and shorter than the sacrals. The ventral margins
of the posterior articular facets, where the chevrons articulate, are
bevelled. In lateral view, the posterior portions of the centra are
projected ventrally relative to the anterior parts. Neural spines are
high, showing broad bases and getting thinner toward the apex. The
transverse processes are directed upward and backward, supported below
by two buttresses that delimit the infraprezygapophyseal,
infradiapophyseal and infrapostzygapophyseal fossae. The prezygapophyses
are elongated and sharply pointed in lateral view. The postzygapophyses,
clearly observable only on the last preserved caudal, are
dorso-ventrally elongated facing laterally and downward. The neural
canal is rounded, margined by low pedicels, and decreases to half of its
diameter from anteriormost preserved sacral until eightieth caudal.

MN 4743-V lacks two synapomorphic features of the Ceratosauria: a
synsacrum and a shallow groove on the ventral margin of the anterior
caudal centra. The new specimen has the sacral centra ventrally arched,
differing from the flattened condition observed in coelurosaurs.
Furthermore, MN 4743-V lacks another coelurosaurian synapomorphy:
anterior chevron with proximal end elongated anteroposteriorly,
flattened and platelike. The  pleurocoelous sacral centra of SMNS 58203
apparently have the ventral margin of anterior articular facet ventrally
projected relative to posterior facet, a condition quite distinct from
that observed in the material described here.

Although the relationships of MN 4743-V cannot be established at the
time being, the presence of two buttresses below transverse processes of
anterior caudals, similar to the condition observed in Baryonyx walkeri,
suggests spinosaurid affinities. Furthermore, there are some
similarities with the undescribed sacral vertebrae of MN 4802-V, also
tentatively attributed to the Spinosauridae. In any case, based on the
current knowledge, the comparisons with the five theropod specimens from
the Romualdo Member in which sacral and/or caudal vertebrae were
recovered indicate the presence of perhaps four different species. This
suggest that the theropod diversity of the Santana Formation is higher
than previously thought.

In: III Simpósio Brasileiro de Paleovertebrados, Resumos. Rio de
Janeiro, RJ.