A single, fragmentary dinosaur bone from Brazil may show that one of the Triassic’s oldest predators survived a major ecological turnover—and could even be the youngest herrerasaurid ever found. Herrerasauridae comprises the oldest large-bodied predatory dinosaurs and is currently known exclusively from the Carnian, or Late Triassic, of South America.

Previously, herrerasaurid occurrences in Argentina and Brazil were restricted to rhynchosaur-dominated assemblages, with no records from younger strata characterized by abundant Exaeretodon. The specimen is CAPPA/UFSM 0539, a partial left pubis recovered from Exaeretodon-dominated strata of the Hyperodapedon Assemblage Zone in southern Brazil.

Its combination of pubic features, including a well-developed pubic boot with anterior and posterior distal expansions, is currently unique to herrerasaurids. A phylogenetic analysis supports referring it to an indeterminate herrerasaurid, making it the first Herrerasauridae record from these Exaeretodon-dominated strata.

Herrerasaurid dinosaurs were the oldest large predatory dinosaurs, reaching body lengths of over five meters. Known herrerasaurids share a relatively robust skull with ziphodont teeth, forelimbs with large trenchant manual unguals, a perforate acetabulum, and an obligate bipedal posture.

Their phylogenetic history has been marked by competing hypotheses. The absence of herrerasaurid records from Exaeretodon-rich strata could reflect faunal changes that prevented the group’s persistence. One possibility is that herrerasaurids specialized in preying on rhynchosaurs, so the decline of those herbivores may have harmed their survival.

But rhynchosaurs become less abundant in the younger assemblages without disappearing, so that explanation alone may not be enough. CAPPA/UFSM 0539 is the first herrerasaurid dinosaur described from a locality typically assigned to the Exaeretodon sub-AZ of southern Brazil, extending the clade’s known temporal range.

The specimen was collected at the Várzea do Agudo, or Janner, site in the municipality of Agudo, Rio Grande do Sul, Brazil. The fossil-bearing strata belong to the Santa Maria Formation in the Paraná Basin and are assigned to the Candelária Sequence of the Santa Maria Supersequence.

In panel A, the orange marker places Várzea do Agudo along the regional road and river network, while panel B shows the site as a broad, steep red-earth exposure. Panel C narrows this to the fossil-bearing slope and marks CAPPA/UFSM 0539 directly on the outcrop; panel D adds an artistic reconstruction of the animal represented by the specimen.

The Várzea do Agudo fossil assemblage is Carnian in age, although its uppermost fossil-bearing beds may extend into the earliest Norian. The site’s age is currently inferred from biostratigraphic evidence, including the co-occurrence of Hyperodapedon and Exaeretodon, which is characteristic of the Hyperodapedon Assemblage Zone.

The succession extends over approximately ten meters, and Siriusgnathus has been recorded exclusively where CAPPA/UFSM 0539 was collected ex situ. Because Exaeretodon has not yet been recovered from that same level, the exposed succession may include a lower Hyperodapedon AZ interval and an upper, younger interval characterized so far only by Siriusgnathus.

The exposed strata form a succession approximately twelve to thirteen meters thick, with fossils restricted to the lower ten meters. The lowermost three meters are red mudstones that have yielded only Isodapedon varzealis. The overlying seven meters of orange mudstones have produced Hyperodapedon, Exaeretodon riograndensis, Trucidocynodon riograndensis, Stenoscelida aurantiacus, Dynamosuchus collisensis, Pampadromaeus barberenai, and Bagualosaurus agudoensis.

The uppermost orange-mudstone levels have also yielded Siriusgnathus niemeyerorum, currently the only formally recognized taxon from that portion. Although CAPPA/UFSM 0539 is extremely fragmentary, it is a bone element with specific features found in only a few archosaur groups, so a more resolved placement may still be possible.

The specimen was included in a phylogenetic analysis to investigate its affinities and assess its taxonomic assignment. It was scored in a combined matrix with broad avemetatarsalian archosaur diversity, emphasizing Triassic forms. The updated matrix contains two hundred ninety-three morphological characters and seventy-nine operational taxonomic units, and it was analyzed in TNT version one point six to recover the most parsimonious trees.

Euparkeria capensis rooted the trees, which were recovered using random addition, tree bisection reconnection, one thousand Wagner-tree replicates, and branch-swapping. The specimen preserves the complete distal end of a left pubis. The lateral surface is fully exposed, while some matrix remains on the medial surface because the element is exceptionally thin.

The bone surface is relatively well preserved, with rugose structures along the lateral margin. At its proximalmost preserved portion, the pubic shaft is kidney-shaped in cross-section. The bone wall is extremely thin, measuring two point five millimeters at its thickest and one point two millimeters at its thinnest, leaving a proportionally wide medullary cavity.

The unexpanded shaft measures nineteen millimeters anteroposteriorly and twenty-one point four millimeters transversely. The shaft is laterally convex and tapers medially into the pubic apron, an anteroposteriorly thin, plate-like structure. The bone projects posteriorly to form a remarkably elongate, triangular process called the pubic boot, as in other herrerasaurids.

This well-developed expansion differs from the poorly expanded, knob-like distal pubis seen in several Carnian saurischians. The pubic boot has a gradual transition from the shaft, and its main axis approaches a ninety-degree angle with the shaft’s long axis. Unlike a posterior-only expansion, CAPPA/UFSM 0539 also has a subtle anterior expansion that is smaller and projects at a more gradual angle.

The pubic boot is sixty-nine point six millimeters long and only three millimeters thick ventrally; the absence of a medial expansion leaves the distal end L-shaped in ventral view. The phylogenetic analysis recovered five hundred forty most parsimonious trees with one thousand one hundred thirty-one steps.

The new specimen was recovered within Herrerasauridae in all of those trees. In the strict consensus tree, CAPPA/UFSM 0539 forms a polytomy with an indeterminate Brazilian herrerasaurid, Gnathovorax cabreirai, Herrerasaurus ischigualastensis, and Sanjuansaurus gordilloi.

That clade is supported by five synapomorphies, including an anteriorly expanded distal end of the pubis, and it is the sister group to Staurikosaurus pricei. Herrerasauridae is recovered within Herrerasauria, which is the sister group to Theropoda plus Sauropodomorpha.

The specimen’s preserved shaft and boot are visibly incomplete, but its triangular, expanded end can still be compared directly with the neighboring examples. The Bremer support was one, and bootstrap values from one thousand replicates were below fifty percent for the nodes of interest.

Those low values are not surprising given the fragmentary nature of the material investigated and of other specimens in the matrix. Despite its fragmentary nature, the preserved portion allows confident referral to Herrerasauridae because it combines morphological features currently known to be exclusive to that clade.

A small distal pubic expansion, or knob, occurs in several archosaur groups, whereas a well-expanded distal pubic end, or pubic boot, is restricted among Triassic archosaurs to some pseudosuchians, theropods, and herrerasaurids. In non-poposauroid suchians, the pubic expansion is mediolaterally thick and the expansions meet at the midline, unlike CAPPA/UFSM 0539, whose distal expansion is transversely thin.

Some poposauroids have a transversely flattened expansion restricted to the lateral pubic margin, producing separated expansions and an L-shaped pubis in ventral view, as in CAPPA/UFSM 0539. But in those poposauroids the posterior expansion is hook-shaped and directed dorsally, unlike CAPPA/UFSM 0539, where it does not project dorsally.

In Triassic theropods with a pubic boot, the posterior expansion is far less developed than in CAPPA/UFSM 0539. CAPPA/UFSM 0539 also differs from non-herrerasaurid herrerasaurians and Staurikosaurus pricei because it possesses an anterior expansion, while those herrerasaurians expand exclusively posteriorly.

The figure’s lower row makes the contrast especially clear: panel I shows the pubic boot extending into a long, sharply tapered posterior process, whereas panels G and H retain shorter, broader distal outlines. Across the panels, the anterior extension is visibly distinct in A and D, while it is absent from most of the illustrated comparison taxa.

The combination of pubic characters in CAPPA/UFSM 0539 is known only among some Herrerasauridae within Triassic archosaurs, and the phylogenetic analysis also recovered it within Herrerasauridae. The specimen cannot be confidently assigned to any of the remaining recognized species because it lacks features uniquely shared with those species.

Referral to an indeterminate herrerasaurid is therefore the most plausible hypothesis, with no alternative interpretation currently supported by comparable evidence. A placement elsewhere within Herrerasauria remains possible if future discoveries show that the anterior pubic expansion was more widely distributed, but that hypothesis is currently less plausible.

No Herrerasauridae record had previously been documented from the Exaeretodon sub-AZ, despite the growing number of new herrerasaurid specimens from Brazilian Triassic beds. CAPPA/UFSM 0539 demonstrates that herrerasaurid dinosaurs persisted in ecosystems marked by declining rhynchosaurs and increasing traversodontid cynodont abundance.

The occurrence may suggest ecological plasticity, allowing herrerasaurids to persist in ecosystems different from those typically associated with the Hyperodapedon Acme Zone. The prey exploited by herrerasaurids remains difficult to determine because direct evidence of predator-prey interactions is lacking.

The discovery increases the diversity of relatively large-bodied predators known from Várzea do Agudo and suggests that fully grown herrerasaurids may have occupied apex-predator roles there. CAPPA/UFSM 0539 may also represent the youngest known record of Herrerasauridae worldwide.

If the Brazilian Exaeretodon-rich strata follow the pattern recognized in Argentina, CAPPA/UFSM 0539 would be younger than several previously known herrerasaurids. However, absolute dating of the Brazilian strata remains essential for establishing a more reliable chronological framework.

Look across the central stratigraphic columns: the herrerasaurid reconstruction appears within both the Hyperodapedon acme zone and the overlying Exaeretodon sub-zone. The latter is positioned above the former, while the adjacent time scale marks the interval between roughly two hundred thirty-eight and two hundred twenty-five million years ago.

The specimen preserves a combination of pubic features shared exclusively with members of Herrerasauridae, and both morphological and phylogenetic evidence support its referral to an indeterminate herrerasaurid. CAPPA/UFSM 0539 is the first herrerasaurid dinosaur record from the Exaeretodon-dominated strata of the Hyperodapedon Assemblage Zone.

The finding indicates that herrerasaurids persisted in ecosystems distinct from those typical of the Hyperodapedon Acme Zone and may represent the youngest known Herrerasauridae record worldwide. CAPPA/UFSM 0539 is best identified as an indeterminate herrerasaurid, based on its distinctive pubic anatomy and phylogenetic placement.

It records these predators in Exaeretodon-dominated ecosystems for the first time, although its exact age still needs absolute dating.