Dinosaur Tracks of the Areia do Mastro and Papo-Seco Formations (Lower Cretaceous, Cabo Espichel): Paleobiological and Paleoenvironmental Continuities and Discontinuities ^†

Abstract

New theropod tracks found in the Papo-Seco Formation (lower Barremian, Lusitanian Basin, Portugal) are presented. In 2022, thirteen theropod tracks were identified on the lowermost bed of this formation, preserved as natural cast infillings on the bedding surface. Two different morphotypes of theropod footprints have been identified, which occur at two different levels of this layer. The first morphotype, consisting of smaller footprints with narrow digits, is attributed to theropoda indet; the other morphotype, consisting only of one footprint, is assigned provisionally to the ichnogenus Megalosauripus isp. It is suggested that they were produced by medium-sized theropods, on a carbonate mud substrate, probably in a coastal environment associated with a lagoon. These tracks, and others previously described in the underlying Areia do Mastro Formation, suggest a temporal continuity of the groups of dinosaurs that frequented this area, during the early Barremian. However, the morphotypes now described differ from those of the theropods tracks from the Areia do Mastro Formation, which may indicate a change in faunal types within the theropod group.

1. Introduction

Dinosaur tracksites are abundant in Mesozoic successions, and the fossil record of dinosaurs from the Early Cretaceous of Europe is extensive. During this period, theropods and other dinosaur groups inhabited Europe, including the Iberian Peninsula [1,2,3,4,5,6,7,8,9,10,11,12,13,14]. In Portugal, from the Early Cretaceous, eight ichnosites with footprints of ornithopods, theropods, and sauropods have been described [5,8,14,15]. Of these, four contain theropod footprints (Lagosteiros, Praia de Salema, Praia Grande, and Boca do Chapim/Areia do Mastro) [5,11].

Paleontological research at Cabo Espichel began in the late 19th century [16]. Since then, skeletal remains of crocodiles, turtles, pterosaurs, fish, and dinosaurs have been described in the Papo-Seco Formation [10,17,18,19,20,21]. Dinosaur remains from this region include theropods [10,17,18,20,22], sauropods [10,17,23], and ornithopods [10,17,19,20,24].

Several ichnosites with dinosaur tracks from the Late Jurassic and Early Cretaceous have also been described in the Cabo Espichel area (e.g., [1,3,12,25]). More recently, new dinosaur tracksites have been described between Boca do Chapim and Praia do Guincho (north of Cabo Espichel), in the Areia do Mastro and Papo-Seco formations (lower Barremian) [11,12,13,14].

2. Geological Settings

The tracks described here are located at Boca do Chapim, approximately 2 km north of Cabo Espichel, at coordinates 38°26′02″ N/9°12′43″ W. Municipality of Sesimbra, district of Setúbal (Figure 1).

The exposed sedimentary succession (Figure 2) includes limestone, marls, sandstones, and fine gravel layers, deposited in shallow marine, lagoonal, and estuarine environments. This succession comprises three lithostratigraphic units of the Lower Cretaceous (from bottom to top; formal stratigraphy according to [26]: the Areia do Mastro Formation and the Papo Seco Formation, attributed to the lower Barremian; and the Boca do Chapim Formation, attributed to the middle Barremian (e.g., [27,28,29,30,31]) (Figure 1).

The tracks occur in the first layer of the Papo Seco Formation (PS1; lower Barreman), which lies above a succession consisting of the uppermost strata of the Areia do Mastro Formation, where dinosaur tracks have also been described [12,13,14]. The basal layers of the Papo Seco Formation consist of fine, dark yellow sandstones, 1.0–1.2 m thick, interbedded with some light gray and greenish marls, up to 2.7 m thick. The layer where these tracks occur corresponds to the sandstones, which exhibit sedimentary structures related to hydrodynamics (currents along tidal channels), wood fragments (on some levels), and dolomitic concretions. These beds of the Papo Seco Formation indicate a depositional environment corresponding to a subtidal to intertidal lagoon adjacent to a sandy coastal barrier.

3. Materials and Methods

This study consists of the analysis of 13 theropod footprints discovered at Boca do Chapim, at the base of the Papo Seco Formation. Most of the work was carried out in the field, including the collection of measurements (width and length). Measurements were obtained in the field using a flexible tape measure, following standard procedures for theropod footprint measurement [32,33]. Dimensions include footprint length, width, digit lengths, and divarication angles, all measured along the central axes of the impressions. Aerial photographs were obtained using a DJI Mini 3 drone, manufacturer by SZ DJI Technology Co., Ltd., Guangdong, China, equipped with a high-resolution camera, under stable lighting and weather conditions to ensure optimal image quality, photographic survey using a Canon 100D camera, manufacturer by Canon Inc, Taiwan, and the coding of each footprint. The geographical location and orientation of each track were also recorded. This work was done using the Compass app on the IOS system. A database was created with information about each footprint: code, location, orientation, and measurements. Based on the field tables containing the coordinates, a map (Figure 3) was produced, which allowed for the determination of their spatial distribution.

4. Results

In Boca do Chapim, 13 theropod footprints were identified in 2 different areas (Loc. 1 and Loc. 2; Figure 3), on the surface of the layers. At Loc. 1, 11 footprints were found: 2 in a lower level (N1), and 9 at the top of the layer (N2). At Loc. 2, two footprints were identified. The layers of Loc. 1; N1 consist of negative relief impressions, corresponding to the depression of the sediment due to the pressure from the animal’s foot; the remaining footprints are in positive relief, forming natural molds. All the footprints are tridactyl, longer than wide.

Two morphotypes of tracks were identified:

The first morphotype consisting of the tracks from Loc. 1—N2. They have a length between 20 and 40 cm, are digitigrade, tridactyl, asymmetric, and elongated and thin digits, with a straight digit III (Figure 4A1–A3). Three of these tracks form a narrow track. In these tracks, it was not possible to identify the impression of the phalanges. The angle of divergence between the digits of these footprints ranges from 44.5° (max.) to 25.7° (min.) between digits II and III, and from 50.4° to 28.9° between digits III and IV. These footprints share some similar characteristics to those of Tuojiangpus shuinanensis [34] and reassigned to Eubrontes isp. [35], from the Early Jurassic of China, which are considered nomen dubium by [36].

The second morphotype, which we attribute to Megalosauripus isp., is represented by the two robust footprints from Loc1N1 and Loc2 (Figure 3 and Figure 4). These tracks are longer than wide and display weak mesaxony, consistent with the diagnostic features of Megalosauripus [37]. (Figure 3). Except for the footprint Loc2T1 (Figure 4B2), these tracks are mostly plantigrade, tridactyl, and less elongated than those of morphotype 1, with wide digits.

The track at Loc1N1T1 has wider digits and is plantigrade, with an impression of the heel, which has a square shape. Loc1N1T2 is digitigrade with a higher degree of interdigital divergence. Loc1N1T1 measures 27 cm in length and 16 cm in width, while Loc1N1T2 measures 26 cm by 17 cm. Loc1N1T2 is digitigrade. Both have a straight impression of digit III. The angle of divergence between digits II–III is 26.8° at Loc1N1T1 and 23.6° at Loc1N1T2, and between digits III–IV is 29.7° at Loc2T1 and 31.7° at Loc1N1T2. Both show distinct claw marks. The footprint at Loc2T1 (Figure 4B1) has thinner digits and a higher degree of interdigital divergence, while Loc2T2 has wider digits, with an impression of the heel, which is square-shaped. Loc2T1 measures 32 cm in length and 31 cm in width, while Loc2T2 measures 38 cm by 25 cm. Loc2T1 is digitigrade, while Loc2T2 is plantigrade. Both have a straight impression of digit III. Loc2T2 (Figure 4B2) shows the ungual impression of digit I, oriented posteriorly and distally. The angle of divergence between digits II–III is 40° at Loc2T1 and 23.5° at Loc2T2, and between digits III–IV is 36° at Loc2T1 and 26.7° at Loc2T2.

In the upper beds of the Areia do Mastro Formation and the lower beds of the Papo Seco Formation, dinosaur and crocodylomorph footprints are found (in the uppermost layer). The first bed of the Papo Seco Formation, which outcrops at Boca do Chapim, contains theropod footprints [23], along with, although in smaller numbers, footprints from other dinosaur groups such as sauropods and ornithopods. The presence of these groups aligns with the fossil bone remains discovered in the Papo Seco Formation, as well as with the set of footprints found in the underlying layers of the Areia do Mastro Formation [10,12,14,23], suggesting a continuity of dinosaur groups that inhabited this region during the early Barremian. However, some theropod morphotypes identified in the Papo Seco Formation (Figure 4) differ from those described in the Areia do Mastro Formation [12,14,23], displaying thinner and longer digit prints. This change may be linked to taphonomic factors or related to differences in sedimentary environments. It is also possible that this reflects a shift in the theropod fauna that occupied the area at that time. Additionally, the footprint distribution in the first layers of the Papo Seco Formation reveals a different carnivore/herbivore ratio compared to that identified in the Areia do Mastro Formation.

The Areia do Mastro Formation consists of blue-grey clayey limestones and marl-limestones, with intercalations of sandstones and silty marls. These sediments result from deposition in marine environments, specifically in an inner continental platform. In contrast, the Papo Seco Formation comprises marls and silty clays with lignite, gypsum, and intercalated sandstones—some fine-grained with a dolomitic matrix and others coarse-grained. These deposits reflect lagoon-type sedimentary environments (lagoon and estuary) alternating with fluvial environments [28,31,38] (Figure 5). Thus, there is a shift from marine environments (Areia do Mastro Formation) to continental environments (Papo Seco Formation) accompanied by an episode of negative eustatic trend.

5. Discussion

We observed a continuity of dinosaur groups between these two formations. However, the Papo Seco Formation shows a greater diversity of vertebrate groups in the osteological record, with remains not only of dinosaurs but also of crocodylomorphs, turtles, pterosaurs, and fish [10]. This broader faunal assemblage is consistent with a marginal to restricted marine setting, as indicated by the associated invertebrate and sedimentological data [15,30]. The number of footprints is, however, much higher in the upper layers of the Areia do Mastro Formation than in the lower layers of the Papo Seco Formation, suggesting distinct taphonomic and ecological controls on track preservation.

In general, fossilization tends to favour the preservation of certain organisms and trace fossils depending on sedimentary and environmental conditions [33,39,40]. The Papo Seco Formation records deposition in more restricted, possibly brackish or lagoonal environments, whereas the overlying Areia do Mastro Formation represents more open, littoral to beach settings [30,38]. These environmental differences likely influenced both the behaviour of the trackmakers and the preservation potential of their footprints. Brackish and muddy substrates may have been less suitable for the formation of well-defined and durable tracks, while the firmer, sandy surfaces of the Areia do Mastro palaeobeach favoured track registration and subsequent preservation.

Despite the occurrence of similar dinosaur groups in both formations, the ichnological record from the Papo Seco Formation is dominated by theropod tracks. This prevalence of carnivorous dinosaur footprints may reflect behavioural and ecological factors rather than purely taphonomic bias [33,39,40]. The scarcity of herbivorous tracks could be related to the absence of extensive vegetation or grazing areas in the more saline and unstable coastal-lagoonal environments of the Papo Seco Formation [30,31]. In contrast, herbivorous dinosaurs may have frequented more inland or vegetated zones, which are not preserved in the current outcrops.

Theropods, on the other hand, may have been attracted to these marginal environments in search of alternative food resources, such as stranded marine animals, fish, or small vertebrates inhabiting the brackish areas. Such behaviour, including opportunistic scavenging near coastal zones, has been proposed for other Mesozoic tracksites in similar paleoenvironmental contexts [41,42]. Interestingly, the smaller size of the theropod tracks from the Papo Seco Formation compared to those from the Areia do Mastro Formation may suggest either ontogenetic or taxonomic differences among the trackmakers, or behavioural segregation related to habitat preference and resource exploitation.

6. Conclusions

These footprints from the Papo Seco Formation expand our knowledge about the occurrence of mew theropod footprints in the Lower Cretaceous of Portugal, and the study of their producers complements the understanding of carnivorous dinosaur faunas in the Barremian of Cabo Espichel. In the same layer, although in fewer numbers, footprints from other dinosaur groups (sauropods and ornithopods) also occur. The occurrence of these groups is consistent with the fossilized bone remains found in the Papo Seco Formation, as well as with the set of footprints identified in the layers immediately beneath it, belonging to the Areia do Mastro Formation. However, the morphotypes now identified differ from those described for theropods in the Areia do Mastro Formation, which may indicate a change in faunal types within the theropod group.

These footprints of the Papo Seco Formation expand the knowledge about the occurrence of theropod footprints in the Lower Cretaceous in Portugal and the study about their producers complements the knowledge of carnivorous dinosaur faunas in the Barremian of Cabo Espichel. In the same layer, footprints of other groups of dinosaurs (sauropods and ornithopods) occur, although in smaller numbers. The occurrence of these groups is consistent with the fossil bone remains found in the Fm. of Papo Seco, as well as with the set of footprints identified in the layers immediately underlying it, belonging to the Fm. of Areia do Mastro. However, the morphotypes of these tracks differ from those described for the theropods tracks of the Fm. de Areia do Mastro, which may indicate a change in faunal types within the theropod group.