[D] Xenotarsosaurus bonapartei [sG] [T]
Describer
Martinez, Gimenez, Rodriguez & Bochatey, 1987
Time
Cretaceous Late Maastrichtian
Classification
Saurischia Theropoda Tetanurae Carnosauria Allosauridae
Diet
Carnivore
Fossilsite
Bajo Barreal Formation, Chubut, Argentina
Info
Genus - Typespecies
\\\"lizard with a strange ankle\\\" The astragalus and calcaneum are fused, and both are fused to the tibia. Martinez et al,1987 list several resemblances of the femur and dorsal to those of Carnotaurus sastrei. These features may imply that Xenotarsosaurus bonapertei is an abelisaurid . Xenotarsosaurus may be much more primitive, as mentioned by Coria, Salgado, and others in abstracts in Ameghiniana.
Vertebrae, hindlimb elements. The single species, Xenotarsosaurus bonapartei is based on a femur, tibiotarsus, fibula and associated dorsal vertebrae.
Coria, R. A. and J. Rodríguez. 1993. Sobre Xenotarsosaurus bonapartei Martínez, Giménez, Rodríguez, & Bochatey , 1986; un problematico Neoceratosauria (Novas, 1989) del Cretacico de Chubut. Ameghiniana 30 (3):326-327. [On Xenotosaurus bonapartei, Giménez, Rodríguez and Bochatey, 1986, A problematic neoceratosauria (Novas, 1989) form the Cretaceous of Chubut
Translated by Matthew C. Lamanna April 2001
In 1986, Martinez et al., recognized the species Xenotarsosaurus bonapartei, based on appendicular and vertebral remains of a medium sized theropod, coming from the Bajo Barreal Formation (Senonian pre-Maastrichtian). These authors assigned it to the Family Abelisauridae due to similarities that it presented with Carnotaurus. Later, Novas (1989), although maintaining the family assignment, and diagnosing the species based on autapomorphies, recognized that the relationships of Xenotarsosaurus with the remaining Abelisauridae (Abelisaurus and Carnotaurus) are problematic. It proposed then, as a possible synapomorphy of Carnotaurus and Xenotarsosaurus, the presence of a relatively deep prespinal basin (1), although maintaining the possibility that this character is in fact a synapomorphy of Noasaurus + Abelisauridae. The discovery of a theropod in Neuquén, identified as PVPH-35, communicated in previous contributions (Coria et al., 1991) and that represents the sister group of Noasaurus + Abelisauridae (redefined in consequence to the [Neoceratosauria] (= Ceratosaurus + [Noasaurus + Abelisauridae]; sensu Novas 1989, 1992): a) confirms in part the hypothesis of Novas, since the character (1) is not exclusive to Abelisauridae but a synapomorphy of the group (PVPH-35 + [Noasaurus + Abelisauridae]); and b) it weakens the assignment of Xenotarsosaurus to Abelisauridae. It is proposed to preliminarily consider Xenotarsosaurus bonapartei as [Neoceratosauria] incertae sedis, until new materials are known in order to establish with more precision its affinities within the clade. The family Abelisauridae is in this way composed by Abelisaurus comahuensis and Carnotaurus sastrei, a group whose monophyly is strongly supported by characters, at the moment, exclusively cranial not present in PVPH-35 and unknown in Xenotarsosaurus.
Martínez, R., Ginmenez, O., Rodriguez, J., and Bochatey, G. (1986). Xenotosaurus bonapartei gen. et. sp. nov. (Carnosauria, Abelisauridae), un nuevo Theropoda de la Formación Bajo Barreal, Chubut, Argentina. IV Congr. Argentino Paleontol. Bioestratigr. Actas 2, 23-31 [ Xenotarsosaurus bonapartei nov. gen. et sp. (Carnosauria, Abelisauridae), a new Theropoda from the Bajo Barreal Formation, Chubut, Argentina]
Translated by: M. Carrano
Abstract
Part of a right lower limb (femur, tibia, fibula and astragalo-calcaneum) and two incomplete dorsal vertebrae pertaining to a carnosaur from the Bajo Barreal Formation (pre-Maastrichtian Senonian), Province of Chubut, are described as the holotype of Xenotarsosaurus bonapartei. The anterior dorsal vertebrae are amphicoelous, with very large parapophyses, very excavated lateral surfaces of the vertebral centra, and tall prezygapophyses. The femur is gracile, with an anteromedially projecting head, greater trochanter that is continuous with the femoral head, relatively small lesser trochanter, and posteromedially projecting fourth trochanter.
The tibia, with a relatively delicate proximal end, is fused distally with the astragalo-calcaneum. The astragalus and calcaneum are fused with the tibia without a visible boundary in distal and posterior view. The fibula has a compressed shaft and extensive longitudinal contact with the tibia. The similarities with similar elements of Carnotaurus sastrei and their differences with Tyrannosaurus rex and Piatnitzkysaurus floresi, indicate that Xenotarsosaurus bonapartei belonged to the family Abelisauridae. Anatomical characters which suggest phylogenetic relationships with the family [Ceratosauridae] Ceratosauria are pointed out.
Introduction
In recent exploration trips to the sites of the Bajo Barreal Formation existing on the \\\"Ocho Hermanos\\\" ranch, in association with the \\\"Vertebrates of the Bajo Barreal Formation - Cenomanian - Chubut - Argentina\\\" investigatory plan of the Facultad de Ciencias Naturales of the Universidad Nacional de la Patagonia San Juan Bosco, the first diagnostic remains of Theropoda were obtained, probably from the pre-Maastrichtian Senonian (J. C. Sciutto, pers. comm.). A commission from MACN had found fragmentary remains of the same specimen in 1980, which were made available for this study. These remains of carnosaurs, associated with those of the titanosaurid Epachthosaurus sciuttoi Powell (1986), agree with the first taxa recognized from the Local Fauna of \\\"Ocho Hermanos\\\". The fossiliferous locality is relatively rich and we can point out numerous discoveries of small pieces among which are included the remains of fish, anurans, turtles, crocodilians, etc.
Systematics and description
Order Saurischia Seeley, 1888
Suborder Theropoda Marsh, 1881
Infraorder Carnosauria von Huene , 1920
Family Abelisauridae Bonaparte and Novas , 1985
Xenotarsosaurus gen. nov.
Derivation of Name
From the Greek xenos: strange, peculiar; tarsus: foot; saurus: lizard. Alludes to the peculiar arrangement of the astragalo-calcaneum and its relation with the tibia.
Type Species
Xenotarsosaurus bonapartei sp. nov.
Known Distribution
Upper part of the lower member of the Bajo Barreal Formation, pre-Maastrichtian Senonian of Chubut, Argentina.
Diagnosis
Carnosauria of moderate size, with anterior dorsal vertebrae bodies wider than tall, anterior face concave and not flat as in Carnotaurus sastrei, neural arch more elevated and the fossae located above the neural canal deeper than in Carnotaurus sastrei. Femur very similar to that of Carnotaurus sastrei. Tibia with proximal region slighter than in Carnotaurus sastrei, with cavities in the base of the cnemial crest. Fibula and tibia united in a narrow contact with indications of strong ligaments. Astragalus and calcaneum fused, undifferentiable. Contact zone between the astragalus and tibia not visible in posterior view. Fibular shaft more compressed mediolaterally. Femur-tibia ratio 1:0.94.
Xenotarsosaurus bonapartei sp. nov.
Derivation of Name
In honor of Dr. José Fernando Bonaparte, eminent student of Mesozoic vertebrates.
Holotype
Two incomplete cervico-dorsal vertebrae, complete femur, tibia, fibula, and astragalo-calcaneum from the right side, pertaining to the same individual; paleovertebrates collection of the Universidad Nacional de la Patagonia San Juan Bosco (Comodoro Rivadavia - U.N.P.S.J.B. - Pv. 184 and U.N.P.S.J.B. - Pv. 612).
Stratigraphi and geographicprovenance
Upper part of the lower member of the Bajo Barreal Formation, 6 km north of the compound of the \\\"Ocho Hermanos\\\" ranch, Departamento Sarmiento, Province of Chubut, Argentina.
Diagnosis
The same as for the monotypic genus.
Description
Vertebrae
Two incomplete vertebrae are available from the anterior dorsal region. One of these shows a concave anterior face on the vertebral centrum, different from the same vertebrae of Carnotaurus sastrei Bonaparte (1985), which is flat. The parapophyses, very large, occupy almost the entire anterolateral edge of the vertebral body. The lateral face of the vertebral centrum is very excavated with a pronounced posterior border and a strong depression where a small pleurocoel is located. The neural arch is incomplete, but reveals a basic design similar to that of Carnotaurus sastrei. The prezygapophyses, incomplete, are tall and do not surpass the anterior border of the vertebral body. Underneath the prezygapophyses, in anterior view, observe two wide cavities located over the neural canal, which are deeper than in Carnotaurus sastrei .
Hind Limb
The complete femur, tibia, fibula and astragalo-calcaneum are available. The tibia and fibula were found articulated. The proportions between the length of the femur and the tibia without the astragalo-calcaneum are practically equal, since the ratio is 1:0.94.
Femur
Relatively gracile aspect. Shaft almost straight in posterior view, while medially observe a fairly obvious curvature, whose convexity corresponds to the dorsal surface of the shaft and whose concavity to the ventral surface of the same. The head has an anteromedial orientation of approximately 45° relative to the major axis of the femur; it is semispherical and presents a smooth articular surface, with a clear neck. The greater trochanter is continuous with the femoral head. The lesser trochanter is well defined and projects anteriorly. At the base of the lesser trochanter, on the lateral face, there is a bony edge that serves for muscular insertion, as in Ceratosaurus nasicornis (Marsh, 1884).
The fourth trochanter rises as a prominent, thick, posteromedially-directed crest. In the distal end the condyles project well posteriorly. The internal tibial condyle, of globular form and compressed laterally equal to the external tibial and fibular, stands out due to its size with respect to the other two. The external tibial condyle, the smallest, is also inclined laterally. There is a very pronounced intercondylar groove on the posterior face, which continues anteriorly in a less marked form. In medial view, in the distal third, note a wide ridge that clearly delimits the medial and anterior faces. This ridge divides the anterior border from the internal tibial condyle and extends down nearly a third of the length of the bone, also defining a large depression in anterior view and another less accentuated in medial view, a character that is also present in Carnotaurus sastrei and Ceratosaurus nasicornis.
Tibia , astragalo - calcaneum and fibula
These bones are described as a whole due to their more or less accentuated fusion. The right tibia, in a good state of preservation, seems somewhat shorter than the femur and relatively slender. The shaft is straight in anterior view. The distal end shows approximately 90° of torsion with respect to the proximal end. The proximal articular surface is irregular and the internal border of the cnemial crest is convex, while that of the external border is concave.
It has a globular lateral articular condyle for articulation with the femur, which is separated from the rest of the articulation by a furrow in the posterior part. The process for the fibular, in a more proximal position, incomplete, terminates distally at the same height as the distal border of the cnemial crest.
On the lateral face of the cnemial crest there is a well marked cavity on the posterior border for the fibular crest. The shaft shows in posteromedial view, in the upper third, marks for muscular insertions. In lateral view observe on the length of approximately three quarters part of the shaft, the area of insertion for the tibio-fibular ligaments. Due to its extension, it indicates an almost total lack of mobility between the two bones.
Distally, the tibia is enlarged in form abruptly, receiving the astragalus and calcaneum, which are fused and with no observable delimitation between them. In lateral view the distal end of the fibula has a clear contact with the astragalo-calcaneum. In the distal end of the element, in lateral view and continuing anteriorly, observe a furrow which is a proper feature of the astragalo-calcaneum, as in Ceratosaurus nasicornis, which nevertheless keeps both bones well individualized. A furrow is not observed in the union of the astragalo-calcaneum and the tibia, neither in posterior nor in distal view, which can result from an absolute fusion that already leaves the surface of the bone without visible sutures.
On the external zone of the anterior and lateral views, observe clearly the separation between the astragalo-calcaneum and the distal end of the fibula. The tibia, distally and in posterior view presents a lateral sector more prominent than the medial, from which it is separated by a slight depression. The preservation of the astragalo-calcaneum is not good, both bones being fused and undifferentiable. Observe the ascending process of the astragalus, small, which was attached to the anterior face of the tibia. The astragalo-calcaneum has its anterior and distal face concave, with the internal sector more distal than the rest.
The fibula is a bone whose proximal end is well expanded. The femoral articular surface is relatively concave with some rugosities, appearing wider than tall in front. In medial view it has a deep, well-delimited concavity which extends from the proximal end to the length of the first third of the bone. This cavity is covered in part by a bony fold that rises from the anterior border and which in turn delimits a small external depression. In medial view of the shaft observe a rugose surface for ligamentous insertion, which indicates a very firm relationship between the tibia and fibula.
Anteriorly the proximal end is very convex and the anterolateral process stands out, which is located within the proximal third of the element. The shaft is compressed mediolaterally. The distal end is expanded and rounded, with a convex area of attachment for the calcaneum, seeming medially concave. There is a contact between the distal and anterior section of the fibula with the tibia and ascending process of the astragalus, and another posteriorly with the external expansion of the tibia.
Comparison
The comparisons are limited because only an incomplete right hind limb and a vertebrae with good diagnostic characters are available. The hind limbs of Theropoda are very indicative of the grade of specialization obtained, since they were principally locomotor organs; along the history of these carnivores note good characters indicative of the specialization attained.
Among the Argentine forms, the available Theropoda are Piatnitzkysaurus floresi from the Middle Jurassic of Chubut (Bonaparte, 1979) and Carnotaurus sastrei from the Albian? of the same province (Bonaparte, 1985). With the first the differences are the grade of evolution since Piatnitzkysaurus floresi shows an astragalus not fused to the tibia, and the fibula is well separated from the major part of the tibia. It is recorded that this carnosaur is from the Middle Jurassic, although the species we describe is from the Upper Cretaceous. With Carnotaurus sastrei of which we have a practically complete skeleton, note a clear general affinity in the femur and proximal portion of the tibia.
With non-South American forms, we are warned of some suggestive affinities with Ceratosaurus nasicornis (see Gilmore, C. W., 1920), united by distinctive characters, since this form from Laurasia is from the Upper Jurassic. With forms from the Upper Cretaceous, such as Tyrannosaurus rex (see Osborn, H. , 1917) note as strong differences in the femur as in the general structure of the hind limb. This indicates to us that they are families with accentuated differences in the hind limb.
Comparis on with Piatnitzkysaurus floresi
The anterior dorsal vertebrae of Piatnitzkysaurus floresi are opisthocoelous or platycoelous and, in lateral view, have very excavated vertebral centra; the disposition of the laminae of the neural arch is similar, with deeper cavities than in Xenotarsosaurus bonapartei. The parapophyses of Piatnitzkysaurus floresi are smaller and the pleurocoels more developed. Although the femur has a head projecting as in Xenotarsosaurus bonapartei, the location and morphology of the greater and lesser trochanters are distinct. The proximal end of the tibia is more massive than in X. bonapartei, and the fibular process more distal. The astragalus, calcaneum and tibia are not fused, and the fibula is very separated from the tibia. These differences suggest that the species compared are from distinct families.
Comparis on with Ceratosaurus nasicornis
In Ceratosaurus nasicornis (see Gilmore, C. W., 1920), the grade of amphicoely of the anterior dorsal vertebrae is comparable to that of Xenotarsosaurus bonapartei, but the centra are not laterally excavated and the parapophyses are reduced. The femur has a similar lesser trochanter and, on its base, an osseous border comparable to that of Xenotarsosaurus bonapartei.
The orientation and form of the femoral head and the location of the greater and fourth trochanters are different. In general design the tibia is similar. The relationships between the astragalo-calcaneum (fused), fibula and tibia are similar distally, but reveal a more advanced grade of fusion in Xenotarsosaurus bonapartei. The astragalo-calcaneum of Ceratosaurus nasicornis has two furrows, in anterior view, similar to those of Xenotarsosaurus bonapartei. The ascending process is also similar. These affinities, difficult to adequately evaluate, could indicate some phylogenetic relationship between the families Abelisauridae and [Ceratosauridae] Ceratosauria.
Comparison with Carnotaurus sastrei
The anterior dorsal vertebrae of Carnotaurus sastrei reveal a comparable design, with some characteristics more accentuated than in Xenotarsosaurus bonapartei, such as the deeper lateral cavities of the centra, larger parapophyses, and more marked cavities located below the prezygapophyses. The anterior face of the vertebral centrum is flat in Carnotaurus sastrei , while in Xenotarsosaurus bonapartei it is concave. That which is preserved of the femora of Carnotaurus sastrei demonstrates that they were practically the same, with traits such as the semispherical femoral head, lesser trochanter similar in position, extension and projection; very similar morphology and disposition of the distal condyles, while the distal crest that delimits the anterior and medial face is similar in its morphology, position and extension. The differences that can be pointed out are: in Carnotaurus sastrei the greater trochanter is more differentiated from the shaft and is proportionally longer. The tibia of Carnotaurus sastrei is heavier in construction proximally, with less pronounced cavities in the base of the cnemial crest than in Xenotarsosaurus bonapartei.
It is then very probable that both forms correspond to the same family: Abelisauridae.
Comparison with Tyrannosaurus rex
The design of the anterior dorsal vertebrae reveal differences with respect to Xenotarsosaurus bonapartei, such as their amphiplatyan or slightly opisthocoelous articular faces, instead of being amphicoelous; smaller and ventrally displaced parapophyses, more centrally placed pleurocoels than in Xenotarsosaurus bonapartei and the vertebral centra little excavated in lateral view. The femur is of distinct characteristics, which are revealed in the medial orientation of the head, the morphology and placement of the lesser trochanter which projects more proximally, reaching the height of the greater trochanter; the fourth trochanter an the intercondylar depression are distinct. The fibular process is more distal. The relationships in the distal part of the tibia between the astragalus, calcaneum and fibula are distinct, since each bone is well individualized. The ascending process of the astragalus is very pronounced in T. rex while that in Xenotarsosaurus bonapartei is more reduced.
Conclusions
From the comparison with Carnotaurus sastrei arises a series of common traits in the vertebrae and which are preserved in the femur and tibia, which indicate that Xenotarsosaurus bonapartei is located taxonomically within the family Abelisauridae. The existing differences justify the recognition of a new genus. Lamentably it is not possible to effect comparisons with Abelisaurus comahuensis (Bonaparte and Novas, 1985), because it is represented only by the skull. The differences with Tyrannosaurus rex confirm that these are phylogenetically distinct forms, but of comparable habits. The similarities observed with Ceratosaurus nasicornis in the few available skeletal remains, could indicate a phylogenetic relationship with the families Abelisauridae and [Ceratosauridae] Ceratosauria , or a grade of convergence between forms of distinct families.
Acknowledgement
In the first place, we wish to thank Dr. José F. Bonaparte for his guidance and for having kindly placed at our disposition the material necessary for the comparisons. Likewise we are indebted to the U.N.P.S.J.B. for the economic support necessary for our work in Buenos Aires, to Lic. Omar Césari for the constant stimulus that he gave us with his help of all kinds, and to Lic. Juan C. Sciutto for his valuable observations on the geology of the region. Also we wish to thank the Valbuena family for their hospitality at the \\\"Ocho Hermanos\\\" ranch.
Source: Polyglot Paleontologist
Martinez, Gimenez, Rodriguez & Bochatey, 1987
Time
Cretaceous Late Maastrichtian
Classification
Saurischia Theropoda Tetanurae Carnosauria Allosauridae
Diet
Carnivore
Fossilsite
Bajo Barreal Formation, Chubut, Argentina
Info
Genus - Typespecies
\\\"lizard with a strange ankle\\\" The astragalus and calcaneum are fused, and both are fused to the tibia. Martinez et al,1987 list several resemblances of the femur and dorsal to those of Carnotaurus sastrei. These features may imply that Xenotarsosaurus bonapertei is an abelisaurid . Xenotarsosaurus may be much more primitive, as mentioned by Coria, Salgado, and others in abstracts in Ameghiniana.
Vertebrae, hindlimb elements. The single species, Xenotarsosaurus bonapartei is based on a femur, tibiotarsus, fibula and associated dorsal vertebrae.
Coria, R. A. and J. Rodríguez. 1993. Sobre Xenotarsosaurus bonapartei Martínez, Giménez, Rodríguez, & Bochatey , 1986; un problematico Neoceratosauria (Novas, 1989) del Cretacico de Chubut. Ameghiniana 30 (3):326-327. [On Xenotosaurus bonapartei, Giménez, Rodríguez and Bochatey, 1986, A problematic neoceratosauria (Novas, 1989) form the Cretaceous of Chubut
Translated by Matthew C. Lamanna April 2001
In 1986, Martinez et al., recognized the species Xenotarsosaurus bonapartei, based on appendicular and vertebral remains of a medium sized theropod, coming from the Bajo Barreal Formation (Senonian pre-Maastrichtian). These authors assigned it to the Family Abelisauridae due to similarities that it presented with Carnotaurus. Later, Novas (1989), although maintaining the family assignment, and diagnosing the species based on autapomorphies, recognized that the relationships of Xenotarsosaurus with the remaining Abelisauridae (Abelisaurus and Carnotaurus) are problematic. It proposed then, as a possible synapomorphy of Carnotaurus and Xenotarsosaurus, the presence of a relatively deep prespinal basin (1), although maintaining the possibility that this character is in fact a synapomorphy of Noasaurus + Abelisauridae. The discovery of a theropod in Neuquén, identified as PVPH-35, communicated in previous contributions (Coria et al., 1991) and that represents the sister group of Noasaurus + Abelisauridae (redefined in consequence to the [Neoceratosauria] (= Ceratosaurus + [Noasaurus + Abelisauridae]; sensu Novas 1989, 1992): a) confirms in part the hypothesis of Novas, since the character (1) is not exclusive to Abelisauridae but a synapomorphy of the group (PVPH-35 + [Noasaurus + Abelisauridae]); and b) it weakens the assignment of Xenotarsosaurus to Abelisauridae. It is proposed to preliminarily consider Xenotarsosaurus bonapartei as [Neoceratosauria] incertae sedis, until new materials are known in order to establish with more precision its affinities within the clade. The family Abelisauridae is in this way composed by Abelisaurus comahuensis and Carnotaurus sastrei, a group whose monophyly is strongly supported by characters, at the moment, exclusively cranial not present in PVPH-35 and unknown in Xenotarsosaurus.
Martínez, R., Ginmenez, O., Rodriguez, J., and Bochatey, G. (1986). Xenotosaurus bonapartei gen. et. sp. nov. (Carnosauria, Abelisauridae), un nuevo Theropoda de la Formación Bajo Barreal, Chubut, Argentina. IV Congr. Argentino Paleontol. Bioestratigr. Actas 2, 23-31 [ Xenotarsosaurus bonapartei nov. gen. et sp. (Carnosauria, Abelisauridae), a new Theropoda from the Bajo Barreal Formation, Chubut, Argentina]
Translated by: M. Carrano
Abstract
Part of a right lower limb (femur, tibia, fibula and astragalo-calcaneum) and two incomplete dorsal vertebrae pertaining to a carnosaur from the Bajo Barreal Formation (pre-Maastrichtian Senonian), Province of Chubut, are described as the holotype of Xenotarsosaurus bonapartei. The anterior dorsal vertebrae are amphicoelous, with very large parapophyses, very excavated lateral surfaces of the vertebral centra, and tall prezygapophyses. The femur is gracile, with an anteromedially projecting head, greater trochanter that is continuous with the femoral head, relatively small lesser trochanter, and posteromedially projecting fourth trochanter.
The tibia, with a relatively delicate proximal end, is fused distally with the astragalo-calcaneum. The astragalus and calcaneum are fused with the tibia without a visible boundary in distal and posterior view. The fibula has a compressed shaft and extensive longitudinal contact with the tibia. The similarities with similar elements of Carnotaurus sastrei and their differences with Tyrannosaurus rex and Piatnitzkysaurus floresi, indicate that Xenotarsosaurus bonapartei belonged to the family Abelisauridae. Anatomical characters which suggest phylogenetic relationships with the family [Ceratosauridae] Ceratosauria are pointed out.
Introduction
In recent exploration trips to the sites of the Bajo Barreal Formation existing on the \\\"Ocho Hermanos\\\" ranch, in association with the \\\"Vertebrates of the Bajo Barreal Formation - Cenomanian - Chubut - Argentina\\\" investigatory plan of the Facultad de Ciencias Naturales of the Universidad Nacional de la Patagonia San Juan Bosco, the first diagnostic remains of Theropoda were obtained, probably from the pre-Maastrichtian Senonian (J. C. Sciutto, pers. comm.). A commission from MACN had found fragmentary remains of the same specimen in 1980, which were made available for this study. These remains of carnosaurs, associated with those of the titanosaurid Epachthosaurus sciuttoi Powell (1986), agree with the first taxa recognized from the Local Fauna of \\\"Ocho Hermanos\\\". The fossiliferous locality is relatively rich and we can point out numerous discoveries of small pieces among which are included the remains of fish, anurans, turtles, crocodilians, etc.
Systematics and description
Order Saurischia Seeley, 1888
Suborder Theropoda Marsh, 1881
Infraorder Carnosauria von Huene , 1920
Family Abelisauridae Bonaparte and Novas , 1985
Xenotarsosaurus gen. nov.
Derivation of Name
From the Greek xenos: strange, peculiar; tarsus: foot; saurus: lizard. Alludes to the peculiar arrangement of the astragalo-calcaneum and its relation with the tibia.
Type Species
Xenotarsosaurus bonapartei sp. nov.
Known Distribution
Upper part of the lower member of the Bajo Barreal Formation, pre-Maastrichtian Senonian of Chubut, Argentina.
Diagnosis
Carnosauria of moderate size, with anterior dorsal vertebrae bodies wider than tall, anterior face concave and not flat as in Carnotaurus sastrei, neural arch more elevated and the fossae located above the neural canal deeper than in Carnotaurus sastrei. Femur very similar to that of Carnotaurus sastrei. Tibia with proximal region slighter than in Carnotaurus sastrei, with cavities in the base of the cnemial crest. Fibula and tibia united in a narrow contact with indications of strong ligaments. Astragalus and calcaneum fused, undifferentiable. Contact zone between the astragalus and tibia not visible in posterior view. Fibular shaft more compressed mediolaterally. Femur-tibia ratio 1:0.94.
Xenotarsosaurus bonapartei sp. nov.
Derivation of Name
In honor of Dr. José Fernando Bonaparte, eminent student of Mesozoic vertebrates.
Holotype
Two incomplete cervico-dorsal vertebrae, complete femur, tibia, fibula, and astragalo-calcaneum from the right side, pertaining to the same individual; paleovertebrates collection of the Universidad Nacional de la Patagonia San Juan Bosco (Comodoro Rivadavia - U.N.P.S.J.B. - Pv. 184 and U.N.P.S.J.B. - Pv. 612).
Stratigraphi and geographicprovenance
Upper part of the lower member of the Bajo Barreal Formation, 6 km north of the compound of the \\\"Ocho Hermanos\\\" ranch, Departamento Sarmiento, Province of Chubut, Argentina.
Diagnosis
The same as for the monotypic genus.
Description
Vertebrae
Two incomplete vertebrae are available from the anterior dorsal region. One of these shows a concave anterior face on the vertebral centrum, different from the same vertebrae of Carnotaurus sastrei Bonaparte (1985), which is flat. The parapophyses, very large, occupy almost the entire anterolateral edge of the vertebral body. The lateral face of the vertebral centrum is very excavated with a pronounced posterior border and a strong depression where a small pleurocoel is located. The neural arch is incomplete, but reveals a basic design similar to that of Carnotaurus sastrei. The prezygapophyses, incomplete, are tall and do not surpass the anterior border of the vertebral body. Underneath the prezygapophyses, in anterior view, observe two wide cavities located over the neural canal, which are deeper than in Carnotaurus sastrei .
Hind Limb
The complete femur, tibia, fibula and astragalo-calcaneum are available. The tibia and fibula were found articulated. The proportions between the length of the femur and the tibia without the astragalo-calcaneum are practically equal, since the ratio is 1:0.94.
Femur
Relatively gracile aspect. Shaft almost straight in posterior view, while medially observe a fairly obvious curvature, whose convexity corresponds to the dorsal surface of the shaft and whose concavity to the ventral surface of the same. The head has an anteromedial orientation of approximately 45° relative to the major axis of the femur; it is semispherical and presents a smooth articular surface, with a clear neck. The greater trochanter is continuous with the femoral head. The lesser trochanter is well defined and projects anteriorly. At the base of the lesser trochanter, on the lateral face, there is a bony edge that serves for muscular insertion, as in Ceratosaurus nasicornis (Marsh, 1884).
The fourth trochanter rises as a prominent, thick, posteromedially-directed crest. In the distal end the condyles project well posteriorly. The internal tibial condyle, of globular form and compressed laterally equal to the external tibial and fibular, stands out due to its size with respect to the other two. The external tibial condyle, the smallest, is also inclined laterally. There is a very pronounced intercondylar groove on the posterior face, which continues anteriorly in a less marked form. In medial view, in the distal third, note a wide ridge that clearly delimits the medial and anterior faces. This ridge divides the anterior border from the internal tibial condyle and extends down nearly a third of the length of the bone, also defining a large depression in anterior view and another less accentuated in medial view, a character that is also present in Carnotaurus sastrei and Ceratosaurus nasicornis.
Tibia , astragalo - calcaneum and fibula
These bones are described as a whole due to their more or less accentuated fusion. The right tibia, in a good state of preservation, seems somewhat shorter than the femur and relatively slender. The shaft is straight in anterior view. The distal end shows approximately 90° of torsion with respect to the proximal end. The proximal articular surface is irregular and the internal border of the cnemial crest is convex, while that of the external border is concave.
It has a globular lateral articular condyle for articulation with the femur, which is separated from the rest of the articulation by a furrow in the posterior part. The process for the fibular, in a more proximal position, incomplete, terminates distally at the same height as the distal border of the cnemial crest.
On the lateral face of the cnemial crest there is a well marked cavity on the posterior border for the fibular crest. The shaft shows in posteromedial view, in the upper third, marks for muscular insertions. In lateral view observe on the length of approximately three quarters part of the shaft, the area of insertion for the tibio-fibular ligaments. Due to its extension, it indicates an almost total lack of mobility between the two bones.
Distally, the tibia is enlarged in form abruptly, receiving the astragalus and calcaneum, which are fused and with no observable delimitation between them. In lateral view the distal end of the fibula has a clear contact with the astragalo-calcaneum. In the distal end of the element, in lateral view and continuing anteriorly, observe a furrow which is a proper feature of the astragalo-calcaneum, as in Ceratosaurus nasicornis, which nevertheless keeps both bones well individualized. A furrow is not observed in the union of the astragalo-calcaneum and the tibia, neither in posterior nor in distal view, which can result from an absolute fusion that already leaves the surface of the bone without visible sutures.
On the external zone of the anterior and lateral views, observe clearly the separation between the astragalo-calcaneum and the distal end of the fibula. The tibia, distally and in posterior view presents a lateral sector more prominent than the medial, from which it is separated by a slight depression. The preservation of the astragalo-calcaneum is not good, both bones being fused and undifferentiable. Observe the ascending process of the astragalus, small, which was attached to the anterior face of the tibia. The astragalo-calcaneum has its anterior and distal face concave, with the internal sector more distal than the rest.
The fibula is a bone whose proximal end is well expanded. The femoral articular surface is relatively concave with some rugosities, appearing wider than tall in front. In medial view it has a deep, well-delimited concavity which extends from the proximal end to the length of the first third of the bone. This cavity is covered in part by a bony fold that rises from the anterior border and which in turn delimits a small external depression. In medial view of the shaft observe a rugose surface for ligamentous insertion, which indicates a very firm relationship between the tibia and fibula.
Anteriorly the proximal end is very convex and the anterolateral process stands out, which is located within the proximal third of the element. The shaft is compressed mediolaterally. The distal end is expanded and rounded, with a convex area of attachment for the calcaneum, seeming medially concave. There is a contact between the distal and anterior section of the fibula with the tibia and ascending process of the astragalus, and another posteriorly with the external expansion of the tibia.
Comparison
The comparisons are limited because only an incomplete right hind limb and a vertebrae with good diagnostic characters are available. The hind limbs of Theropoda are very indicative of the grade of specialization obtained, since they were principally locomotor organs; along the history of these carnivores note good characters indicative of the specialization attained.
Among the Argentine forms, the available Theropoda are Piatnitzkysaurus floresi from the Middle Jurassic of Chubut (Bonaparte, 1979) and Carnotaurus sastrei from the Albian? of the same province (Bonaparte, 1985). With the first the differences are the grade of evolution since Piatnitzkysaurus floresi shows an astragalus not fused to the tibia, and the fibula is well separated from the major part of the tibia. It is recorded that this carnosaur is from the Middle Jurassic, although the species we describe is from the Upper Cretaceous. With Carnotaurus sastrei of which we have a practically complete skeleton, note a clear general affinity in the femur and proximal portion of the tibia.
With non-South American forms, we are warned of some suggestive affinities with Ceratosaurus nasicornis (see Gilmore, C. W., 1920), united by distinctive characters, since this form from Laurasia is from the Upper Jurassic. With forms from the Upper Cretaceous, such as Tyrannosaurus rex (see Osborn, H. , 1917) note as strong differences in the femur as in the general structure of the hind limb. This indicates to us that they are families with accentuated differences in the hind limb.
Comparis on with Piatnitzkysaurus floresi
The anterior dorsal vertebrae of Piatnitzkysaurus floresi are opisthocoelous or platycoelous and, in lateral view, have very excavated vertebral centra; the disposition of the laminae of the neural arch is similar, with deeper cavities than in Xenotarsosaurus bonapartei. The parapophyses of Piatnitzkysaurus floresi are smaller and the pleurocoels more developed. Although the femur has a head projecting as in Xenotarsosaurus bonapartei, the location and morphology of the greater and lesser trochanters are distinct. The proximal end of the tibia is more massive than in X. bonapartei, and the fibular process more distal. The astragalus, calcaneum and tibia are not fused, and the fibula is very separated from the tibia. These differences suggest that the species compared are from distinct families.
Comparis on with Ceratosaurus nasicornis
In Ceratosaurus nasicornis (see Gilmore, C. W., 1920), the grade of amphicoely of the anterior dorsal vertebrae is comparable to that of Xenotarsosaurus bonapartei, but the centra are not laterally excavated and the parapophyses are reduced. The femur has a similar lesser trochanter and, on its base, an osseous border comparable to that of Xenotarsosaurus bonapartei.
The orientation and form of the femoral head and the location of the greater and fourth trochanters are different. In general design the tibia is similar. The relationships between the astragalo-calcaneum (fused), fibula and tibia are similar distally, but reveal a more advanced grade of fusion in Xenotarsosaurus bonapartei. The astragalo-calcaneum of Ceratosaurus nasicornis has two furrows, in anterior view, similar to those of Xenotarsosaurus bonapartei. The ascending process is also similar. These affinities, difficult to adequately evaluate, could indicate some phylogenetic relationship between the families Abelisauridae and [Ceratosauridae] Ceratosauria.
Comparison with Carnotaurus sastrei
The anterior dorsal vertebrae of Carnotaurus sastrei reveal a comparable design, with some characteristics more accentuated than in Xenotarsosaurus bonapartei, such as the deeper lateral cavities of the centra, larger parapophyses, and more marked cavities located below the prezygapophyses. The anterior face of the vertebral centrum is flat in Carnotaurus sastrei , while in Xenotarsosaurus bonapartei it is concave. That which is preserved of the femora of Carnotaurus sastrei demonstrates that they were practically the same, with traits such as the semispherical femoral head, lesser trochanter similar in position, extension and projection; very similar morphology and disposition of the distal condyles, while the distal crest that delimits the anterior and medial face is similar in its morphology, position and extension. The differences that can be pointed out are: in Carnotaurus sastrei the greater trochanter is more differentiated from the shaft and is proportionally longer. The tibia of Carnotaurus sastrei is heavier in construction proximally, with less pronounced cavities in the base of the cnemial crest than in Xenotarsosaurus bonapartei.
It is then very probable that both forms correspond to the same family: Abelisauridae.
Comparison with Tyrannosaurus rex
The design of the anterior dorsal vertebrae reveal differences with respect to Xenotarsosaurus bonapartei, such as their amphiplatyan or slightly opisthocoelous articular faces, instead of being amphicoelous; smaller and ventrally displaced parapophyses, more centrally placed pleurocoels than in Xenotarsosaurus bonapartei and the vertebral centra little excavated in lateral view. The femur is of distinct characteristics, which are revealed in the medial orientation of the head, the morphology and placement of the lesser trochanter which projects more proximally, reaching the height of the greater trochanter; the fourth trochanter an the intercondylar depression are distinct. The fibular process is more distal. The relationships in the distal part of the tibia between the astragalus, calcaneum and fibula are distinct, since each bone is well individualized. The ascending process of the astragalus is very pronounced in T. rex while that in Xenotarsosaurus bonapartei is more reduced.
Conclusions
From the comparison with Carnotaurus sastrei arises a series of common traits in the vertebrae and which are preserved in the femur and tibia, which indicate that Xenotarsosaurus bonapartei is located taxonomically within the family Abelisauridae. The existing differences justify the recognition of a new genus. Lamentably it is not possible to effect comparisons with Abelisaurus comahuensis (Bonaparte and Novas, 1985), because it is represented only by the skull. The differences with Tyrannosaurus rex confirm that these are phylogenetically distinct forms, but of comparable habits. The similarities observed with Ceratosaurus nasicornis in the few available skeletal remains, could indicate a phylogenetic relationship with the families Abelisauridae and [Ceratosauridae] Ceratosauria , or a grade of convergence between forms of distinct families.
Acknowledgement
In the first place, we wish to thank Dr. José F. Bonaparte for his guidance and for having kindly placed at our disposition the material necessary for the comparisons. Likewise we are indebted to the U.N.P.S.J.B. for the economic support necessary for our work in Buenos Aires, to Lic. Omar Césari for the constant stimulus that he gave us with his help of all kinds, and to Lic. Juan C. Sciutto for his valuable observations on the geology of the region. Also we wish to thank the Valbuena family for their hospitality at the \\\"Ocho Hermanos\\\" ranch.
Source: Polyglot Paleontologist