1 Introduction

There are three extant species of the genus Morus: Morus bassanus, Morus capensis and Morus serrator. Morus species are large seabirds that live on coastal areas and normally breed in large colonies, along offshore islands or in mainland coastal cliffs. After the breeding season, these birds disperse over a wide area (Ospina-Alvarez, 2008; Svensson et al., 2009; Veron & Lawlor, 2009). Nowadays, Morus bassanus lives in both sides of the North Atlantic, the range of the Morus capensis extends from the coastal waters off the Gulf of Guinea on the west coast of Africa, to Mozambique, on the east coast. Morus serrator lives in Western Australia, and in the North and South Islands of New Zealand (Medway, 1993; Ospina-Alvarez, 2008; Stephenson, 2007; Veron & Lawlor, 2009).

In all world there are eleven species of Morus described in the fossil record of America and Europe. This work reports another presence of Morus in Europe and constitutes the first to be reported in the Miocene of Iberian Peninsula. The European Cenozoic deposits have yielded a considerable number of avian remains, including those of seabirds (Mlíkovský, Mayr, 2005; Tyrberg, 1998, 1999; Warheit, 2002). Seabirds are an ecologically important group of birds characterized by their dependence to the marine environment (Harrison, 1985; Schreiber & Burger, 2002; Gaston, 2004), whose fossil record in Europe, although scarce, ranges from the late Paleocene to the Recent (Mlíkovský, 2009). The oldest sulid record in Europe is from the Messel Pit WHS (Germany), in a middle Eocene lacustrine formation, where was described the species Masillastega rectirostris (Mayr, 2002; Mlikovsky, 2007). From the Miocene there are five sulid species (including the genus Morus) described in Europe, France, Austria and Romania. Unfortunately, European Miocene sulids lack a modern taxonomic revision, which prevents a closer evaluation of this record (Mlikovsky, 2009). Eleven species within the genus Morus have been described: eight in North America; one, in South America and two, in Europe (France and Romania) (Stucchi et al., 2016).

Despite Miocene avian remains from the Iberian Peninsula are in small numbers and in a fragmentary state, which sheds uncertainties to subsequent taxonomic tasks (Sánchez Marco, 2021) remains of several groups of birds have been described, include marine birds: Galliformes; Charadriiformes; Passeriformes, Accipitriformes, Strigiformes, Gruiformes, Odontopterygiformes and Anseriformes (Sánchez Marco & Sastre Páez, 2001; Figueiredo, 2018; Sánchez Marco, 1995a, 1995b, 1999, 2021). No sulid taxa have so far been described in the Miocene of the Iberian Peninsula.

There are ten paleontological Cenozoic sites with fossil birds described in Portugal: one from the lower Eocene of Silveirinha, where Harrison (1983) has identified a charadriiform (Fluviatilavis antunesi); and nine from the Miocene. In Praia do Penedo Norte, an unidentified bird was reported. These are fragmented bones of birds, whose conservation did not allow them to be identified (Figueiredo, 2010, 2018; Sánchez Marco, 1995a). However, the fact that they were preserved in marine sediments has allowed to interpret them as the remains of indeterminate seabirds (Mourer-Chauviré & Antunes, 2003). Unidentified bird bone fragments were described at Aveiras de Baixo (Zbyszewski & Ferreira, 1967) and at Amor, near Leiria (Zbyszewski and Ferreira, 1967; Antunes & Mein, 1981); Anser sp. has recently been identified at Aveiras de Baixo (Figueiredo, 2018). In Lisbon, three Miocene bird remains were described: Palaeoperdix media, in Olival da Susana; a gruidae, in Quinta das Pedreiras (Mourer-Chauviré & Antunes, 2003) and Miocorax sp., at Quinta das Freiras (Figueiredo, 2018). In Costa da Caparica, one sternum of Pelagornis miocaenus was discovered in 1976 and described by Mayr et al (2008). In Anchino (Vila Nova da Rainha) was described a turdidae and Miocorax sp. (Figueiredo, 2018). Finally, from Barreiro do Oleiro (Vila Nova da Rainha), a passeriform indet. and cf. Palaeoperdix sp were described by Figueiredo (2018).

In the present study, we describe a proximal fragment of a coracoid of a sulid bird from the middle Miocene of Portugal (Figs. 1), discovered by one of the authors (CNC), in 1996, at the basal beds of the cliff of the Praia do Penedo Norte (Sesimbra, Setúbal Peninsula).

Fig. 1
figure 1

Regional geological map of the Cabo Espichel, SW Setubal Peninsula area with relevant location of the studied Praia do Penedo Norte site: A—Portuguese Cenozoic deposits. The numbers shows the location of paleontological sites with bird body- and traces fossils: 1—Silveirinha (lower Eocene); 2—Amor; 3—Aveiras de Baixo, Anchino and Barreiro do Oleiro; 4—Olival da Susana, Quinta das Fereiras and Quinta da Pedreira; 5—Costa da Caparica; 6—Praia do Penedo Norte (2–6, Miocene); 7—Ilha do Pessegueiro (Upper Pleistocene); B—Miocene deposits of the Tagus Basin and Setúbal peninsula; C—Geology of Cabo Espichel area (southern sector of Setúbal Peninsula and northern slope of the Arrabida anticline).; (*)—Location of the paleontological site of Praia do Penedo Norte. Graphical scales: A—20 km; B—10 km; C—2 km (cartographic source: Carta Geológica de Portugal, LNEG)

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2 Geological setting

The Praia do Penedo Norte coastal cliffs are mainly cut in Miocene fine silty-clay sandstones with basal units more cemented as very fossiliferous biocalcarenites. These units belong to the southern sector (Setúbal Peninsula) of the Mio-Pliocene Lower Tagus basin and are part of the northern limb of the fold-fault of the Betic-oriented Arrábida mountain chain, that develops further south (Fig. 1). The Praia do Penedo Norte lowermost units are particularly enriched in the greenish phyllosilicate glauconite which indicates sediment starvation conditions associated to the main transgressive sequence of the beginning of the middle Miocene marine cycle of the Neogene of Portugal (Cachão & Silva, 2000).

The lithostratigraphy of Praia do Penedo Norte is constituted composed mainly by dark silty-clay fine sandstones, yellowish micaceous fine sandstones, locally known as “areolas”, and cemented biocalcarenites. The coracoid was found on bed C 11 (Fig. 2), on a condensed section constituted by glauconite-rich biocalcarenites, with abundant body fossils of both marine vertebrates and invertebrates, nowadays subjected to intense anthropic erosion due to ongoing activity of private collectors of Miocene shark teeth.

Fig. 2
figure 2

Stratigraphic column of Praia do Penedo Norte. Abbreviations: m—meters; S—calcareous nannofossil samples (PI–PVI—Penedo I–Penedo VI); B—beds; Lt—lithology

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3 Materials and methods

This study is based on a coracoid collected at Praia do Penedo Norte (Sesimbra) and curated in the paleontology collection of CPGP (numbered CPGP.13.96.2). In the laboratory (LAP), the bone fragment was photographed and measured followed by biometric, taxonomic and taphonomic studies, through a comparison, analysis of the main characteristics of the studied fossil with two coracoids (LAP.2016.3 and LAP.2017.1) from the extant northern gannet Morus bassanus and stored in the anatomical comparison collection of CPGP. The osteological nomenclature used follows Baumel and Witmer, 1993. The measurements were made in millimetres.

To determine the bird fossil’s age 6 samples were equally spaced and sequentially collected from the base to the uppermost layers of the stratigraphic sequence of Praia do Penedo Norte. For each sample a smear slide was prepared following procedures specific for siliciclastic-rich sediments (see Johnson et al., 2012). One to two 30 mm columns of the smear slide were screened with a petrographic microscope (with cross polars or nicols) at × 1250 magnification, to characterize the assemblage of calcareous nannofossils and register the presence of index fossils. Taxonomic identifications follow Nannotax (www.mikrotax.org/Nannotax3) diagnostical guidelines.

Institutional Abbreviations: CPGP Centro Português de Geo-História e Pré-História; LAP Laboratório de Arqueozoologia e Paleontologia.

4 Paleoenvironmental context

4.1 Paleoecology

The Praia do Penedo Norte succession is cropping out in the Miocene cliffs, with a set of a diversified assemblage of fossils, both of invertebrates and marine vertebrates, where marine facies are present (Manuppella et al., 1999). Water column may had been significant depth due to the quantity of nektonic vertebrate groups represented ranging from a wide variety of sharks and fishes to cetaceans. However, the coast should not be very far due to the presence of dark glauconite-rich internal molds of early Miocene bivalves reworked and included in some of the basal units of the sequence. Samples collected for micropaleontology revealed concentrations of calcareous nannofossils (less than 10 per field of view along the rippled smear slides) compatible with a neritic marine paleoenvironment, over middle continental shelf. The lowermost sample from the section has the highest abundancies and diversity of calcareous nannofossils (see Supplementary data), compatible with the deepest paleobathymetry of the entire section. Abundance and diversity diminish gradually towards the top of the section in response to the regressive trend that characterizes the uppermost units of the middle Miocene of the Lower Tagus Basin (Cachão & Silva, 2000).

4.2 Biostratigraphy of calcareous nannofossils

Despite the scarcity on oceanic index species such as Discoaster spp. due to paleoenvironmental reasons, the lowermost sample from the section (Penedo I) contained common Helicosphaera ampliaperta associated to rare H. magnifica. These helicoliths co-occurence restrain this assemblage the calcareous nannofossil biozone NN3 (Martini, 1971), more or less equivalent to the middle Burdigalian (see Fig. 3).

Fig. 3
figure 3

Biostratigraphic framework of the 6 samples (I–VI) collected along the entire section of Praia do Penedo Norte for calcareous nannofossils

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Penedo II sample discloses the occurrence of (very rare) Sphenolithus heteromorphus which, in association with the still present and common H. ampliaperta (commom), indicates a younger age, compatible with biozone NN4 (Martini, 1971), equivalent to the late Burdigalian to early Langhian (see Fig. 3).

Penedo III already contains the small helicolith Helicosphaera walbersdorfensis while has no trace of H. ampliaperta and no S. heteromorphus. This, together with the presence of small to medium size reticulofenestrids (i.e., still without large Reticulofenestra pseudoumbilicus) indicates an age roughly compatible with biozone NN5 (Martini, 1971) (see Fig. 3).

The subsequent samples Penedo IV, V and VI all disclose similar relatively poor calcareous nannofossil assemblages (without open ocean discoasterids and very rare sphenoliths) with the co-occurrence of H. walbersdorfensis, R. pseudoumbilicus and very large (~ 15 mm) Coccolithus miopelagicus. Biostratigraphically, this assemblage is compatible with the late middle Miocene (Serravallian) calcareous nannofossil biozone NN6 (Martini, 1971) (see Fig. 3).

5 Results

Systematic palaeontology

AVES Linnaeus 1758

SULIFORMES Sharpe 1891

SULIDAE Reichenbach 1849

MORUS Vieillot 1816

MORUS SP. Olson and Warheit 1988

Material proximal portion of a left coracoid (CPGP.13.96.2) (see Fig. 4).

Fig. 4
figure 4

Proximal portion of a left coracoid (CPGP.13.96.2): Abbreviations: Cs—cotyla scapularis; Fac—facies articularis clavicularis; Fah—facies articularis humeralis; Ff – Furcular facet; lea—lamina elliptica articularis; la – lig. Acrocoracohumeralis; Pa—process acrocoracoideus; Pf—pneumatic foramina; Pp—process procoracoideus; A dorsal view; B ventral view; C anterior view. The arrow shows the broken border of the cotyla scapularis

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Locality and horizon Praia do Penedo Norte Bed C11 from the depositional sequence, Langhian in age based on the calcareous nannofossil biozonation (see above).

Measurements: the fossil has a maximum length of 41.5 and a maximum width of 18.1 mm. Five osteological measures (see Fig. 5) were defined in order to compare with the bones of the extant remains of Morus bassanus (Table 1).

Fig. 5
figure 5

Identification of the defined measures. DW distal width, Lfah Length of facies articularis humeralis, Ltc length of triosseal channel, Wla width of lig. acrocoracohum; Lfac Length of facies articularis clavicularis. (author: Alexandre Fonseca)

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Table 1 Measurements, in mm, of CPGP.13.96.2, compared with the same measures of LAP.2016.3 and LAP.2017.1
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Description and comparisons: CPGP.13.96.2 (Fig. 3) has a pointed border of the cotyla scapularis. The sacapularis cotyla has a sub oval shape, with a pointed distal end, the process acrocoracoid is narrow and the lamina elliptica articularis has a drop shape. It has four pneumatic foramina in the triosseal canal. The process acrocoracoideus is not complete (Fig. 6), because this area is broken, probably caused by necrophagy (Fig. 7).

Fig. 6
figure 6

A 1 and 2: Morus bassanus left coracoids (1: LAP.2016.3; 2—LAP.2017.1); 3—CPGP.13.96.2 and 4 – drawing of the CPGP.13.96.2 (author: Alexandre Fonseca). A dorsal view, B ventral view

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Fig. 7
figure 7

Comparison of the gnawing traces observed in LAP.2017.1 (left) and in the CPGP.13.96.2 (right)

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A comparison was made with two extant Morus bassanus coracoids (Fig. 7 2). The pointed border of the cotyla sacapularis is a derived feature which support the classification of the CPGP.13.96.2 as a Morus (Cohen & Serjeantson, 1996; Van Tets et al., 1988).

Compared to other fossil Morus, CPGP.13.96.2 coracoid is narrower and thinner than those of the Morus peninsulares (Mourer-Chauviré & Geraads, 2010); the facies articularis clavicularis is drop-shaped in CPGP.13.96.2, as in the Morus bassanus (Fig. 8), while in the Morus peninsulares is round (Mourer-Chauviré & Geraads, 2010). The long axis of furcular facet is parallel with shaft, like Morus, and not tilted medially from the axis of the shaft, as in Sula and Papasula (Benson & Erickson, 1997). The enlargement of the head of coracoid, observed in CPGP.13.96.2, is a characteristic of the Morus genus (Benson & Erickson, 1997). The sterno-lateral corner of facies articularis humeralis in CPGP.13.96.2 has a square-shape and pointed and narrow, which is a characteristic of the genus Morus (Steadman et al., 1988; Van Tets et al., 1988), it is also more concave, like the Morus coracoid, while in Papasula and Sula the facies articularis humeralis is less concave, as observed in Steadman et al. (1988) and in Van Tets et al. (1988).

Fig. 8
figure 8

Possible reconstitution of the individual Morus sp. to which the coracoid CPGP.13.96.2 may have belonged (art—Alexandre Fonseca)

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6 Discussion and conclusion

The broken part (see Fig. 6) of the processus acrocoracoideus do not seem to be the result of erosion resulting from postmortem transport, as this is one of the most resistant bone zones for erosion. The fact that it presents the same characteristics of similar traces in recent bones, resulting from gnawing, we think that this breakage was the result of the action of necrophagous animals. This necrophagy action may also have disassembled the skeleton.

The presence of seabirds in the Miocene of Praia do Penedo Norte is consistent with the fossil record of this paleontological site, where remains of marine vertebrates abound. The marine paleoenvironments over the continental shelf, as the calcareous nannofossils found in the sequence Praia do Penedo Norte indicate, are also in agreement with the environments frequented by birds of the genus Morus. The biostratigraphic calcareous nannofossil framework dates this specimen as Langhian (middle Miocene, 16–14 Ma).

The fossil coracoid now studied presents the main characteristics observed in coracoids of the genus Morus, as observed in the comparative study: pointed border of the cotyla scapularis, oval shape of the sacapularis cotyla, the narrow process acrocoracoid and the drop shaped facies articularis clavicularis.