Research Article |
Corresponding author: Oliver Hampe ( oliver.hampe@mfn.berlin ) Academic editor: Torsten Scheyer
© 2023 Manja Voss, Oliver Hampe, Kristin Mahlow, Joan C. Vilanova.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Voss M, Hampe O, Mahlow K, Vilanova JC (2023) New findings of Prototherium ausetanum (Mammalia, Pan-Sirenia) from paving stones in Girona (Catalonia, Spain)? Fossil Record 26(1): 135-149. https://doi.org/10.3897/fr.26.99096
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Taxonomic and morphological approaches on Eocene sirenians from Catalonia (Spain) benefit from a newly discovered specimen found in a quite unusual locality, the pedestrian zone in the city of Girona. Two fossil-bearing limestone slabs from middle Eocene (Bartonian) layers of a quarry in the wider surrounding area north-west of Barcelona, were CT-scanned in the Clínica Girona to enhance more detailed investigations. Post-processing of the scans and, as far as possible, 3D-reconstruction of the preserved elements in the slabs was performed at Museum für Naturkunde Berlin. Thereby, a skull of a Dugong specimen was used as a reference point. Based on the combined analysis of macroscopic and CT-data, the specimen most likely represents Prototherium ausetanum Balaguer & Alba, 2016 and complements the available information of the holotype and hitherto only known specimen of that species. The Girona specimen is an adult, but small individual that corroborates P. ausetanum as a generally small-sized species compared to other known Prototherium taxa.
3D-visualisation, CT-scanning, Eocene, morphology, taxonomy, urban fossil
In a preceding study (
Two limestone slabs from the late middle Eocene of Sant Vicenç de Castellet (Catalonia, NE Spain, Fig.
Panoramic view of the SW side of the limestone quarry southeast of Sant Vicenç de Castellet showing the massive blocks of Bartonian carbonates bound by micritic matrix, and containing besides macroforaminifers, such as nummulites and discocyclinids, bryozoans, corals, and bivalves vertebrate remains (here described: MAC GIR-042375).
The objective of this follow-up article is to verify the preliminary results from
For this subsequent and complementing article on fossil remains of a sea cow found in two slabs in the pedestrian zone in the city of Girona, CT-scans made at the Clínica Girona were analyzed in detail as best as possible. The CT used here was a GE light speed Optima CT520 (General Electric Company, GE Healthcare, Chalfont St Giles/UK) and was utilized under the following protocol parameters: slice thickness 0.625 mm; space 0.31 mm; pitch 0.56 mm; single collimation width 0.625 mm; pixel spacing 0.4 × 0.4; matrix 512 × 512; field of view 250 mm; convolution kernel: Boneplus; tube voltage and stabilizer 120 kVp, 140 mA. Post-processing of the scanned slabs, i.e., segmentation of the individual bones and the combination of the separate section planes in 3D, is hampered by inclusions of iron-rich minerals and by the characteristic thick and dense sirenian bones itself keeping the contrast between bone and surrounding limestone relatively low. Nevertheless, the individual bones could be segmented and combined in 3D allowing further morphological examination of the skull. Analysis and visualization of the CT-data were performed with VGStudio MAX 2.2.
The taxonomic-morphological comparisons of the Girona specimen MAC GIR-042375 are based almost solely on personal investigations of the taxa for comparison. This refers to P. veronense De Zigno, 1875 and P. intermedium Bizzotto, 1983 from the Bartonian and Priabonian of North Italy, respectively, stored at MGPD (MV). In each case, the holotype and associated specimens were investigated as is provided by
MAC, Museu d’Arqueologia de Catalunya, Girona, Spain; MGPD, Museo di Geologia e Paleontologia, Padova, Italy; MGSB, Museu Geològic del Seminari de Barcelona, Spain; RGHP, Réserve géologique de haute Provence, Digne-les-Bains, France;
Class Mammalia Linnaeus, 1758
Afrotheria
Tethytheria McKenna, 1975
Pan-Sirenia
Genus Prototherium de Zigno, 1887
The present systematic palaeontology considers the quite recently established clade Pan-Sirenia,
MAC GIR-042375, partial skull and rib remains documented in urban paving stones of Girona, Catalonia, Spain.
Limestone quarry south of Sant Vicenç de Castellet, central Catalonia, Spain.
Late Bartonian, middle Eocene. The age of the find is corrected from early Bartonian (
The sirenian remains are embedded in a shallow carbonate platform sediment consisting of an alternation of biomicritic limestones, lutites and bluish-grey claystones (Figs
Modified extract from the geological map (Mapa geològic de Catalunya 1:25 000, Monistrol de Montserrat 392-1-1 (71-29)) showing the position of the limestone quarry (circle: SE Sant Vicenç de Castellet, E of Llobregat river) from which the pavement slabs containing the MAC GIR-042375 Prototherium cf. ausetanum remains came from. All sediments from this area are dated to the late Bartonian.
The first new observation refers to the orientation of the single fossil-bearing slabs. With the help of an adult skull of the extant dugong (Dugong dugon, ZMB_MAM 69306), the partial skull was aligned in an anatomically correct position mainly considering the slope of both, the jugal and the rostrum. As a result, each slab shows oblique angles in the transversal plane, hence do not lie one-to-one at that specific level. For slab 1, the angles are 28.22° in lateral view, and 4.3° in frontal view; for slab 2, the angles are 28.22° in lateral view, and 6.0° in frontal view, respectively (Fig.
The distance between both slabs could be ascertained to be 11.4 mm (Fig.
Furthermore, the preliminary results by
Prototherium cf. ausetanum, MAC GIR-042375. A. Photograph; B. Corresponding CT-scan of slab 1 with focus on the basicranium. The unpolished bottom side of slab 1 (A) shows parts of the basisphenoid, the spheno-occipital eminences, and the basioccipital, all of which being firmly fused indicating adulthood. The CT-Scan of the basicranium (B) shows interferences, however, the suture between basisphenoid and basioccipital is unequivocally obliterated and estimated to be more or less level with the vertical interference (indicated by arrows). Abbreviations: BO, basioccipital; BS, basisphenoid; J, jugal; M, maxilla; PF, pharyngeal fossa; PM, premaxilla; P3-4, third and fourth upper permanent premolars; SOE, spheno-occipital eminences; ZOB, zygomatic-orbital bridge of maxilla.
For relative age determination, it is also highly indicative whether sutures between certain bones are open, fused or even obliterated (
With respect to the taxonomic assignment,
Prototherium cf. ausetanum, MAC GIR-042375. A. Photograph showing the dorsal side of slab 2; B. Corresponding CT-scan showing the ventral side of slab 2 focusing on the dentition; C. CT-scans of anatomically correct combined slab 1 and 2 in lateral view. The CT-scan of the lower side of slab 2 (B) reveals the crowns of the upper molars 1–3 (M1-3) with the M3 fully erupted and clearly in wear. Abbreviations: BS, basisphenoid; C, upper canine; FI, foramen incisivum; I1, first upper incisor; J, jugal; M, maxilla; M1-3, first to third upper molars; P1-4, first to fourth permanent upper premolars; PM, premaxilla; ZOB, zygomatic-orbital bridge of maxilla.
Nevertheless, the present results support the Girona specimen as belonging to the genus Prototherium. As such, its identification on the species level focusses on comparisons with three Prototherium species generally considered valid: P. veronense De Zigno, 1875 from the late middle Eocene (Bartonian) of Verona (Italy), P. intermedium Bizzotto, 1983 from the late Eocene (Priabonian) of Possagno (north-east from Verona, Italy), and the recently established P. ausetanum Balaguer & Alba, 2016 from the Bartonian of Tona (Catalonia, Spain). Additionally, the taxonomic-morphological comparisons are complemented by two further Prototherium species from the Bartonian of Catalonia, P. solei
The subsequent new results on the morphology and taxonomy of the Girona specimen include additional data on the premaxilla, the maxilla (zygomatic-orbital bridge), the basicranium, and the dentition. All comparative descriptions rely on the preserved features only.
For the premaxilla, plesiomorphic conditions of the dental formula have been assumed by
Further preserved diagnostic features of the premaxilla refer to the length of the alveolus for the first upper incisor (I1). Slab 2 reveals the alveoli of both I1 (Fig.
Another morphological feature with diagnostic value is related to the position of the zygomatic-orbital bridge of the maxilla with respect to the palate (e.g.,
The morphology of the basicranium may have very likely diagnostic value. As stated above, the basisphenoidal-basioccipital region is preserved, which is best visible in macroscopic view of slab 1 (Fig.
At this point also a correction as to the presence of a hypoglossal foramen is made (
Moreover, our examination of the preserved M1-3 crowns in deeper levels of slab 2 further helps to substantiate the taxonomic identity of the Girona specimen (Fig.
Mesiodistal length of molars M1-3 of the “Girona specimen” MAC GIR-042375 compared to other Prototherium species: data for P. veronense and P. intermedium are taken from
mesiodistal length in mm | |||
---|---|---|---|
M1 | M2 | M3 | |
P. veronense (cast of holotype) | 16 | 21 | 23 |
P. intermedium (holotype) | 17 | 19 | / |
P. ausetanum (holotype) | 13.8 l / 15.5 r | 16.3 l / 16.6 r | 21.6 l / 21.4 r |
Girona specimen (MAC GIR-042375) | 13.5 | 17.2 | 20.5 |
P. montserratense (holotype) | / | 17 | 21 |
P. solei (holotype) | / | 15 | 18 |
Extending the comparisons upon the preserved M2-3 in P. solei and P. montserratense from Spain unveils interesting results. The M2-3 in MAC GIR-042375 (and in P. ausetanum) is about 13% and 12% larger than in P. solei, respectively, making the latter species the smallest in terms of the molar dimensions. However, compared with P. montserratense, the M2-3 are almost as large as in MAC GIR-042375 (Table
Another molar feature to distinguish MAC GIR-042375 refers to the shape of the M2-3. Except for P. ausetanum, all taxa for comparison show a trapezoid M2, with the protoloph each wider than the metaloph, whereas in both, MAC GIR-042375 (Fig.
In the following, the description of specimen MAC GIR-042375 is complemented with regard to the preserved morphology visible in slab 1 (Fig.
The CT-scan of the slab represents a relatively deep level of the upper part of the skull (Fig.
After post-processing, the nasal area could be made well visible by reconstructing the extension of the vomer. The vomer is a mediolaterally thin element and triangular in cross section as in other sirenians. It passes through most of the internal narial passage, contacts the maxilla laterally (and most likely the palatine, that, however, cannot be distinguished here) and extends in a longitudinal groove on the dorsal surface of the maxilla slightly forward into the mesorostral fossa (external nasal opening). Anteriorly, the vomer builds up the nasal septum. Posteriorly, the vomer runs indistinctly out into the basicranium.
On the left side, parts of the frontal are visible with the slightly downturned supraorbital process. A complex of remnants of the left jugal anteriorly and the left zygomatic process posteriorly indicate the zygomatic arch of the squamosal.
The analysis of the CT-Scans resulted in a genuine added value in terms of supplemented morphological data that allow for a more coherent taxonomic assignment of the Girona sea cow.
First of all, this specimen is undoubtedly considered as representing a Prototherium species. As mentioned above, the key characteristic for the genus Prototherium is the lack of a persistent DP5/ DP5 (e.g.,
Taking into account observations in more derived sea cows, i.e. members of the Dugonginae that show mesial drift of their molars and, as a consequence, lose preceding teeth in lifetime, one can argue that there may have been an unreplaced DP5 that was worn out and shed given the advanced age of the Girona specimen. The gap between P4 and M1 would then have been closed by mesial drift. Although this is an interesting perspective, it is likewise a difficult one. For example, there are two specimens known from Eocene strata representing adults of “Halitherium” taulannense that show irregular resorption of DP5/DP5 (holotype skull RGHP D040 and paratype mandible RGHP E.7.096a (
Traces of resorption are commonly present in specimens that are known to have lost teeth due to mesial drift, especially amongst members of the subfamily Dugonginae that are characterized by the reduction of the antemolar dentition (e.g.,
Although the data basis remains limited not at least due to the lack of material between and in each of the two fossil-bearing slabs, the present study provides a combination of characters that argues with the utmost probability for the Girona specimen as being a representative of P. ausetanum. Two characters corroborate that hypothesis on the species level and refer to the shape of the M2 and M3. As outlined above, only MAC GIR-042375 and P. ausetanum share a rectangular M2 as well as an oval M3. Three further characters that do not argue unambiguously for a taxonomic assignment to P. ausetanum, but can be put into perspective, refer to the mesiodistal length of the molar crowns, the position of the zygomatic-orbital bridge, and the shape of the spheno-occipital eminences. Of all taxa for comparison, the mesiodistal length especially of M2-3 is most similar between MAC GIR-042375, P. ausetanum, and P. montserratense. A zygomatic-orbital bridge nearly level with the alveolar margin is congruent only in MAC GIR-042375, P. ausetanum, and P. solei. While P. montserratense differs from MAC GIR-042375 in having convex spheno-occipital eminences (a morphological feature that is also present in P. veronense), personal observations of the holotype specimen also reveal that it is not fully grown. This is indicated by the non-obliterated spheno-occipital suture and the only slightly worn M3. As such, the subadult age may qualify the current results on the mesiodistal length of M2-3 in P. montserratense. The material basis of P. solei does not provide further morphological characters that can be compared here. However, three differing dental characters as mentioned above versus one shared cranial character, i.e. the level of the zygomatic-orbital bridge, question a taxonomic assignment of MAC GIR-042375 to P. solei. Against that background, the feature of concave spheno-occipital eminences in the Girona specimen may potentially supplement the morphological information of the holotype of P. ausetanum, which lacks respective data. In fact, these eminences are likewise concave in P. intermedium, but the latter differs from MAC GIR-042375 in the dental features outlined above and in that the zygomatic-orbital bridge is clearly elevated above the alveolar margin. As such, the fossil sea cow remains from Girona resp. Sant Vicenç de Castellet contribute to more and new information that enhance and supplement our knowledge on the morphology and, considering the similar age and area, distribution of P. ausetanum.
In addition to the morphology, the stratigraphy and palaeogeography implicate, at least in part, new hypotheses on the taxonomic identity of the species for comparison, which, conversely, support the assignment of MAC GIR-042375 to P. ausetanum. In the Bartonian, potentially four Prototherium taxa are present in southern Europe: P. ausetanum (including MAC GIR-042375), P. montserratense and P. solei, each from Spain, and P. veronense from Italy. For the latter species, different and sometimes contradictory information on its geological age exist throughout the literature, and therefore, this issue shall be clarified first. Apart from
As outlined above, the present study provides morphological information from the not yet fully prepared holotype of P. montserratense that is even now valuable for comparison. For the time being, the morphological characters of it presented here argue for a species distinct from P. intermedium, hence, neither supporting a synonymy (
Similarly, P. solei is questioned here to be either a junior synonym (
Further studies could also help to fix the issue of the controversially discussed interrelationships of the genus Prototherium, which is shown as para- and polyphyletic in
This approach is particularly suggested in light of an alternative phylogenetic hypothesis that supports the monophyly of Prototherium by showing a sister group relationship between P. veronense and P. intermedium (
Considering the bigger picture, better-resolved intra- and intergeneric relationships based on new informative fossils such as MAC GIR-042375 have the potential to contribute to a better understanding of the interrelationships of Eocene taxa and sirenians in total. Although this task is beyond the scope of this paper, it shall be outlined here that the classification of Sirenia has long been controversial. In other words: the traditional classification concept of Sirenia, which distinguishes four families (e.g.,
In order to solve this drastic paraphyly problem,
Fig.
Palaeogeographic map of Europe during the Eocene based on templates of Ron
There is a rough SW-NE direction or tendency of distribution regarding the geological ages of the records. In the southwestern Tethyan area, ear bones from the early Eocene (Ypresian) of Tunisia represent the oldest sea cow remains (
In light of this stratigraphic and taxonomic background, the hypothesis on a Tethyan origin of sea cows is reinforced. Pan-Sirenia form a well-substantiated clade called Tethytheria together with their extant sister group Proboscidea (
Although a fast dispersal remains a solid hypothesis in the evolutionary history of stem pan-sirenians (e.g.,
The present study incorporates the non-invasive analysis of CT-scans and macroscopic data generated from a sirenian find embedded in two paving slabs. On the basis of these data, and supplementing the results from
Considering the distribution of stem pan-sirenians across Eurasia in total, the records not only reflect a rough southwest-northeast oriented trend towards younger geological ages, but also mark the Pyrenean Basin as one of the centers of abundant and well-preserved pan-sirenians, including nearly complete skulls and skeletons even of a plesiomorphic evolutionary stage like Sobrarbesiren cardieli (
The paving stones containing the fossil sea cow remains are being exhibited in the Museu d’Arqueologia de Catalunya (Archaeology Museum) in Girona and have the collection number MAC GIR-042375.
All CT images of this sirenian are archived in the Museum für Naturkunde and accessible through the Department of Science Data Management under https://doi.org/10.7479/c65w-cj22 (Voss et al. 2022; Cranial CT-data of Eocene Prototherium ausetanum from Catalonia compared to a Dugong skull [Dataset], shared under a CreativeCommons CC-BY license).
MV and OH conceived the research, processed the morphological analyses and wrote the manuscript. JCV run the CT analyses at the Clínica Girona. KM finalised segmentation of the CT data and provided the 3D reconstruction. MV prepared Figs
The authors declare that they have no conflict of interest.
We are deeply thankful to Roger Mata Lleonart (Caldes de Malavella, Spain), Jordi Ferrer Lopez and Marta Picó (both Girona, Spain) – without whose kind assistance, this work could not have been realised. We are indebted to the Mayor of the City of Girona, Maria Madrenas who warmly welcomed us and supported our research operation. We are grateful to Daryl P. Domning (Washington, D.C.) who provided valuable comments that were considered in the discussion on the dentition. Jan Philipp Kundt (Berlin, Germany) prepared initial CT segmentations, David M. Alba and Jordi Balaguer Bruguera (both Barcelona, Spain) kindly helped with the interpretation of the basicranium of the holotype of P. ausetanum. Finally, we deliver our thanks to the two reviewers, Iyad Zalmout (Jeddah, Saudi Arabia) and one anonymous, who provided very constructive comments that significantly improved our article.