Retymaijychampsa beckerorum, Müller, 2025

Retymaijychampsa beckerorum sp. nov.

Fig. 3 View Fig .

ZooBankLSID:urn:lsid:zoobank.org:act:80A4B7E7-6C18-4A81-B668- B623A9D416A4

Etymology: In honor of the family Becker , the owners of the property were the Linha Várzea 2 site is located.

Holotype: CAPPA/UFSM 0430 , a complete and articulate right hindlimb. GoogleMaps

Type locality: Linha Várzea 2 (= Becker ) site ( 29°44′03″ S, 53°09′07″ W), Paraíso do Sul, Rio Grande do Sul, Brazil (Fig. 1).

Type horizon: Santa Maria Formation ( Schultz et al. 2020); PinheirosChiniquá Sequence ( Horn et al. 2014) of the Santa Maria Supersequence ( Zerfass et al. 2003), Paraná Basin . This third-order sequence is associated with the Dinodontosaurus Assemblage Zone , which is Ladinian to early Carnian in age ( Ezcurra et al. 2017; Schultz et al. 2020; Novas et al. 2021; Müller and Garcia 2022).

Material.— Holotype only.

Diagnosis.— Retymaijychampsa beckerorum gen. et sp. nov. differs from all other known proterochampsids with comparable skeletal material based on the following local autapomorphies: absence of an extensor fossa on the anterior surface of the distal portion of the femur, resulting in a straight anterior margin in distal view; fibular condyle of the femur projects more ventrally than tibial condyle, forming an uneven distal surface in anterior view; and markedly robust tibia, with a ratio of 0.37 between the maximum width of the proximal end and the total length of the bone.

Description.—Although the femur lacks the proximal portion, it cleary had a sigmoid shape ( Fig. 3A View Fig 1 View Fig ). The preserved segment of the shaft includes the distal half of the fourth trochanter, which forms an elongated ridge ( Fig. 3A View Fig 6 View Fig ). The distal portion of the trochanter merges smoothly with the shaft. Whereas the femur is not completely preserved, it is evident that the fourth trochanter is distally elongated, similar to the condition seen in most proterochampsids ( Trotteyn et al. 2013). Conversely, the fourth trochanter of Stenoscelida aurantiacus Müller et al., 2022 (CAPPA/UFSM 0293; Müller et al. 2022) and Tropidosuchus romeri Arcucci, 1990 (PVL 4601; Arcucci 1990) is short. The distal portion of the femur expands relative to the shaft and is arched posteriorly. The popliteal fossa is not elongated. Unlike other proterochampsids ( Trotteyn 2011; Trotteyn and Ezcurra 2020; Müller et al. 2022; Paes-Neto et al. 2024), this specimen lacks an extensor fossa on the anterior surface of the distal portion of the femur, resulting in a straight anterior margin in distal view ( Fig. 3A View Fig 4 View Fig , A 8). In addition, compared to other proterochampsids, the fibular condyle projects more ventrally than the tibial condyle, forming an uneven distal surface in anterior view. The tibiofibular crest is not strongly expanded and there are no distinguishing features between the lateral surface of the tibiofibular crest and the lateral condyle.

The complete tibia ( Table 1 View Table 1 ) is notably robust, with a ratio of 0.37 between the maximum width of the proximal end and the total length of the bone. For comparison, the ratio for the holotype of Stenoscelida aurantiacus (CAPPA/UFSM 0293) is 0.25 and for the holotype of Kuruxuchampsa dornellesi Paes-Neto et al., 2024 (UFRGS-PV-0877-T) is 0.26. In lateral/medial view, the proximal portion of the tibia is expanded both anteriorly and posteriorly relative to the shaft ( Fig. 3A View Fig 5 View Fig ). The anterior projection comprises the cnemial crest, which tapers to a point and is straight ( Fig. 3A View Fig 6 View Fig ). The posterior projection is the medial condyle, which is positioned further posteriorly relative to the lateral condyle. The proximal articular surface is flat. The shaft of the tibia is straight and expands distally, with the expansion predominantly oriented anteroposteriorly. This condition differs the specimen from Tropidosuchus romeri (PVL 4601; Arcucci 1990), in which the distal end of the tibia is not expanded. The posterior margin of the distal end of the tibia of CAPPA/ UFSM 0430 is gently concave ( Fig. 3A View Fig 7).

The fibula is slender, with a shaft that has approximately half the transverse width of the tibial shaft ( Fig. 3A View Fig 6 View Fig ). The fibula is slightly shorter than the tibia ( Table 1 View Table 1 ). The proximal end is expanded posteriorly relative to the shaft and is compressed transversely ( Fig. 3A View Fig 6 View Fig ). The shaft is straight along its length, differing from the arched shaft of the fibula in most pseudosuchians ( Nesbitt 2011; Ezcurra 2016). The width of the shaft remains consistent along the entire length of the fibula, whereas in Stenoscelida aurantiacus (CAPPA/ UFSM 0293; Müller et al. 2022) the proximal half of the shaft is wider. There is no tubercle or ridge for the attachment of the iliofibularis muscle, such as in Proterochampsa barrionuevoi Reig, 1959 (PVSJ 606; Trotteyn et al. 2013). The distal end expands relative to the shaft, and the distal articular surface is flat.

The astragalus is transversely expanded ( Fig. 3A View Fig 7). The dorsal margin of its medial surface is concave, while its ventral surface of the bone is convex anteroposteriorly. There is no anterior expansion at the anteromedial corner. The posterior margin lacks a dorsal process. The calcaneum is lateromedially shorter than the astragalus ( Fig. 3A View Fig 6 View Fig , A 7). The facet for the fibula is oriented dorsomedially. The calcaneal tuber is well-developed and perpendicular to the main body of the calcaneum ( Fig. 3A View Fig 6 View Fig ). There is a depression between the main body of the bone and the posterior surface of the calcaneal tuber ( Fig. 3A View Fig 6 View Fig , A 7).

The configuration of the metatarsals is consistent with that in other proterochampsids ( Table 1 View Table 1 ): metatarsal I is short; metatarsal II is slightly longer than metatarsal I and the stoutest of the elements; metatarsal III is longer than metatarsal II and more slender; metatarsal IV is quite slender and as long as metatarsal III; and metatarsal V is reduced and tapers distally. Pedal digit I includes two phalanges ( Fig. 3A View Fig 5 View Fig ). The specimen differs from Tropidosuchus romeri (PVL 4601; Arcucci 1990) and Stenoscelida aurantiacus (CAPPA/UFSM 0293; Müller et al. 2022) in that both of the latter taxa have a shorter digit I. Whereas pedal digit II is longer than digit I, it is shorter than digit III ( Fig. 3A View Fig 6 View Fig ). Digit IV preserves one phalanx, which is quite slender ( Fig. 2A View Fig 5 View Fig , A 6 View Fig ). As this phalanx is not an ungual bone, the total number of phalanges in this digit remains uncertain. There are no phalanges in the digit V. This condition is shared with other proterochampsids, whereas a putative vestigial phalanx has been reported for Stenoscelida aurantiacus (CAPPA/UFSM 0293; Müller et al. 2022).

Stratigraphic and geographic range.— Type locality and horizon only.

Phylogenetic analysis

The phylogenetic analysis recovered a single most parsimonious tree ( Fig. 4 View Fig ) of 914 steps each, with a consistency index of 0.521 and a retention index of 0.557. Retymaijychampsa beckerorum gen. et sp. nov. is found in a trichotomy with both species of Proterochampsa , based on the elongated fourth trochanter of the femur (ch. 329: 0→1) and the ratio between the maximum width of the proximal end and the total length of the tibia (ch. 339: 2→3). Sphodrosaurus pennsylvanicus is the sister taxon to the node comprising R. beckerorum gen. et sp. nov. plus both species of Proterochampsa . The clade comprising these four OTUs is named here Proterochampsinae and is supported by five synapomorphies: ratio between the maximum width of the skull and length of the presacral vertebral column ranging from 0.336 to 0.440 (ch. 12: 1→3); distinct coarse ornamentation on lateral surface of the surangular and angular (ch. 187: 0→1); in medial view, the area ventral to the internal mandibular fenestra shows a larger contribution from the angular bone compared to the prearticular, or both bones contribute equally (ch. 188: 0→1); prearticular with ventral margin posterior to its contact with the splenial straight or ventrally curved on the anterior half of the bone in medial or lateral view (ch. 189: 1→0); and presence of ventral keel on the centrum of the ninth presacral vertebra (ch. 231: 0→1). Stenoscelida aurantiacus is recovered as the sister taxon to Proterochampsidae . The inner composition of Proterochampsidae and Doswelliidae follows previous analyses of the original data matrix ( Paes-Neto et al. 2024).

An interesting aspect to note is that, prior to the advent of computational phylogenetic analyses, Bonaparte (1971) divided proterochampsians into two main families: “ Proterochampsidae ” and “Cerritosauridae”. Although far fewer proterochampsian species were known at the time, this subdivision closely aligns with the results obtained in the present analysis