Argentinean theropods: first morphological analysis on isolated teeth

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Figure 1: Map showing the localities where the sample of isolated teeth come from (see Table 1 for age, locality, signature of the sampled teeth, associated dinosaur fauna, and references).

BUSCALIONI, A. D. 1; GASPARINI, Z.2; ; PÉREZ-MORENO, B. P.1; SANZ, J. L.1
1.Departamento de Biología, Paleontología. Fac. Ciencias. Universidad Autónoma de Madrid. Cantoblanco 28049, Spain
2.Departamento de Paleontología Vertebrados. Museo de La Plata. 1900 La Plata. Argentina.

Abstract

A sample of dinosaur theropod isolated teeth from Argentina, ranging from the Mid-Jurassic up to the Upper Cretaceous, is studied. The results based on denticle morphology reveal two distict morphotypes: chisel-like and hooked-like. These morphotypes are compared with those with similar characteristic from Laurasia. The comparison reveals an unknown combination of features within hooked denticle theropod teeth.

Introduction

Archosauria has been diagnosed by their peculiar tooth morphology -crowns laterally compressed with serrated margins- (Benton, 1990). This dentition is frequently parallelized within archosaurs (phytosaurs, rauisuchids, basal crocodyliforms, and theropods dinosaurs among others show this dentition). The general framework in which might be regarded this particular study on gondwanic theropod dinosaurs is to dilucidate what is the spectrum of diversity in crocodile and theropod serrated teeth.

Theropod teeth have some degree of taxonomic utility, but their intraspecific variation (mainly based on the position of the crown along the dental series) have dealt with the proliferation of new specific and generic taxa. However, recent studies coincide that some particular theropods, principally the small ones, might be taxonomically identified based on isolated teeth (Currie et al., 1990; Fiorillo & Currie, 1994). Important differences have been discussed between large and small theropods based on shape, size and number of denticles. Up to now, the main contributions come from the Upper Cretaceous laurasiatic faunas, particularly those from North America (Currie et al., 199O, Farlow et al., 1991; Abler; 1992; Fiorillo & Currie, 1994; Rauhut & Kriwet, 1994). However, none of the obtained results were tested in other theropod faunas, such as those from the southern continents. Thus, the main purpose of this study is to have a detailed description of a diverse sample of argentinean theropod teeth ranging temporally from the Middle Jurassic up to Maastrichtian, and to show a brief discussion on the similarities with the known laurasiatic and gondwanic record.

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Figure 2. Structural and functional terminology of denticles (from Abler, 1992; Currie et al., 1990). A. Denticles chisel-like. B. hooked-like denticles. Abbreviations: ca, cauda, also denominated blooded grooves; ce, cella (gap or chamber between neighboring denticles); dh, denticle height; dl, denticle length (taken at the base of the denticle, between diaphyses);dy, diaphysis (junction between denticles); is, interdenticle slits; pi, pit at the base of the denticle (where the diaphysis ends); o, outer end of the denticle. Main differences between hooked-like and chisel-like denticles are depicted -is, and ce- Scale bar 200 micrometers. SEM photographs. Right: chisel-like denticles of the specimen MACN 18172, from the Albian of Chubut. Above, posterior margin taken at the base of the crown; below, anterior margin of the same specimen taken at the middle of the crown (crown height, ca. 40 mm). Left: hooked-like denticles of two specimens. Above, posterior margin taken at the middle of the crown (specimen MACN- RN 1002 from Los Alamitos site, Rio Negro, Campanian); below, anterior margin (specimen UNC-M 310 from Lago Pelegrini, Rio Negro, Campanian).

Material and Method

The sample is composed by isolated teeth (see Fig. 1 and Table 1 for localities and age). The teeth were cleaned with ultrasonic and for the morphological observations a Scanning Electro Microscope (SEM) was used. Most of the teeth lack roots and, since in some specimens the crowns are broken, observations on denticle shape regarding their position along the crown have not been systematized, and no morphometric crown-denticle size ratios were either established. The taxonomical attribution of the specimens is still tentative, and further comparisons with the dinosaur associated fauna have to be carried out.

 

 

 

 

Localities, Age and Signatures Dinosaur Associated Fauna Source of data
Cerro Cóndor (Chubut); CallovianMACN-CH1851 *Piatnitzkysaurus floresiVolkheimeria chubutensisPatagosaurus fariasi Bonaparte, 1979 1986,
La Amarga, (Neuquén); Hauterivian MACN-N57-62 Ligabuenoi andesi Amargasaurus cazaui Stegosauria indet. Bonaparte, 1986, 1996;
Gorro Frigio, (Chubut); AlbianMACN 18172 Theropoda indet. Bonaparte, Novas & Coria, 1990
El Chocón, (Neuquén);Albian -Cenomanian UNC-PV6 *Giganotosaurus caroloniiAndesaurus delgadoi “Diplodocid-like” Calvo & Bonaparte, 1991; Coria & Salgado, 1995; Bonaparte, 1996
Los Alamitos, (Río Negro); U. Campanian MACN-R999-1011 Theropoda indet.Aeolosaurus rionegrinusKritosaurus australis Bonaparte, 1987, 1996;
Lago Pelegrini, (Río Negro); U. Campanian UNC-M308-312 *Abelisaurus comahuensisTitanosaurus araukanicusLambeosaurids Bonaparte & Novas, 1985; Bonaparte, 1996

TABLE 1. Information of the sampled theropod teeth. Localities are correlatively ordered as in Fig. 1 for geographic location of the sites.

Former approaches to this issue have already produced a proper nomenclature on denticle morphology (see authors above mentioned). To establish the basis of comparison with our sample we have gathered this information adjusting it for elaborating a common character list (see Fig 2 for identifying the used terminology, and appendix for characters). The results on the application of these characters (see Table 2) has revealed within the sample two principal morphotypes (denoted by an asterisk in the table).

Sample a b c d e f g h i j k *
MACN-CH1851 + ? + + + + + + ? A
MACN-N 57-62 + + + + + + + N + ? A
UNCPV6 + + + + + + N + ? A
MACN 18172 + + + + + N + + A
MACN-RN 999-1011 N + + B
UNC-M308-12 N + + B

TABLE 2. Data matrix of the denticles morphology of the analyzed sample (see appendix for characters a-k; and Fig. 1 and Table 1 for localities and age of the sampled teeth)

The theropod argentinean sample: morphotypes and comparisons.

From the above data matrix two distinct morphotypes might be recognized. The morphotype A is characterized by the combination of the following features (Fig. 2A): equivalent denticles on the anterior and posterior margins (in shape and size) (a), chisel-like denticles (c) conforming a series of inflated cubes (d), denticles close each other leaving narrow and high cellae (g). The morphotype A corresponds to that observed in large theropods. These large theropods incorporate laterally compressed (the labiolingual diameter is about 1/3 the mesiodistal one) crowns, ranging their height from 20 to 180 mm. Crowns are also distally curved and apically sharp, and its surface is smooth without any ridge or ornamentation. The denticles in large theropod have a similar length to height denticle ratio (1:1.5); theropod denticles tend to have also a longer labiolingual axis than the proximodistal one (1:0.6).
Within this morphotype several differences might be determined in the sample: a) The development of the caudae. The general trend in large theropods is the absence of a pronounced caudae. Tyrannosaurids are an exception, in which caudae are pronounced and consistent (Abler, 1992; Fiorillo & Currie, 1994). The absence of a developed caudae has been discussed as a probable primitive trait for theropods. Our sample follows the same trend (absence of caudae). However, the teeth from the Callovian (Middle Jurassic) has caudae as pronounced as those of tyrannosaurids. If the tentative taxonomical attribution of this specimen to Piatnitzkysaurus is confirmed regarding the associated theropod fauna (see Fig. 1), then the presence of consistent caudae should be considered convergent, at least, in Tetanurae (sensu Holtz, 1995). theropods. Furthermore, caudae are present in other european (Portugal) Upper Jurassic large theropods (Rauhut & Kriwet, 1994) ( b) The outer margin of the denticles. The general trend in chisel-like denticles is having a straight outer end. Nevertheless, denticles that taper slightly distally have been also described for particular taxa. This is the case of the north american taxa Richardoestesia (Currie et al., 1990), and two cases of our sample, one specimen from the Albian (MACN 18172) (See Fig. 2) and the other from the Callovian (MACN-CH 1851). c)Serration coarseness. Size-related changes in denticle coarseness are meaningful only if tooth size is taken into account (Farlow, 1987). In this sense, again the specimen MACN-CH 1851 differs from the remainder theropods of the sample with equivalent crown height. The specimens have denticle units relatively long and high (in relation with its crown height), the serration does not grade into the body of the tooth crown but ends abruptly, and finally the denticle units are widely separate each other when compared with other theropods.

The morphotype B is characterized by the combination of the following features (Fig. 2B): the anterior and posterior denticles are very distinct in size and shape (a), the posterior denticles are hooked (c) with wide interdenticle slits. The chamber between neighboring denticles (cellae) (g) is reduced and occasionally shallow. Teeth with hooked denticles are characteristic of small theropods. These theropods have particularly more variable tooth crown morphologies i.e. the strong marginal compression, depressions on the root, root-crown constriction. Their crown height ranges up to 10 mm. They possess ornamented crowns with evident caudae and, in some cases, ridges running along the crowns.

The argentinean sample of hooked denticle teeth come from the North of Patagonia, two sites of similar age (Late Campanian see Fig. 1), and do not possess differences between them. Both sites Lago Pelegrini and Alamitos are sharing a common taxa. Since hooked denticles teeth appears in the same levels than the theropod Abelisaurus a precise comparison have to be carried out.

Several differences might be established when our sample is compared with north american small theropods. a) The crown heights. North american small theropods (the Dromaeosauridae Saurornitholestes and the Troodontidae Troodon) possess smaller crowns (up to 10 mm) than the argentinean teeth gathered as morphotype B (up to 20 mm). This difference has implications on the serration coarseness. The former taxa have relatively longer and higher denticles with respect to their crown height. b) Ratios of the anterior and posterior denticles. In the north american hooked denticle teeth the anterior denticles are usually less than half the height and length of the posterior denticles. Anterior denticles in dromaeosaurids and troodontids are minute or even might be absent. In our sample the anterior denticles are half the height of the posterior ones, but the length is equivalent in both or even slightly longer than the posterior ones. c) Presence of wide pits. Pits are common in small theropods. However, in the sample have not been found pits as large as those described in Troodon.

Up to now, no hooked denticle teeth was described except those belonging to the clade Maniraptoriformes (sensu Holtz, 1995). The argentinean evidence is significant because the denticle morphology and/or coarseness are distinguished from that of Maniraptoriformes, thus, two implications might be remarked. First, if those teeth belongs to Abelisaurus, then it will prove convergent patterns of hooked denticles teeth within Theropoda. And second, the evidence could indicate the presence of an yet unknown maniraptoriform. This latter supposition is not improbable since, recently, an isolated Droameosaurid-like teeth with similar denticle morphology (see Fig 2B) to that of Argentina, has been reported for the Cenomanian of Sudan in Africa (Rauhut & Werner, 1995). Nevertheless, the argentinean teeth are still problematic for a maniraptoriform because of their peculiar crown heights.

In conclusion, when the argentinean sample is compared with the described teeth from North American, the spectrum of diversity in theropod serrated teeth is in general maintained. Some features that present specimens of morphotype B (teeth with hooked denticles) are the most remarkable differences. A precise comparison of the specimens studied with associated faunae will confirm the presence of a new maniraptoriform during the Late Cretaceous in Argentina.

Acknowledgement and abbreviations used

We would like to thanks J. Bonaparte and L. Salgado for providing us most of the specimens studied. We also thanks Esperanza from the SIDI of Universidad Autónoma Madrid who enthusiastically shoot the SEM photographs. ABBREVIATIONS: Museo Argentino de Ciencias Naturales (MACN), Museo de la Universidad Nacional del Comahue (UNC.PV; UNC-M). This study corresponds with a sabatical stay DGICYT SAB95-0258 of Z. G. at the Universidad Autónoma of Madrid.

Appendix

Proposed denticle morphology characters:

a. Anterior and posterior denticles equivalent in size and shape (+); Anterior and posterior denticles differing in size and shape (-). In this latter case the anterior denticles are longer than higher (2:1), and the posterior as high as long (1:1).

b. Anterior denticles in a greater number than the posterior ones (+).

c. Shape of the posterior denticles chisel-like (+) or hooked-like (-).

d. The body of the chisel-like denticles is an inflated cube (+).

e. The outer border of the chisel-like denticles is expanded forming a domed contour. This border is formed by the enamel of one side (labial one) (+). This trait seems to be variable in tyrannosaurid (Abler, 1992).

f. Chisel denticles with an outer oblique contour (+) , the distal tip of the denticle (the one directed toward the apex of the crown) is higher than the proximal one (the one directed towards the base).

g:Cellae long and narrow, evident from the outer edge of the denticle to half the denticle high (+); reduced (-).

h: Caudae not pronounced (+); specially evident on the surface (lingual) of the crown (-).

i: Caudae obliquely oriented toward the base of the crown (+); perpendicular to the major axis (apicobasal) of the crown (-).

j: Maximal convexity of the denticle perpendicular to the major axis of the crown (+); oblique (-).

k: Presence of pits at the base of the denticle (+); distinctly wide (-).

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