A new silesaurid from Carnian beds of Brazil fills a gap in the radiation of avian line archosaurs

Comprising the oldest unequivocal dinosauromorphs in the fossil record, silesaurs play an important role in the Triassic radiation of dinosaurs. These reptiles provide the main source of information regarding the ancestral body plan of dinosaurs, as well as the basis for biogeographic models. Nevertheless, the co-occurrence of silesaurs and the oldest unequivocal dinosaurs is rare, which hampers reliable ecological inferences. Here we present the first species of silesaur from the oldest unequivocal dinosaur-bearing beds from Brazil. Amanasaurus nesbitti gen. et sp. nov. possesses a unique set of femoral traits among silesaurs, including the oldest occurrence of an anterior trochanter separated by the femoral shaft by a marked cleft. Its femoral length indicates that the new species rivals in size with most coeval dinosaurs. This find challenges the assumption that in faunas where silesaurs and unambiguous dinosaurs co-occurred, silesaurs were relatively smaller. Moreover, the presence of dinosaur-sized silesaurs within ecosystems with lagerpetids, sauropodomorphs and herrerasaurids reinforces the complex scenario regarding the early radiation of Pan-Aves. Silesaurs—independent of their phylogenetic position—persisted during most of the Triassic Period, with its plesiomorphic body size advancing through the dawn of dinosaurs, instead of silesaur lineages decrease in body size through time.


Phylogenetic analysis.
In order to access the phylogenetic affinities of the new silesaur, it was scored in the data matrix of Norman et al. 10 , which is a modified version of the data matrix published by Müller & Garcia 9 .This is the most comprehensive dataset regarding silesaurs.Furthermore, we inserted Gamatavus antiquus 8 , a recently described silesaur from Brazil.Its scoring was performed through first-hand examinations of the holotype (UFSM 11348a, a partial right ilium) and a referred specimen (UFSM 11348b, a partial left femur).Chilesaurus diegosuarezi was removed from the data matrix because of its controversial affinities 26 .The most parsimonious trees were recovered in the software TNT v. 1.5 27 .All characters received the same weight and characters 4, 13, 18, 25, 63, 82, 84, 87, 89, 109, 142, 166, 174, 175, 184, 186, 190, 201, 203, 205, 209, 212, 225,  235, 236, 239, 250 and 256 were treated as additive (ordered).Euparkeria was used to root the most parsimonious trees, which were constructed using random addition sequence + tree bisection reconnection (TBR), which included 1000 replicates of Wagner trees (with random seed = 0), TBR and branch-swapping (holding 20 trees saved per replicate).
Femoral length estimation criteria.The total femoral length of CAPPA/UFSM 0374 and CAPPA/UFSM 0375 was estimated according to two ordinary least squares linear regressions employing the dataset of Barrett et al. 28 .This dataset includes measurements of 31 femora of distinct Triassic and Lower Jurassic ornithodirans.The femoral length of CAPPA/UFSM 0374 was estimated using the proximal long axis of the femoral head as the independent variable, whereas the femoral length of CAPPA/UFSM 0375 was estimated using the distal long axis as the independent variable.Holotype.CAPPA/UFSM 0374 (Table 1), a proximal portion of a right femur.

Etymology.
The genus combines the Tupi word "amana" (= rain) and the Greek "saurus" (= lizard), referring to the Carnian pluvial episode.The specific epithet honors Dr. Sterling J. Nesbitt, a prominent North American paleontologist, for his contribution and studies on silesaurs and Triassic archosaurs.Description.The femoral head of Amanasaurus nesbitti is well-preserved (Fig. 2a-e).The bone surface preserves fine details and the specimen shows no evidence of sedimentary compression.Similar to other silesaurs, the femoral head is poorly expanded transversely.This condition differing the Am.nesbitti from most dinosaurs and pterosauromorphs 33,34 .It is triangular in proximal view, with a deep straight groove separating the anterior and posterior surfaces (Fig. 2c).This groove is absent in Lewisuchus admixtus 20,34 .The medial articular surface is straight, whereas in L. admixtus and Asilisaurus kongwe it is rounded 19,35 .The anterolateral tuber is well-developed, as well as the anteromedial tuber.The later forms the posteromedial margin of the femoral head (Fig. 2e), delimiting the distalmost extension of the articular surface of the proximal portion of the femoral head, such as in Sacisaurus agudoensis 14 .Distinct from the other tubers, the posteromedial tuber is poorly developed, lacking a sulcus for ligamentum captis femoris in proximal view.The posteromedial tuber of L. admixtus, As. kongwe and Eucoelophysis baldwini is well-developed 19,35 , differing from the new specimen.There is a reduced fossa trochanterica (Fig. 2c), resembling As. kongwe 35 .In L. admixtus it is well-developed 19 , whereas in other silesaurs it is absent 7,14,21,36 .The greater trochanter is angled, whereas in lagerpetids it is rounded 15,37,38 .The specimen lacks the "overhang structure" on the proximal surface, which is reported for some specimens of Silesaurus opolensis 39 .
The typical "notch" between the ventral transition from the femoral head to the femoral shaft is present (Fig. 2a).This differs from the concave emargination that marks the transition in most dinosaurs 33 .In addition, the medial articular surface of the femoral head bears a transverse scar above the notch (Fig. 2d).In Si. opolensis there is a similar scar that forms the ventral margin of the attachment point of the iliofemoral ligament 40 .There is a smooth surface on the homologous surface of Sa. agudoensis 14 .Its surface is reduced in the Am.nesbitti.There is an unusual sub-circular scar on the posterior surface of the femoral heart (Fig. 2e), slightly below the fossa trochanterica.An identical scar was not reported for other silesaurs.
The anterior surface of the dorsolateral trochanter is sharp and merges with the shaft well-below the proximal articular surface (Fig. 2a).In contrast, this trochanter is rounded for all the ontogenetic stages sampled for As.kongwe 41 .A proximodistally oriented scar runs on the lateral surface of this trochanter (Fig. 2b), whereas a transverse scar extends from the trochanter to the posterior margin of the femoral shaft (Fig. 2e).A raised anterolateral scar is absent, whereas it is reported for L. admixtus, As. kongwe, and Si.opolensis 19,20,39,41 .Indeed, there are faint striations on the homologous surface.The anterior trochanter is finger-like, extending proximodistally.Its proximal tip is separated from the femoral shaft by a cleft (Fig. 2b), such as in several silesaurs, prionodontians, and theropods 10,11,14,42 .On the other hand, in L. admixtus and As.kongwe the proximal tip merges smoothly with the femoral shaft, lacking the cleft 19,20,35 .Whereas the proximal tip of the anterior trochanter is not connected to the shaft in the new specimen, it is far less expanded than the wing-like trochanter of several prionodontians 10,43,44 .The condition of the new specimen is also distinct from the pyramidal anterior trochanter of some theropods 42,45 .The proximal portion of the linea intermuscularis cranialis rests medial to the anterior trochanter (Fig. 2a).It is absent in Sa. agudoensis 14 .A foramen pierces the femoral shaft medial to the proximal portion of the cranial intermuscular line (Fig. 2d).The trochanteric shelf is absent, a condition shared with Sa. agudoensis, E. baldwini, Kwanasaurus williamparkeri, Diodorus scytobrachion, and prionodontians 7,10,11,14,36 .
The referred distal portion of a left femur bears a well-preserved bone surface (Fig. 2f-i).On the other hand, the bone is fragmented.Distinct from lagerpetids 15,37 , the anterior surface is convex in distal view (Fig. 2i), lacking any evidence of an extensor grove.There is a raised scar extending from the anterior to the lateral surface of the bone (Fig. 2f-h), which is common for dinosauromorphs.The lateral margin of the lateral condyle is rounded in distal view and there is a depression between this condyle and the crista tibiofibularis.The exact size and shape of the crista tibiofibularis and the medial condyle are uncertain.Whereas the popliteal fossa is not entirely preserved, it is proximodistally elongated (Fig. 2h), resembling the condition of most silesaurs and aphanosaurs 4,46 .A raised scar runs from the surface above the crista tibiofibularis into the popliteal fossa.

Phylogenetic analysis.
The heuristic search recovered 1728 most parsimonious trees (MPTs) of 1074 steps each, with a consistency index of 0.298 and a retention index of 0.689.The general topology of the strict consensus tree (Fig. 3a) follows that recovered by Norman et al. 10 , where silesaurs are nested in low diversity groups in the branch that leads to Prionodontia (i.e., traditional ornithischians).Amanasaurus nesbitti nests as a parapredentatan within Ornithischia in all the MPTs.The new taxon nests in a trichotomy with Ignotosaurus fragilis and Silesarus opolensis, which is supported by a fossa on the ventral surface of postacetabular part of ilium (ch.174: 1 → 2), iliac lamina two times deeper or more than the acetabulum (ch.187: 0 → 1), and ligament sulcus of the femoral head does not form a medial excavation in proximal view (ch.204: 0 → 1).Only the latter character is coded for Am.nesbitti.Following the phylogenetic definition proposed by Nesbitt et al. 4 , the clade supporting Am. nesbitti, I. fragilis and Si.opolensis receives the name Silesauridae.
Regarding Gamatavus antiquus, it nests as the basalmost member of Sulcimentisauria, an arrangement supported by the absence of the femoral trochanteric shelf (ch.215: 0 → 1) and facies articularis antitrochanterica not ventrally descended (ch.216: 0 → 1).Furthermore, Saltopus elginensis nested as sister taxon of Lewisuchus admixtus and is recovered as an ornithischian for the first time.This result should be taken carefully, however, given the fragmentary and difficult-to-interpret nature of the holotype.In previous iterations of this dataset Sa. elginensis was recovered as an early-diverging saurischian sister to Eodromaeus murphi 9 and as a non-dinosaur dinosauromorph 15 .

Discussion
The holotype of Amanasaurus nesbitti possesses typical traits of silesaurs, such as the presence of a notch between the ventral transition from the femoral head to the femoral shaft and a straight medial articular facet of the proximal portion in proximal view 4,11,14,28,36 .Therefore, Am. nesbitti can be safely assigned to a silesaur.Although the referred specimen lacks overlapping material with the holotype, the preserved distal portion of the femur resembles that of other silesaurs (e.g., presence of an elongated popliteal fossa 4 ) and lacks typical features of other related groups.So, this specimen is referred to Am. nesbitti on the basis of the geological context and the shared morphology with other silesaurs.
Regarding the general morphology of Am. nesbitti, it bears a unique set of plesiomorphic and apomorphic traits for silesaurs, which is consistent with its phylogenetic and stratigraphic position.It retains a fossa trochanterica, a trait presents in older silesaurs, such as Lewisuchus admixtus and Asilisaurus kongwe 19,35 .Conversely, the posteromedial tuber is extremely reduced, resembling the condition observed in late diverging forms, such as in Sacisaurus agudoensis 14 and Kwanasaurus williamparkeri 7 .Perhaps, the "transitional" shape of the anterior trochanter comprises one of the most interesting features of the new taxon.The anterior trochanter of dinosaurs and close related groups is usually regarded as the insertion point for the m.iliofemoralis 40,47 .In early silesaurs (e.g., L. admixtus; As.Kongwe; Gamatavus antiquus), the proximal tip is completely connected to the femoral shaft (Fig. 3e), whereas in late diverging forms (e.g., Eucoelophysis baldwini) its tip is completely separated from the shaft by a marked cleft 14 (Fig. 3c).This condition is interpreted as an early stage of the "wing-like" anterior trochanter of prionodontians (Fig. 3b) and provided further support for the ornithischian affinities of silesaurs 10,14 .In Am. nesbitti, the anterior trochanter is less pronounced than in post-Carnian silesaurs; however, it bears the cleft separating the proximal tip from the femoral shaft (Fig. 3d).The new taxon comprises the oldest silesaur expressing this condition, revealing a Carnian origin for this feature.
The new silesaur provides further support for the presence of silesaurs in the Hyperodapedon Assemblage Zone (AZ) of Brazil.These reptiles are reported for three of the four AZs assigned to Middle and Upper Triassic 8,17 .The current scenario depicts the total absence of silesaurs and other Pan-Aves solely in the Santacruzodon AZ (Fig. 3f), which is poorly sampled in comparison with other AZs and its geographical distribution is limited 29 .In addition, the presence of the new silesaur in Carnian beds of Southern Brazil reinforces the co-occurrence of distinct Pan-Aves groups during the initial evolution of dinosaurs (ca.230 Ma).The new silesaur comes from an outcrop area that yielded lagerpetids, early sauropodomorphs, and herrerasaurids 15,16,48 .This diversity of Pan-Aves surpasses that of older AZs from Brazil (i.e., Dinodontosaurus AZ and Santacruzodon AZ), being comparable to that of the Riograndia AZ (Fig. 3f).A similar diversity is also reported for the coeval Ischigualasto Formation 18 , where a silesaur is also reported (i.e., Ignotosaurus fragilis).It is reasonable to conclude that the landscapes that witnessed the early evolution of dinosaurs supported a wide range of avian line archosaurs as well.Moreover, according to the femoral length estimations performed here, Am. nesbitti reached the same size of early sauropodomorphs.The estimated femoral length of the holotype (CAPPA/UFSM 0374) is 121 mm (Fig. 3g), whereas for the referred specimen (CAPPA/UFSM 0375) it is 143 mm (Fig. 3h).For comparison, the femoral length of specimens of the early sauropodomorph Buriolestes schultzi ranges from 89 mm (ULBRA-PVT056 49 ) to 138 mm (ULBRA-PVT280 49 ).These specimens were excavated from correlate strata that are 500 m distant from the Pivetta site.It is the first time that silesaurs rivaling in size with early dinosaurs are recovered from the oldest unequivocal dinosaur-bearing beds, challenging the idea that in faunas where silesaurs and unambiguous dinosaurs co-occurred, silesaurs were relatively smaller 28 .This discovery reinforces the complex scenario regarding the radiation of Pan-Aves during the Triassic.Surely, the body plan of early diverging forms being surpassed by late diverging dinosaurs does not fit within the current models anymore.Actually, silesaurs -independent of their phylogenetic position-persisted during most of the Triassic Period, with its plesiomorphic body size advancing through the dawn of dinosaurs, instead of silesaur lineages decrease in body size through time.

Figure 1 .
Figure 1.Provenance of Amanasaurus nesbitti gen.et sp.nov.(a) Surface distribution of the geologic units in the area depicting the location of the Pivetta site.(b) General view of the Pivetta site.(c) hypothetical reconstruction of the skeleton of Amanasaurus nesbitti gen.et sp.nov.depicting (in orange) the preserved portions.(d) CAPPA/UFSM 0374 (holotype), a proximal portion of a right femur in anterior view.(e) CAPPA/ UFSM 0375 (referred specimen), a distal portion of a left femur in anterior view.Figures were generated withGIMP 2.8 (https:// www.gimp.org/).
Figure 1.Provenance of Amanasaurus nesbitti gen.et sp.nov.(a) Surface distribution of the geologic units in the area depicting the location of the Pivetta site.(b) General view of the Pivetta site.(c) hypothetical reconstruction of the skeleton of Amanasaurus nesbitti gen.et sp.nov.depicting (in orange) the preserved portions.(d) CAPPA/UFSM 0374 (holotype), a proximal portion of a right femur in anterior view.(e) CAPPA/ UFSM 0375 (referred specimen), a distal portion of a left femur in anterior view.Figures were generated withGIMP 2.8 (https:// www.gimp.org/).

Figure 3 .
Figure 3. Results of the analyzes.(a) Time-calibrated reduced strict consensus tree depicting the phylogenetic position of Amanasaurus nesbitti gen.et sp.nov.(b) Left (reversed) femur of Scutellosaurus lawleri (MNA 175) in lateral view.(c) Left (reversed) femur of Eucoelophysis baldwini (NMMNH P-22298) in lateral view.(d) Right femur of Amanasaurus nesbitti gen.et sp.nov.(CAPPA/UFSM 0374) in lateral view.(e) Right femur of Lewisuchus admixtus (CRILAR-Pv 552) in lateral view.(f) Occurrence of Pan-Aves according the Middle to Upper Triassic Assemblage Zones of Brazil; (g) Plot of log 10 -transformed proximal long axis of the femur versus log 10 -transformed femoral length of distinct ornithodirans depicting (purple triangle) the estimated femoral length for the holotype of Amanasaurus nesbitti gen.et sp.nov.(h) Plot of log 10 -transformed distal long axis of the femur versus log 10 -transformed femoral length of distinct ornithodirans depicting (purple triangle) the estimated femoral length for the referred specimen of Amanasaurus nesbitti gen.et sp.nov.Figures were generated withGIMP 2.8 (https:// www.gimp.org/).
Figure 3. Results of the analyzes.(a) Time-calibrated reduced strict consensus tree depicting the phylogenetic position of Amanasaurus nesbitti gen.et sp.nov.(b) Left (reversed) femur of Scutellosaurus lawleri (MNA 175) in lateral view.(c) Left (reversed) femur of Eucoelophysis baldwini (NMMNH P-22298) in lateral view.(d) Right femur of Amanasaurus nesbitti gen.et sp.nov.(CAPPA/UFSM 0374) in lateral view.(e) Right femur of Lewisuchus admixtus (CRILAR-Pv 552) in lateral view.(f) Occurrence of Pan-Aves according the Middle to Upper Triassic Assemblage Zones of Brazil; (g) Plot of log 10 -transformed proximal long axis of the femur versus log 10 -transformed femoral length of distinct ornithodirans depicting (purple triangle) the estimated femoral length for the holotype of Amanasaurus nesbitti gen.et sp.nov.(h) Plot of log 10 -transformed distal long axis of the femur versus log 10 -transformed femoral length of distinct ornithodirans depicting (purple triangle) the estimated femoral length for the referred specimen of Amanasaurus nesbitti gen.et sp.nov.Figures were generated withGIMP 2.8 (https:// www.gimp.org/).

Table 1 .
Measurements (in mm) of the femur of Amanasaurus nesbitti gen.et sp.nov.