Lagosuchus talampayensis, in its most restricted form, can only be described based on the incomplete holotype skeleton. It was a lightly built archosaur, with long, slender legs and well-developed feet - features it shares with certain dinosaurs. With its short forelimbs, long shin bones, and narrow stance, it was likely an agile
biped adapted for running.
Thomas Holtz estimated that
Lagosuchus could have obtained a total length of 1.7 ft (51 cm) and a weight similar to that of a
pigeon (50-500 g). However, these estimates may have been based on specimens referred to
Marasuchus, some of which were significantly larger than the
Lagosuchus holotype.
Vertebrae and forelimbs The
dorsal (trunk)
vertebrae had
centra about three times longer than tall, slightly more elongated than those referred to
Marasuchus. Yet the dorsals also had many traits in common with
Marasuchus, such as large excavations below the
transverse processes. In addition, they both have trapezoidal
neural spines with thickened upper edges which expand forwards and backwards to contact those of adjacent vertebrae. The contemporary
silesaurid Lewisuchus has a similar neural spine morphology, but it possesses
osteoderms on its neural spines, unlike
Lagosuchus and
Marasuchus. The two
sacral (hip) vertebrae had large and slightly tapering transverse processes. This is also the case for the first four
caudal (tail) vertebrae at the base of the tail. Further back, the caudals have much shorter transverse processes and more elongated centra, like those of
Marasuchus. The
scapula (shoulder blade) was narrow, with a slightly expanded upper tip and a thick longitudinal ridge on its inner surface. The
humerus was also quite narrow, with a subtriangular
deltopectoral crest in its upper part. The deltopectoral crest extends about 31% down the length of the humerus, making it somewhat less extensive than that of other
avemetatarsalians (including
Marasuchus). The
radius and
ulna are thin, simple, and unusually short, only about 65% the length of the humerus. In contrast,
Marasuchus has a larger ulna with a strong olecranon process. The forelimbs in general were much smaller than the hindlimbs.
Hip and hindlimbs The
pelvis (hip) is similar to that of
Marasuchus, with a thin
pubis and a plate-like
ischium which has a large ridge on its rear edge. The
femur is elongated and has a slightly inturned femoral head. The femoral head is characteristically 'globose', with a strong projecting convex surface stretching up along its inner and upper portions. This trait is shared with
Marasuchus and
lagerpetids. The femur also possessed a knob-like anterior trochanter and a distinct
fourth trochanter. The
tibia and
fibula were narrow and about 10% longer than the femur. Like other dinosauriforms, the tibia had a strong cnemial crest at the knee and a lateral groove near the ankle. The ankle has a small distal
tarsal 3, a larger distal tarsal 4, and a rounded
astragalus, but the
calcaneum is missing from the fossil. The middle three
metatarsals are elongated, with metatarsal III as the longest. Metatarsal V is short and tapers to a sharp tip. Metatarsal I is also short, though still longer than V (unlike
Marasuchus).
Phalanges are slender, and an isolated pedal ungual (toe claw) is weakly curved and flattened sideways. ==Palaeobiology==