NEW TITANOSAUR WITH UNUSUAL HAEMAL
ARCHES FROM THE UPPER CRETACEOUS OF
NEUQUÉN PROVINCE, ARGENTINA
1División Paleontología de Vertebrados, Museo de La Plata. Paseo del Bosque s/n, B1900FWA, La Plata, Argentina. alexandros.otero@gmail.com 2Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET)
3Área Laboratorio e Investigación, Museo “Ernesto Bachmann”, Centro Cívico s/s, 8311 Villa El Chocón, Argentina. juanignaciocanale@hotmail.com; jujuyaspis@yahoo.com
4Centro Paleontológico Lago Barreales, Universidad Nacional del Comahue, Ruta Provincial 51, Kilómetro 65, Neuquén, Argentina. jorgecalvo@proyectodino.com.ar ALEJANDRO OTERO1,2, JUAN IGNACIO CANALE2,3, ALEJANDRO HALUZA3 and JORGE ORLANDO CALVO4
Key Words: Sauropoda. Titanosauria. Chevron. Caudal series. Neuquén Province. The Neuquén Basin has yielded a highly diverse fauna of
sauropod dinosaurs. This group includes diplodocoids (e.g., Salgado and Bonaparte, 1991; Salgado et al., 2004; Haluza
et al., 2009) and titanosaurs (e.g., Lydekker, 1893; Calvo and
Bonaparte, 1991; Bonaparte and Coria, 1993; Salgado and Azpilicueta, 2000; Apesteguía, 2004; González Riga, 2003; Filippi et al., 2011).
The plentiful record of titanosaur sauropods in the Up-per Cretaceous of northern Patagonia is mostly restrict-ed to derivrestrict-ed titanosaurs includrestrict-ed in the Titanosauridae (=Lithostrotia sensu Upchurch et al., 2004). Derived titano-saurs were traditionally characterized by the possession of strongly procoelous anterior-middle caudal vertebrae. This feature was used as a diagnostic trait to differentiate them
from basal forms of titanosaurs with amphiplatyan or am-phicoelous caudal vertebrae (Salgado et al., 1997). However recent discoveries have demonstrated that a great variation exists throughout the titanosaurian caudal series (Salgado and Calvo, 1993; Wilson et al., 1999; Calvo and González Riga, 2003; González Riga et al., 2009). In this sense, pro-coely appears to be non-continuous along the tail, implying a very different morphological scenario.
In this paper we report a series of sauropod caudal verte-brae collected from the upper levels of the Candeleros For-mation (Neuquén Group) near Villa El Chocón, Neuquén Province, Argentina. The materials, previously referred ten-tatively to Andesaurus (Otero et al., 2006), include a discon-tinuous series of mid- and mid-posterior caudal vertebrae,
Figure 1. 1, General location map of Neuquén Province; 2, location of MMCH-Pv 47 within Villa El Chocón, Neuquén Province; 3, simplified
sec-tion of the site showing the stratigraphical posisec-tion of MMCH-Pv 47 (modified from Leanza et al., 2004)/ mapa general de la provincia del Neuquén;
2, ubicación de MMCH-Pv 47 en Villa El Chocón, provincia del Neuquén; 3, sección simplificada mostrando la ubicación estratigráfica de MMCH-Pv 47
(modificado de Leanza et al., 2004).
1 2 3
most of them including their haemal arches. The latter ele-ments present an unusual morphology hitherto not reported in other sauropod dinosaurs.
Institutional Abbreviations. MAU, Museo Municipal “Ar-gentino Urquiza”, Rincón de los Sauces, Neuquén Province, Argentina. Mcf-PVPH, Museo “Carmen Funes”, Plaza Huincul, Neuquén Province, Argentina. MMCH, Área Laboratorio e Investigación, Museo “Ernesto Bachmann”, Villa El Chocón, Neuquén Province, Argentina. MUCPv, Centro Paleontológico Lago Barreales, Neuquén Province, Argentina.
SYSTEMATIC PALEONTOLOGY Order Saurischia Seeley, 1888 Suborder Sauropodomorpha Huene, 1932
Infraorder Sauropoda Marsh, 1878 Unranked Titanosauria Bonaparte and Coria, 1993
Gen. et sp. indet.
Referred material. MMCH-Pv 47, a discontinuous series of nine caudal vertebrae, distributed in a series of five mid-cau-dal vertebrae with their associated haemal arches, and four associated mid-posterior caudal vertebrae with two associ-ated haemal arches, and an isolassoci-ated posterior haemal arch.
Locality and horizon. Cañadón del Cocodrilo, near Li-may River, Villa El Chocón, Neuquén Province, Argentina. The material was recovered from upper levels of Candeleros
Formation (Albian?–Cenomanian according to Calvo and Bonaparte, 1991, or lower Cenomanian, Upper Cretaceous according to Leanza et al., 2004, and Garrido, 2010). The Candeleros Formation is composed of red and purple mas-sive, coarse- and medium-grained sandstones and conglom-erates. It was deposited in a meandering river of low-flow regime in a braided, aeolian environment (Spalletti and Gazzera, 1989). The remains were recovered from alternat-ing sand and clay levels. Paleosols are also common (Fig. 1).
Description
Caudal vertebrae. The description is based mainly on the second vertebra of the series (MMCH-Pv 47/2, Fig. 2.1b– 3b), which is the most complete and least altered element and corresponds closely to the anteriormost one of the series.
The centrum is as wide as high (see Table 1 for measure-ments), sub-quadrangular in lateral view and with slightly concave cranial and caudal faces. The lateral and ventral faces are also craniocaudally concave. The ventral surface carries a longitudinally hollow flanked by the chevron facets, the latter placed at each corner of the ventral surface of the centrum. The transverse processes are poorly developed and are placed at the dorsal margin of the centrum, close to the midpoint of the craniocaudal length. The anteriormost five caudals of the series (i.e., MMCH-Pv 47/1/2/3/4/5) show a slight depres-sion with a pleurocoel-like fossa in lateral view. Two thin and TABLE 1 - Measurements of caudal vertebrae of MMCH-Pv 47 (in millimeters)(table adapted from Mannion and Calvo, 2011)
*Preserved measurement. Abbreviations: CL = centrum length; ACH = anterior centrum height (excluding chevron facets); ACW = anterior centrum width; PCH = posterior centrum height (excluding chevron facets); PCW = posterior centrum width; DFA = distance from anterior end of centrum to anterior end of neural arch; DFP = distance from posterior end of centrum to posterior end of neural arch; NAH = neural arch height (measured from the dorsal surface of the centrum up to the base of the postzygapophyses); NSH = neural spine height (measured from the base of the postzyg-apophyses upwards); NSL = neural spine maximum anteroposterior length; NSW = neural spine transverse width; PRL = prezygapophysis length (measured along medial surface up to the anterior surface of the neural spine).
Vertebra CL ACH ACW PCH PCW DFA DFP NAH NSH NSL NSW PRL
1 157 148 154 142 156 18 47 39 _ _ _ _ 2 155 149 160 147 149 15 49 36 153* 145 19 70 3 158 139 159 136 139 22 50 52 _ _ 21 92 4 164 141* _ _ _ 20 83 37 _ _ _ _ 5 175 130* _ 122 164 19 66 27 _ _ 21 76 6 169 134* _ 125* _ 17 47 34 80* _ 20 79 7 178 _ _ _ _ 16 54 24 82* _ 17 76 8 172 90* 154* 78* 156 29 64 19 58 _ 12 49 9 172 90 156 _ _ _ 67 21 49 137 9 47
craniocaudally elongated furrows run inside this fossa. These structures are not present in the four posterior vertebrae of the series (i.e., MMCH-Pv 47/6/7/8/9). The neural arch is placed along the anterior half of the centrum. The neural arch height is 0.23 times the centrum length; it is laterally com-pressed and has a craniocaudally well-developed base. None of the neural spines of the mid-caudal series are complete, with the second element of the preserved series carrying the most complete neural spine (MMCH-Pv 47/2, Fig. 2.1b–3b). The neural spine is laminar and craniocaudally expanded, with the anterior projection expanded as a lobe. The maximum craniocaudal length preserved of the neural spine corresponds to specimen MMCH-Pv 47/2 and measures 0.93 times the
centrum length. The prezygapophyses coalesce at the base of the neural spine and diverge slightly craniodorsally, exceeding the cranial margin of the centrum. Between the prezygapoph-yses there is a deep interprezygapophyseal fossa at the base of the neural spine. The postzygapophyses are short and placed at the base of the neural spine, with their faces pointing lat-erally in the anteriormost preserved caudals and becoming oblique, facing lateroventrally in the mid-posterior caudals.
The centra of the mid-posterior caudal series are wider than higher. This sequence presents a more craniocaudally elongated centrum, as is common throughout the distal tail of sauropods (Wilson, 2002). Neural spines are low, cranio-caudally elongated, with a slightly concave dorsal edge. The
Figure 2. Caudal vertebrae series of MMCH-Pv 47. 1–3, middle and mid-posterior caudal series showing the arrangement as found in the field,
from left (anteriormost) to right (posteriormost) in cranial (1), caudal (2), and left lateral (3) views. The anteriormost corresponds to MMCH-Pv 47/1, whereas the posteriormost corresponds to MMCH-Pv 47/9; 4–5, haemal arches of MMCH-Pv 47 as found in the field, from left (anteriormost) to right (posteriormost) in left lateral (4), and cranial (5) views. The anteriormost corresponds to MMCH-Pv 47/0-1, whereas the posteriormost corre-sponds to an isolated posterior element. Scale bar= 10 cm/ vértebras caudales de MMCH-Pv 47. 1–3, serie caudal media y media posterior mostrando la disposición como fuera encontrada en el campo, de izquierda (la más anterior) a derecha (la más posterior) en vistas craneal (1), caudal (2) y lateral izquierda (3). La más anterior corresponde a Pv 47/1, mientras que la más posterior corresponde a Pv 47/9; 4–5, arcos hemales de MMCH-Pv 47 como fueran encontrados en el campo, de izquierda (el más anterior) a derecha (el más posterior) en vistas lateral izquierda (4) y craneal (5). El más anterior corresponde a MMCH-Pv 47/0-1, mientras que el más posterior corresponde a un elemento aislado posterior. Escala= 10 cm.
1a 1b 1c 1d 1e 1f 1g 1h 1i 2a 2b 2c 2d 2e 2f 2g 2h 2i 3a 3b 3c 3d 3e 3f 3g 3h 4a 4b 4c 4d 4e 4f 4g 5a 5b 5c 5d 5e 5f 5g
prezygapophyses exceed the cranial margin of the centrum. The postzygapophyses are reduced, as in the mid-caudal se-quence (Fig. 2.1f–i, 2.2f–i, 2.3f–i).
Haemal arches. There are seven almost complete haemal arches preserved (MMCH-Pv 47/0-1/1-2/2-3/3-4/4-5/5-6/ isolated, Fig. 2.4–5, see Table 2 for measurements), which were found closely associated with the caudal vertebrae, six of them in anatomical correlation with the anteriormost caudal series and the last one disarticulated and probably belong-ing to the posterior series. The anteriormost haemal arch (MMCH-Pv 47/0-1) is incomplete, lacking the two proxi-mal rami. It shows a well-developed, blade-like distal process. The preservation of the ventral concavity of the haemal canal and the preserved portions of both rami suggest an almost craniocaudal orientation of the haemal distal spine. The mor-phology of the following haemal arches of MMCH-Pv 47 are V-shaped in anterior view, whereas in lateral view they pres-ent cranial and caudal processes rising from the distal blade, although they not correspond to the forked morphology de-scribed for basal sauropods (e.g., Barapasaurus,
Mamenchisau-rus, Upchurch et al., 2004) and diplodocoids (e.g., Diplodo-cus, Osborn, 1899; Barosaurus, McIntosh, 2005). There are
also cranial and caudal processes merging from the proximal rami progressively from the second (MMCH-Pv 47/1-2) to the sixth (MMCH-Pv 47/5-6) chevron of the series, in which processes from the proximal rami as well as from the distal blade are present (Fig. 2. 4b–f). The distalmost chevron of the series (MMCH-Pv 47/isolated) is composed of two unfused halves. The preservation of an isolated left half of
a distal chevron, showing well developed cranial and caudal processes, with a completely reduced distal blade, suggests the presence of an open haemal canal in the distal part of the tail. DISCUSSION AND CONCLUSIONS
The morphology of the caudal series described herein shows several primitive attributes among titanosaurs. The presence of amphicoelus centra is a plesiomorphic feature of Sauropoda (Upchurch, 1998; Upchurch et al., 2004). Gen-tly amphicoelous centra such as those present in specimen MMCH-Pv 47 were reported in several macronarians, as seen in anterior caudal vertebrae of Camarasaurus supremus (Osborn and Mook, 1921) and Giraffatitan brancai (Taylor, 2009). Mannion and Calvo (2011) noted this character as present in mid-caudal vertebrae of Andesaurus (contra Calvo and Bonaparte, 1991). It was also reported in the posterior caudal vertebrae of MAU-Pv-Lj-472 from the Plottier For-mation in Neuquén Province (Filippi et al., 2008) and mid-dle and posterior caudal vertebrae of Tastavinsaurus (Canu-do et al., 2008) . The specimen described herein also shares with Andesaurus the presence of a slight depression with a pleurocoel-like fossa in lateral view, whereas the possession of a ventral longitudinal hollow in mid-caudals is a character shared with most titanosaurs (Wilson, 2002; Curry Rogers, 2005) although the titanosauriform Tastavinsaurus presents middle caudals weakly excavated by the haemal canal (Can-udo et al., 2008). The hollow present in MMCH-Pv 47 is wider than in Andesaurus. Other characters, such as the lam-inar and craniocaudally elongated neural spine (González
Place of the chevron H HDH HLW TWP HSL ALR TWR APD PPD
0-1 _ _ _ _ 165 _ _ _ _ 1-2 116 74 _ _ _ 49 28 30 N/A 2-3 131 81 50 102 85 48 34 43 N/A 3-4 117 69 33 78 90 49 25 53 12 4-5 115 64 38 90 88 46 29 36 6 5-6 104 67 34 72 _ 44 21 40 30
Isolated 42 N/A _ _ N/A 29 19 24 44
TABLE 2 - Measurements of haemal arches of MMCH-Pv 47 (in millimeters)
*Preserved measurement. Abbreviations: H= Height (taken from articular facet to anterior border of haemal spine); HDH= haemal canal dorsoven-tral height; HLW= haemal canal maximun lateral width; TWP= transverse width across proximal end; HSL= haemal spine length; ALR= anteropos-terior lenght of one proximal ramus; TWR= transverse width of one proximal ramus; APD= ananteropos-terior process developement; PPD= posanteropos-terior process developement. N/A= Not applicable.
Riga, 2003; González Riga et al., 2009), and middle caudal prezygapophyses measuring 40–50% the centrum length (González Riga et al., 2009) are shared with the titanosauri-forms Cedarosaurus (Tidwell et al., 1999) and Tastavinsaurus (Canudo et al., 2008) and also with basal titanosaurs such as Andesaurus (MUCPv-132: AO pers. obs.), Mendozasaurus (González Riga, 2003), Malawisaurus (Gomani, 2005), and Mcf-PVPH-162B from the Portezuelo Formation, Neuquén Province, Argentina (Apesteguía, 2007). The neural arch placed close to the cranial margin of the centrum is a feature present in titanosauriforms (Calvo and Salgado, 1995; Up-church, 1995).
The morphology of the neural spine is rather conserva-tive among basal Titanosauriformes and basal Titanosauria, being near quadrangular and posterodorsally inclined (e.g.,
Giraffatitan, Cedarosaurus, Phuwiangosaurus, Mendozasau-rus, AndesauMendozasau-rus, Malawisaurus). An exception is Malargue-saurus, in which the neural spine is rather vertical and has
a concave caudal margin (González Riga et al., 2009). The cranial lobe present in the neural spine of middle caudals of MMCH-Pv 47 may be a probable autapomorphy of the tax-on. Nonetheless, this lobe is only well preserved in MMCH-Pv 47/2; hence the autapomorphic nature of this feature is not definitely supported.
Regarding haemal arches, MMCH-Pv 47 displays primi-tive features such as the backward- and downward-curved distal blades and the presence of a cranial projection (Up-church, 1998). The presence of proximal rami with double faced articular surface is a condition similar to the
anteropos-teriorly convex articular morphology described by Mannion and Calvo (2011) for Andesaurus, regarded by those authors as a titanosauriform character.
Haemal arches with cranial and caudal projections are present in many eusauropods (Fig. 3), including basal forms (e.g., Shunosaurus, Barapasaurus), basal neosaurupods (e.g.,
Jobaria), diplodocoids (e.g., Apatosaurus, Diplodocus), basal
macronarians (e.g., Camarasaurus lentus) (Gilmore, 1925; Wilson and Sereno, 1998; Upchurch et al., 2004) and the titanosauriform Tastavinsaurus (Canudo et al., 2008). Al-though the haemal arches described here carry cranial and
Figure 3. Haemal arches morphology within Sauropoda. 1, “Y”-type
morphology in lateral (left) and cranial (right) views (Andesaurus); 2, transitional haemal arch of Jobaria in left lateral view (modified from Sereno et al., 1999); 3, transitional haemal arch of MMCH-Pv 47 in later-al (left) and cranilater-al (right) views; 4, “fork-shaped” morphology in dorslater-al (upper) and lateral (lower) views (Diplodocus longus) (modified from Wilson and Sereno, 1998). Black arrows show the processes derived from the proximal rami. Scale bar= 10 cm/ morfología de arcos hemales en Sauropoda. 1, Morfología en “Y” en vista lateral (izquierda) y craneal (derecha) (Andesaurus); 2, Arco hemal de Jobaria en vista lateral izqui-erda (modificado de Sereno et al., 1999); 3, Arco hemal de MMCH-Pv 47 en vista lateral (izquierda) y craneal (derecha); 4, morfología “ahorquillada” en vistas dorsal (dorsal) y lateral (ventral) (Diplodocus longus) (modi-ficado de Wilson y Sereno, 1998). Las flechas negras indican los procesos que derivan de las ramas dorsales. Escala= 10 cm.
Character Reference
Distal blade of haemal arches with cranial and caudal projections (Eusauropoda)
Wilson and Sereno, 1998; Wilson, 2002
Caudal neural arch placed close to the cranial margin of the centrum (Titanosauriformes)
Calvo and Salgado, 1995; Upchurch, 1995, 1998; Curry Rogers, 2005
Haemal arches distal blades curved backwards and downwards (Titanosauriformes)
Upchurch, 1998; Curry Rogers, 2005
Laminar and craniocaudally elongated neural spine on caudal vertebrae (Somphospondyli)
González Riga, 2003; González Riga et al., 2009
Middle caudal prezygapophyses between 40-50% the centrum length (Titanosauria)
González Riga, 2003; González Riga et al., 2009
Ventral longitudinal hollow on middle caudal centra (Titanosauria)
Wilson, 2002; Curry Rogers, 2005
TABLE 3 – Distribution of characters present in MMCH-Pv 47 within Sauropoda
caudal processes, these structures do not correspond neither with the fork- nor Y-shaped morphologies. The fork-type present in the distal tail of several eusauropods is charac-terized by two craniocaudally oriented rami fused cranially and caudally and separated from each other by a ventral slit (Upchurch, 1995, p. 381). The other chevron morphol-ogy corresponds to the Y-type, present in the anterior re-gion of the tail of most sauropods and along most of the tail of titanosaurs (Upchurch et al., 2004). This morphology is characterized by a single, bony blade ventrally or caudo-ventrally oriented, with the proximal end divided into two rami which form the ventral and lateral margins of the hae-mal canal (Wilson and Sereno, 1998, p. 50). This results in a Y-shape in anterior view. In MMCH-Pv 47 the cranial and caudal processes rising from the distal blade are differ-ent from fork-shaped chevrons. In this regard, MMCH-Pv 47 presents single non-paired processes. Besides, the cranial projection is smaller than the caudal projection, whereas fork-shaped chevrons present cranial and caudal projections almost equally in length, as seen in some basal eusauropods and some diplodocoids (Upchurch et al., 2004). Finally, the cranial and caudal projections rising from the proximal rami are features not yet reported in sauropods and probably rep-resent an autapomorphy of MMCH-Pv 47.
The haemal arches reported herein show a morphology that leaves some doubt about the homology of the structures present in MMCH-PV 47 in comparison with the above chevron morphologies described for other sauropods. The preserved section of the caudal series shows probable tran-sitional chevrons between Y-shaped and fork-shaped mor-phologies (Fig. 3b,c). These transitional chevrons seem to correspond to the “skid” type described by Upchurch (1998, char. 148); however this author grouped both fork-shaped and skid types in a single definition.
A wide range of variation exists in the morphology of sauropod chevron series, rendering a complex scenario that requires further comparisons. The scarce record of complete caudal series with haemal arches only enables us to interpret that there is a craniocaudal transition from single chevrons (Y-shaped) to more complex structures (fork-shaped chev-rons) with new processes (i.e., cranial processes). This mor-phological change can take place gradually, involving several vertebrae of the mid-section (e.g., Mamenchisaurus Young and Zhao, 1972; Diplodocus Osborn, 1899; MMCH-PV 47 as described in this paper); or never occur, leaving a se-ries of single chevrons, as observed in Camarasaurus lewisi (McIntosh et al., 1996). Thus, the use of a discrete, binary
state character (i.e., fork-shaped chevrons: absent/present) is not recommended; instead, it is necessary to account for the morphological transition described here, which can result in taxonomically informative and well defined characters for almost complete caudal series.
Summarizing, the specimen described herein displays many synapomorphies which place it among basal Titano-sauria (Tab. 3), although the presence of haemal arches with cranial and caudal projections rising from the distal blade represent a primitive character with putative non-titanosau-riform affinities, and the occurrence of projections merging from the proximal rami is a feature not yet reported in sau-ropod dinosaurs.
ACKNOWLEDGEMENTS
The authors are grateful to R.D. Carolini who found the materials studied in this paper. L. Salgado, S. Apesteguía, P. Gallina and J. Wilson provided helpful comments on haemal arch morphology. P. Sereno is thanked for providing unpublished photos of chevrons of Nigersaurus. P. Mannion and J.I. Canudo are thanked for the useful comments which largely improved the manuscript. A.O. and J.I.C. research was supported by the Consejo Na-cional de Investigaciones Científicas y Técnicas (CONICET). This research was partially funded through a Jurassic Foundation grant awarded to A.O.
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doi: 10.5710/AMGH.v48i4(482)
Recibido: 16 de marzo de 2011
