Titanosaur osteoderms from the Upper Cretaceous of Armuña (Segovia, Spain): a reinterpretation in the light of new findings

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Titanosaur osteoderms from the Upper Cretaceous of Armuña (Segovia, Spain): a reinterpretation in the light of new findings

Reference: Vidal, D.; Ortega, F. y Sanz, J.L. (2014) Osteodermos de titanosaurio del Cretácico Superior de Armuña (Segovia): una reinterpretación a la luz de nuevos hallazgos. XXX Jornadas de la Sociedad Española de Paleontología (Teruel).

Introduction - Titanosaurs are the only clade of sauropods that reaches the uppermost stages of the Cretaceous (Curry-Rogers, 2005). Some of the first findings of titanosaurs in Madagascar were found associated to some dermal ossifications which were attributed, with reservations, to these sauropods (Deperét, 1896). In the 1980s the first unequivocal armoured titanosaurs, such as Saltasaurus loricatus were found (Bonaparte & Powell, 1980). The dermal armor of Saltasaurus was interpreted as a mosaic of  dermal ossicles smaller than 1 cm surrounding larger, rounded and keeled dermal plates (Powell, 2003; Salgado, 2003).

The Upper Cretaceous site of Armuña (Segovia, Spain) was one of the first to yield dermal ossifications unequivocally associated to titanosaurs (Sanz & Buscalioni, 1987). These fragmentary specimens, today deposited in the "Unidad de Paleontología" collection at the Universidad Autónoma de Madrid, were interpreted upon the only dermal armor then known: as rounded plates as in Saltasaurus

The findings of new european Upper Cretaceous sites in recent years, specially Lo Hueco (Cuenca, Spain) have revealed that the armor of Laurasian titanosaurs was very different that that of South American titanosars (Le Loeuff et al., 1994; Csiki, 1999; Vidal et al. 2014) as their osteoderms present two well differentiated regions, the so called bulb and root (D'Emic et al. 2009; Vidal et al. 2014). These findings aid in reinterpreting the osteoderms from Armuña as different from those of Saltasaurus loricatus (the original interpretation) and more kin to the european fossil record of titanosaur bulb and root osteoderms.

Types of dermal ossifications in Titanosauria - The osteoderms associated to titanosaurs have been found in all continents except Antartica. There are, however, just little more than a hundred specimens worldwide. This small sample reveals that titanosaurs bore two kind of dermal ossifications: dermal ossicles (irregular in morphology, smaller than 1 cm and without histological remodelling) and osteoderms  (with a well defined morphology, larger than 1 cm and with remodelled bone).

Titanosaur osteoderms can be divided in two main morphotypes: bulb and root (amygdaloid outline, with a rounded regularly ornamented end and a triangular elongated and irregularly ornamented end, both separated by a cingulum) and scutes (rounded outlines, with a regular ornamentation pattern and sometimes with a perlated contour). The former are present in all continents and the second are only present, until now, in Gondwana (Fig. 1-C). To date, no titanosaur skeleton has been found associated to both types of osteoderms, which may indicate that different titanosaur clades would present different types of osteoderms. Being more geographically widespread and first appearing in lower strata, the bulb and root condition is considered to be primitive (Vidal et al. 2014).

The Armuña specimens -  Four osteoderm fragments have been found at Armuña, all referable to titanosaurs.


  • UPUAM 13951 (AR-01): In external side it presents two regions of different texture: one rounded with a regular pattern of fibers and foramina radiating from the center to the margins and a cingulum with an irregular ornamentation of nodules and fibbers. The intracingular external surface is convex, forming a low conical spine. Its internal side is broken and only the internal anatomy of the osteoderm can be seen. It consists of ducts up to 2 cm in diameter. Its great resemblance to the bulb region of some osteoderms from Lo Hueco (HUE-02000, HUE-02326) or the Hateg basin (FGGUB R-1410), whose bulb is convex, help in identifying UPUAM 13951 as a bulb and root with convex bulb (Fig. 2-A). The correlation between the contour of the osteoderm in internal or external view and the convexity of the bulb found by Vidal et al. (2014) would help in reconstructing UPUAM 13951. Its bulb convexity is moderate, so is contour would be moderately elongated, with its major axis twice its minor axis.
  • UPUAM 13952 (AR-02): Probably with an amygdaloid outline and with both ends of the major axis broken. Its external side has an irregular pattern of fibers and channels, being taller in is center than the edges. The internal side reveals a interwoven criss-crossed bony fibers, typical of other osteoderms from Amniota (D'Emic et al. 2009). It has a very prominent keel or crest in the internal side, parallel to the major axis of the osteoderm and slightly displaced toward one of the edges, being thus one of the longitudinal halves of the osteoderm more concave than the other. The irregular external side ornamentation, the presence of a keel on the internal side parallel to the major axis and an elongated amygdaloid profile suggest UPUAM 13952 is a fragment of the root part of a bulb and root osteoderm (Fig. 2-C)
  • UPUAM 13956 (AR-06): Small fragment with two types of ornamentation in the external side: one regular of radiating fibers and foramina and one irregular of nodules and fibers. It resembles UPUAM 13951, including part of a cingulum and the regular ornamentation, thus it is interpreted as the same: a small fragment of the bulb region of a bulb and root osteoderm (Fig. 2-A)
  • UPUAM 13957 (AR-07): Small, triangular fragment broken only in the widest end. It is convex in its external side, flat in its internal side devoid of foramina but with interwoven bony fibers. Its triangular morphology, the tapering toward a non broken axis make it most likely the end of the root region of a bulb and root osteoderm (Fig. 2-D)
Discusion - The Iberian Peninsula has yielded titanosaur osteoderms from the sites of Laño (Treviño), Armuña (Segovia) and Lo Hueco (Cuenca). The largest set, collected from Lo Hueco, consists of bulb and root osteoderms in its entirety (Vidal et al. 2014), as well as all the osteoderms from Aude (France) and the Hateg basin (Romania).

The interpretation of the osteoderms from Armuña as bulb and root reinforces the hypothesis that osteoderm morphotypes bulb and root and scutes would be present in different titanosaur clades and never together in the same species. Their paleogeographical distribution also appears to indicate that the clade of titanosaurs that bore scutes was restricted to the southern landmasses, as no scutes has yet been found in Laurasia.

The osteoderms from Laño, associated to the titanosaur Lirainosaurus astibiae are very fragmentary, without any contour or unambiguously interpretable morphology (Díez-Díaz et al. 2013). Their slightly tall profile and irregular ornamentation, similar to UPUAM 13957, would indicate that they could be fragments from the root of a bulb and root osteoderm. However, there are not enough anatomical traits to support an unambiguous interpretation. Their true nature will be determined by new findings or by a better calibration of the presence of osteoderms in the phylogenetic hypothesis of this clade.

Conclusions - This study has allowed to reinterpret four titanosaur osteoderms from the Upper Cretaceous of Armuña as fragments of bulb and root osteoderms as they share common traits with complete specimens of this morphotype, such as two differently shaped and ornamented regions and elongated contours instead of rounded.

This interpretation suggests that the Upper Cretaceous european titanosaurs only bore these kind of osteoderms.

References - 

  • Bonaparte, J. F. y Powell, J. E. 1980. A continental assemblage of tetrapods from the Upper Cretaceous beds of El Brete, northwestern Argentina (Sauropoda–Coelurosauria–Carnosauria–Aves). Mémoires de la Société Géologique de France, nouvelle séries, 139:19–28.
  • Csiki, Z. 1999. New evidence of armoured titanosaurids in the Late Cretaceous Magyarosaurus dacus from the Hateg Basin (Romania). Oryctos 2: 93–99.
  • Curry Rogers, K. A. 2005. Titanosauria. En K. A. Curry Rogers, K. A., y Wilson, J. A. (eds.), The Sauropods: Evolution and Paleobiology. Berkeley, University of California Press, 50-103.
  • D'Emic, M. D., Wilson, J. A., y Chatterjee, S. 2009. The titanosaur (Dinosauria: Sauropoda) osteoderm record: review and first definitive specimen from India. Journal of Vertebrate Paleontology 29(1): 165-177.
  • Depéret, C. 1896. Note sur les dinosauriens sauropodes et théropodes du Crétacé supérieur de Madagascar. Bulletin de la Société Géologique de France, 21:176–194.
  • Díez Díaz, V., Pereda Suberbiola, X. y Sanz, J.L. 2013. Appendicular skeleton and dermal armour of the Late Cretaceous titanosaur Lirainosaurus astibiae (Dinosauria: Sauropoda) from Spain, Palaeontologia Electronica Vol. 16, Issue 2; 19A; 18p;
palaeo-electronica.org/content/2013/502-titanosaur-skeleton
  • Le Loeuff, J., Buffetaut, E., Cavin, L., Martin, M., Martin, V. y Tong, H. 1994. An armoured titanosaurid sauropod from the Late Cretaceous of southern France and the occurrence of osteoderms in the Titanosauridae. Gaia 10:155–159.
  • Powell, J. E. 2003. Revision of the South American titanosaurid dinosaurs: palaeobiological, palaeobiogeographical, and phylogenetic aspects. Records of the Queen Victoria Museum 111: 1–173. 
  • Salgado, L. 2003. Considerations on the bony plates assigned to titanosaurs (Dinosauria, Sauropoda). Ameghiniana, 40: 441–456.
  • Sanz, J. L. y Buscalioni, A. D. 1987. New evidence of armoured titanosaurs in the Upper Cretaceous of Spain. pp. 197–202 en P. J. Currie, y E. H. Koster (eds.), Fourth Symposium on Mesozoic Terrestrial Ecosystems, Short Papers. Tyrrell Museum of Palaeontology, Drumheller.
  • Vidal, D., Ortega, F. y Sanz, J.L. en prensa. Titanosaur osteoderms from the Upper Cretaceous of Lo Hueco (Spain) and their implications on the armor of laurasian titanosaurs. PloS one, en prensa.
Figures -

Figure 1 - The different morphotypes of titanosaur osteoderms that can be identified upon complete specimens and their distribution during the Upper Cretaceous. A) Bulb and root osteoderms in external (up) and lateral (down) views. B) Scute osteoderms in external (left) and lateral (right) views, modified from Salgado (2003). C) Worldwide distribution of the different osteoderm morphotypes during the Upper Cretaceous (modified from http://www.scotese.com). b = bulb, r = root.



Figure 2 - Fragmentary titanosaur osteoderm specimens from Armuña in external view (except for D, in internal view) and their interpretation as a fragment of a complete Bulb and Root osteoderm. A) UPUAM 13951 (AR 01). Scale = 10 cm. B) UPUAM 13956 (AR 06). Scale = 5 cm. C) UPUAM 13952 (AR 02). Scale = 10 cm. D) UPUAM 13957 (AR 07). Scale = 5 cm.

POST-PUBLICATION AUTHOR NOTES


The original specimen numbers from the Armuña fossils were "UP-UAM AR number". Due the collection from the Unidad de Paleontología is being reorganised and labeled, the specimen numbers have been updated to the current ones. The elder references have been kept in parenthetical form.

Also, there are two more fragmentary, non published osteoderms from Armuña. They are so fragmented that can be interpreted as titanosaur osteoderms but not as parts of a complete specimen. Here are some pictures of them. The largest specimen, as it can be seen, has a neat preservation of the internal structure.


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