Gondwanax paraisensis

Reconstruction of Gondwanax paraisensis（感謝Müller RT提供）

Age

Triassic（Ladinian-Early Carnian）

237-232 Ma

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Taxonomy

Kingdom: Animalia

Phylum: Chordata

Class: Sauropsida

Superorder: Dinosauria (?)

Order: Ornithischia (?)

Family: Silesauridae

Genus: Gondwanax

Species: Gondwanax paraisensis

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Morphological description

The holotype of Gondwanax paraisensis consists of a complete right femur (CAPPA/UFSM 0417), displaying a slightly curved profile. The medial articular surface of the proximal femur is subtly convex. The anterior trochanter is directly connected to the femoral shaft, lacking a distinct separation. The posteromedial tuber is poorly developed, and the fourth trochanter on the posterior surface of the femur forms a low, subdued prominence rather than the crest-like structure commonly observed in other silesaurids.

The sacral region is composed of three vertebrae that are not fused at their centra. Their transverse processes extend laterally, forming a platform-like structure similar to that seen in other silesaurids. The increased number of sacral vertebrae suggests a more derived condition in sacral evolution. In the anterior caudal vertebrae, the centra are short and robust, with relatively low neural spines that lack transverse expansion. Shallow depressions are present on the lateral surfaces of the vertebrae. The cervical and dorsal vertebrae are generally parallelogram-shaped in lateral view, with elongated and slender neural spines.

The pelvic girdle consists of a right ilium and a pair of ischia. Although the ilium is incomplete, the inner wall of the acetabulum is preserved and resembles the condition seen in most silesaurids. The ischia are elongated, with a circular cross-section along the shaft and a slightly expanded, oval-shaped distal end. Overall, the pelvic morphology closely resembles that of other silesaurids.

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Etymology

Gondwanax: Derived from "Gondwana" and the Ancient Greek word anax, meaning "ruler" or "lord", referring to the dominance of this lineage during the early stages of the Mesozoic.

paraisensis: Refers to Paraíso do Sul, Rio Grande do Sul, Brazil, specifically the Linha Várzea 2 locality where the fossil was discovered.

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Biological Description

Gondwanax paraisensis is a silesaurid from the Middle to Late Triassic, recovered from the Dinodontosaurus Assemblage Zone of the Santa Maria Formation in southern Brazil, corresponding to the transition between the Ladinian and Carnian stages . This taxon represents one of the oldest known silesaurids possessing three sacral vertebrae, indicating that an advanced sacral configuration had already emerged early in the evolution of this group.

The femoral morphology differs markedly from that of other silesaurids, particularly in the absence of a dorsolateral trochanter and the presence of only a weakly developed fourth trochanter. These features are not observed in other known members of the group. The sacral vertebrae form a platform-like articulation similar to that of silesaurids, which may have enhanced pelvic support and suggests a distinct locomotor adaptation.

This species was discovered in the same assemblage as another silesaurid, Gamatavus antiquus, representing the earliest record of co-occurring silesaurids in South America . This co-occurrence may indicate ecological differentiation between the two taxa within the same environment.

（Author: Shui-Ye You）

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Reference

Müller RT. (2025). A new "silesaurid" from the oldest dinosauromorph-bearing beds of South America provides insights into the early evolution of bird-line archosaurs. Gondwana Research.

-29.7156, -53.1511

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