Genus: Marasuchus Sereno& Arcucci, 1994

Classification: Archosauria - Dinosauromorpha

Etymology: Generic name refers to the mara, or Patagonian cavy (Dolichotis), a solitarily cavimorph rodent that is quite abundant on the plains of the Camp de Talampaya and around the type locality. The mara, which commonly moves with a totting (four-legged, bouncing) gait, closely resembles long-legged rabbits and hares. The name Marasuchus (mara; souchos (Greek), crocodile) thus retains the spirit of Romer’s original genus Lagosuchus (lagos (Creek) Rabbit; souchos (Greek), crocodile).

Synonyms:

Type species: M. lilloensis (Romer, 1972) Sereno& Arcucci, 1994

Other Species: none

Diagnosis: as for species

Species: M. lilloensis (Romer, 1972) Sereno& Arcucci, 1994

Etymology:

Synonyms: = Lagosuchus lilloensis Romer, 1972

Holotype: PVL 3872: Back of skull, complete left leg except for phalanges, complete right leg, a series of vertebrae including sacrum and caudal vertebrae.

Referred Specimens: PVL 4670 Articulated anterior caudal vertebrae lacking chevrons.
PVL 4671: Articulated anterior caudal vertebrae with chevrons.
PVL 4672: Articulated vertebral colum from atlas to the 17th presacral vertebra.
= Lagosuchus talampayensis Bonaparte, 1975 (partim)
PVL 3870: Very back of skull, posterior part of premaxilla, maxilla, 9 cervicals, 11 dorsal vertebrae, ilia, pubi, ischia, femur, tibia,
fibula, metatarsals, phalanges.
PVL 3871: Scapula and coracoid, humerus, ulna radius, femur.

Time: Ladinian, Middle Triassic.

Horizon: Los Chanares Formation.

Location: “area de Los Chanares, 2 km al SE de la Puerta de Talampaya.”, more accurately located 3 km north of the north branch of Chanares River and 5 km southwest of the Puerta de Talampaya, Chanares beds, Province of La Rioja, Argentina.

Total length:

Mass:

Diagnosis: Dinosauriform archosaur characterized by anterodorsally projecting posterior cervical neural spines (presacral vertebrae 6-9), marked fossa ventral to the transverse processes in the posterior cervicals and anterior dorsal vertebrae (presacral vertebrae 6 through 10 or 12), subtriangular neural spines in mid- and posterior dorsal vertebrae that contact each other anteriorly and posteriorly, mid-caudal centra twice the length of anterior caudal centra, elongate anterior chevrons that are more than three times the length of the first caudal centrum, broad scapular blade, transversely concave distal pubic blade, and transversely narrow fibular articular surface on calcaneum. Marasuchus lilloensis is a small, carnivorous dinosauriform that is particularly closely related to Dinosauria, sharing with dinosaurs parallelogram-shaped posterior cervical centra, short forelimbs, a partially open acetabulum, an acetabular antitrochanter, a trochanteric shelf on the proximal femur, and a fibular flange on the tibia (Sereno and Arcucci, 1994).

Comments:Sereno and Arcucci review the morphology and systematics of Lagosuchus from the Middle Triassic Los Chaiiares Formation. Two species have been named, Lagosuchus talampayensis and L. lilloensis. The holotype of Lagosuchus talampayensis is a fragmentary skeleton that does not exhibit any autapomorphies to distinguish it from contemporary dinosauromorphs, and the genus and species are regarded as nomina dubia. In contrast, the holotype of Lagosuchus lilloensis is a well-preserved skeleton that exhibits several distinctive features, allowing reference of additional specimens. Therefore, designated a new genus, Marasuchus, for the species "Lagosuchus" lilloensis.
Lagosuchus talampayensis is based on a poorly preserved skeleton cited by Romer (1971) as MLP 64-XI-14-1 1 (in part) and now recatalogued as UPLR 09. The holotypic skeleton is exposed on a slab that also contains a partial skeleton of the small proterochampsid Tropidosuchus romeri, the holotypic skeleton of Gracilisuchus stipanicicorum, and other disarticulated bones. Romer's (197l) diagnosis of L. talampayensis does not include any unique features. The accompanying figure of the tarsus and pes is a reconstruction based principally on better preserved materials that were later referred to the same species by Bonaparte (1975). Romer (1971) also figured and referred an unnumbered fragmentary pelvis and proximal femur to L. talampayensis (now catalogued as PVL 387 1). Romer (1972) figured the appendicular portion of the holotype skeleton and referred an additional partial skeleton to Lagosuchus talampayensis (cited as MCZ 4116 and now re-catalogued as MCZ 4137). In the same paper, Romer named a second species,
Lagosuchus lilloensis, differentiated only by its larger size. Romer figured the left hind-limb and a series of sacral and caudal vertebrae (PVL 3871). This skeleton, however, also includes most of the presacral vertebrae, the left pectoral girdle and forelimb, and the right hind-limb. Romer did not provide a diagnosis for Lagosuchus lilloensis. In his review of L. talampayensis, Bonaparte (1975) considered L. lilloensis a junior synonym of L. talampayensis. In the same work, Bonaparte referred additional remains to L. talampayensis (PVL 3870, 3872).

Holotype of Lagosuchus talampayensis - UPLR 09, fragmentary, semi-articulated postcranial skeleton including the vertebral column from the mid-dorsal to the anterior caudal vertebrae, fragmentary left pectoral girdle and forelimb including the proximal half of the scapula, humerus, radius, and ulna, pelvic girdle and both hind-limbs including the ventral margin of the left ilium, both ischiae and pubes, both femora, partial right and complete left tibiae and fibulae, and left tarsus and pes lacking most of the phalanges.
The holotype of L. talampayensis is preserved on a slab with the holotype of Gracilisuchus stipanicicorum (UPLR 08) and the remains of the small proterochampsid Tropidosuchus romeri (UPLR unnumbered). Romer (1971; 1972) figured the partial forelimb and fragmentary hind-limbs. Apparently, he originally considered a nearby series of vertebrae as part of the holotype but later regarded them as caudal vertebrae of Gracilisuchus. These vertebrae are clearly too large to belong to the holotype of L. talampayensis and represent cervical and dorsal vertebrae of the small proterochampsid Tropidosuchus romeri. Although not mentioned by Romer, the hind-limbs of the holotype are clearly associated with pelvic fragments and a series of 18 small vertebrae, composed of dorsal vertebrae anteriorly and caudal vertebrae posteriorly. The hind-limbs and fragments of the pelvic girdle are positioned to each side of the presumed sacral region. A partial left forelimb is located at some distance anterior to the vertebrae and is clearly included as part of the holotype by Romer. The forelimb is articulated with a partial scapulocoracoid that was not mentioned or figured by Romer. Given its size and location on the slab, the association of the forelimb and scapulocoracoid with the remainder of the holotype is plausible but not without some doubt.

Based solely on its larger size, Romer (1972) named a second species of the genus Lagosuchus, L. lilloensis. The holotype of L. lilloensis (PVL 387 I), however, exhibits several diagnostic characters, which either are not preserved or are at variance with then holotype of Lagosuchus talampayensis. These species, therefore, cannot be synonymized, as suggested by Bonaparte (1975), and we transfer the species L. lilloensis to the new genus Marasuchus. All but one of the specimens previously referred to L. talampayensis can be referred to Marasuchus lilloensis on the basis of the above diagnosis. The one exception is MCZ 4137, a fragmentary specimen preserving the dorsal and sacral vertebral column and a fragmentary pelvis (field number 174, formerly catalogued as MCZ 4116). The neural spines in the dorsal vertebrae of this specimen alternate in shape between subtriangular and rectangular. An alternating pattern in the shape of the neural spines is unknown elsewhere among archosaurs but is quite common among primitive
tetrapods. Unlike the spine pattern in primitive tetrapods, however, both spine shapes in this specimen are equal in height. Bonaparte (1975) described this condition as "surely pathological," but no part of the column appears abnormal or asymmetrical. Because the anterior dorsal vertebrae, fragmentary ischium, and pubis are similar to the corresponding bones in M. lilloensis, the alternating pattern of neural spine shape either constitutes an unusual case of sexual dimorphism or may indicate the presence of an additional closely related taxon.
Two small bipedal archosaurs in the Middle Triassic Los Chaiiares fauna, Lagerpeton chanarensis and Marasuchus lilloensis, provide the best available evidence of ornithodirans that predate the Late Triassic radiation of dinosaurs. Comparative analysis of L. chanarensis, M. lilloensis, and other ornithodirans strongly suggests that these two contemporary ornithodirans share a more recent common ancestry with dinosaurs (as Dinosauromorpha) than with pterosaurs. Within Dinosauromorpha, M. lilloensis and dinosaurs appear to be more closely related (as Dinosauriformes). Several supporting synapomorphies include a gap in the acetabular wall between the ilium and pubis, an antitrochanter on the posterior wall of the acetabulum, extension of the articular surface on the femoral head onto its ventral aspect, and a posterolateral flange on the distal end of the tibia. These dinosauriform synapomorphies involve novel articular relations and processes for muscle attachments that may indicate modifications in the function of the hind-limb. Although their functional significance remains obscure, these anatomical modifications persist among dinosaurian descendants, many of which attain body sizes more than two orders of magnitude greater
than that of L. chanarensis and M. lilloensis.

Illustrations:

Reconstruction of Marasuchus lilloensis (based on PVL 3870 and 3871). (from Sereno, P. C., and Arcucci A. B., 1994)

References:

Romer, A. S., 1971, The Chanares (Argentina) Triassic reptile fauna X. Two new but incompletely known long-limbed pseudosuchians: Brevoria, n. 378, p. 1-10.

Romer, A. S., 1972, The Chanares (Argentina) Triassic reptile fauna. XV. Further remains of the thecodonts Lagerpeton and Lagosuchus: Breviora, n. 394, p. 1-7.

Sereno, P. C., and Arcucci A. B., 1994, Dinosaurian precursors from the Middle Triassic of Argentina: Marasuchus lilloensis, gen. nov: Journal of Vertebrate Paleontology, v. 14, n. 1, p. 53-73.

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