A Bone Missing for Two Decades Fills a Gap in the Evolution of Triassic Archosaurs in Brazil
When a fossil loses its provenance and stratigraphic information, its scientific value can be greatly diminished, even if the bones themselves are well preserved. This was the case for an archosauriform fossil from Brazil, catalogued as MCP 4186-PV. The specimen consists of a scapula, an ilium, and a femur. The three bones were found in association and are consistent in size, indicating that they belonged to the same individual. However, a portion of the proximal femur bearing the specimen's collection code had been accidentally lost, preventing researchers from verifying its provenance for more than 20 years. Only when the missing fragment was rediscovered in the museum collection could the animal's identity and history finally be pieced together.
The specimen originated from a fossil locality near Dona Francisca in Rio Grande do Sul, southern Brazil. It was recovered from the Pinheiros-Chiniquá Sequence of the Santa Maria Supersequence, which dates to the Ladinian stage of the Middle Triassic. Following a detailed anatomical reassessment and comparison with other archosauriforms, researchers identified the specimen as a previously unknown taxon, naming it Silescelida acristata.
The genus name Silescelida commemorates the specimen's unusual history. It combines the Latin word siles, meaning "silence," with the Greek skelēs, meaning "hind leg," in reference to the missing portion of the proximal femur. The species epithet acristata, meaning "without a crest," refers to one of the animal's distinctive femoral characteristics.
Archosauriformes encompasses crown-group Archosauria and numerous closely related lineages along the archosaur stem. During the Triassic, several archosauriform groups evolved body plans and skeletal features that increasingly resembled those of ancestral archosaurs. Among these groups, Eucrocopoda occupies a particularly important position in understanding this evolutionary transition. Modern crocodilians and birds, along with extinct dinosaurs, pterosaurs, and numerous Triassic reptiles, all belong to lineages within Eucrocopoda. Nevertheless, the phylogenetic relationships among early-diverging eucrocopodans remain poorly resolved.
The scapula of Silescelida acristata lacks a pronounced constriction near the base of the scapular blade, which expands asymmetrically toward its dorsal end. Its posterior margin is relatively straight and forms a distinctive angle near the shoulder joint, while an elliptical tuber for the attachment of the triceps muscle is present immediately above the glenoid fossa. The dorsal margin of the ilium is also relatively straight in lateral view, and a distinct notch occurs between the medial acetabular wall and the ischiadic peduncle. The researchers used this combination of scapular, iliac, and femoral characteristics to establish Silescelida acristata as a new genus and species.
The femur, the most complete element of the specimen, measures approximately 17.3 cm in length. In many archosauriforms, the femur bears a fourth trochanter, which serves as an attachment site for the caudofemoralis muscle and typically forms a prominent mound, crest, or flange. In Silescelida acristata, faint muscle attachment scars are preserved in the corresponding region, but there is almost no elevated crest. This distinctive feature inspired the species epithet acristata.
To determine the phylogenetic position of the new species, the researchers incorporated MCP 4186-PV into a morphological dataset comprising 287 operational taxonomic units and 919 characters, conducting four separate phylogenetic analyses. These alternative analyses were necessary because the taxonomic identities and relationships of several early-diverging eucrocopodans remain uncertain. For example, the resulting tree topology changes considerably depending on whether Osmolskina czatkowicensis is represented solely by its holotype or by the complete hypodigm, including additional specimens referred to the species. Similarly, the results are affected by whether the problematic Chinese taxon "Turfanosuchus shageduensis" and Halazhaisuchus qiaoensis are treated as separate taxa or combined into a single operational taxonomic unit under the hypothesis that they may be conspecific.
Regardless of these alternative configurations, Silescelida acristata was consistently recovered as an early-diverging member of Eucrocopoda, positioned crownward of Erythrosuchidae and outside the clade comprising Proterochampsia plus Archosauria. This aspect of its phylogenetic placement was relatively stable across all four analyses.
Its possible membership in Euparkeriidae, however, remains considerably less certain. Some phylogenetic trees recovered Silescelida acristata within Euparkeriidae, whereas others placed it outside the group, close to other early-diverging eucrocopodans such as Halazhaisuchus qiaoensis, "Turfanosuchus shageduensis," and Marcianosuchus.
This phylogenetic instability also highlights a broader question: whether Euparkeriidae itself constitutes a monophyletic group remains unresolved. The South African species Euparkeria capensis is currently the only unequivocal member of the family, while other taxa previously assigned to Euparkeriidae occupy different positions across alternative phylogenetic analyses. One possibility is that Euparkeriidae contains fewer species than traditionally assumed. Alternatively, several early-diverging eucrocopodan lineages may have retained similar ancestral skeletal characteristics, making it difficult to distinguish their evolutionary relationships based on morphological similarities alone.
Previously, fossils tentatively assigned to Euparkeriidae were primarily known from South Africa, China, Germany, Poland, and Russia. Although South America has yielded a rich diversity of Triassic archosauriforms, unequivocal records of early-diverging eucrocopodans of this evolutionary grade had been lacking. Silescelida acristata therefore represents the first known early-diverging eucrocopodan from the Triassic of Brazil and potentially the first record of Euparkeriidae from South America.
The temporal distribution of these animals raises another question. Early-diverging eucrocopodans from northern Pangaea are primarily known from the Early Triassic and early Middle Triassic. In southern Pangaea, apart from the earlier occurrence of Euparkeria capensis in South Africa, most records date to the late Middle Triassic or early Late Triassic. This difference could indicate that these animals dispersed into southwestern Pangaea relatively late. Alternatively, they may have reached the region considerably earlier, with their initial presence remaining undocumented in the fossil record. Consequently, the timing of their dispersal into South America remains uncertain.
The discovery of Silescelida acristata extends the known geographic distribution of early-diverging eucrocopodans into Brazil and provides new insight into a longstanding taxonomic problem. Before the rise of crown-group archosaurs, Pangaea was already home to multiple archosauriform lineages with similar skeletal features, whose evolutionary relationships remain difficult to resolve.
Author: Shui-Ye You
Reference:
Garcia MS et al. (2026). A new eucrocopodan archosauriform from the Middle Triassic of southern Brazil and the phylogeny of Euparkeriidae. Scientific Reports.
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