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Article

An Ammonite Preserved in the Upper Pliocene Lower Tejo River Deposits of Quinta Do Conde (Southwest Portugal)

by
Carlos Neto de Carvalho
1,2,3,*,
Miguel Barroso
4 and
Sofia Soares
5
1
Geology Office of the Municipality of Idanha-a-Nova, Naturtejo UNESCO Global Geopark, Centro Cultural Raiano, Av. Joaquim Morão, 6060-101 Idanha-a-Nova, Portugal
2
Instituto Dom Luiz, Faculdade de Ciências da Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal
3
Centro Português de Geo-História e Pré-História, Campo das Amoreiras Lt 1 – 2.º O, 1750-021 Lisboa, Portugal
4
Meteoric Cipher, Lda—Geological Consultancy and Services Company, Rua Quinta Nossa Senhora do Sameiro, Nº 25, Foros de Amora, 2845-316 Amora, Portugal
5
LNEG—Laboratório Nacional de Energia e Geologia IP, Estrada da Portela, Bairro do Zambujal, Alfragide, Ap. 7586, 2610-999 Amadora, Portugal
*
Author to whom correspondence should be addressed.
Foss. Stud. 2025, 3(2), 8; https://doi.org/10.3390/fossils3020008
Submission received: 31 January 2025 / Revised: 17 May 2025 / Accepted: 27 May 2025 / Published: 3 June 2025
(This article belongs to the Special Issue Continuities and Discontinuities of the Fossil Record)
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Abstract

:
A cast is an object that results from a fossilization process that is considerably rare in nature. For a cast to be produced, secondary diagenetic processes during and after fossilization are normally involved. Natural casts are formed when minerals are deposited within the fossil mold. Here we describe an exceptional example of the natural cast by gypsum of an ammonite presumably preserved as a limestone-made “half” mold that had previously been transported as an extraclast, deposited and dissolved within Upper Pliocene quartz sandstones of the ancestral Tejo river. Portable X-ray fluorescence was used to analyze and compare the geochemical composition of the ammonite fossil with that of the nodules found within the same bed, reflecting different diagenetic timings. The composition of the ammonite cast reflects the in situ dissolution of limestone and the precipitation of calcium sulfate. High δ34S‰ and Sr values obtained from the ammonite show that the cast was produced by percolating acidic waters in the vadose zone, under marine influence, during the Late Pliocene or already in the Pleistocene. The waters being rich in sulfur resulted more likely from a marine water-influenced water table. Alternatively, it may have resulted from the weathering concentration of sulfur from the Marco Furado ferricretes overlying Santa Marta sandstone. This is, so far, the only testimony of the enormous temporal discontinuity that occurred during the taphonomic history of an ammonite, with a final preservation in the form of a cast made of gypsum, the most didactic example of this type of fossilization ever found in Portugal.

1. Introduction

Casts and molds result from fossilization processes, where the physical characteristics of organisms are impressed onto sediments and rocks. Among the various modes of fossilization that can occur in nature (e.g., [1,2]), the formation of casts is considerably rare. The formation of a natural cast implies the filling of an original fossil, allowing its replication with sedimentary/mineralogical material different from that which constitutes it [3]. This process assumes that casts are not only relatively rare but also implies that they were produced in a period of time different from that which led to the preservation of the organism in the geological record. Depending on the diagenetic processes and the speed of their development, the formation of casts should normally be measured in thousands to millions of years, determining minor or major discontinuities in the taphonomic history of a given fossil record. In several seminal works (resumed in [4]), Adolf Seilacher discussed the ammonoid taphonomy. Preservational features express the varying time relationships between shell solution, compaction, and cementation processes during early diagenesis [5]. The aragonitic shell of the ammonite was dissolved very early in diagenesis while the sediment infill was already hard enough to be preserved as an internal mold. Still, during early diagenesis, aragonite chemical solution would leave spaces that could later be filled with precipitated calcite or pyrite, as the most common casting minerals available; in other cases, the resedimentation of pre-buried and pre-fossilized shells can happen [5,6,7]. These are cases of early diagenesis ammonite casts in the same diagenetic background.
In the present work, we further document a rare case of an ammonite preserved as a natural cast, with a diagenetic history more complex than usual, that was found in the Portuguese fossil record and preliminarily presented [8,9]. An ammonite with origins in Mesozoic deposits from the Lusitanian Basin was transported as a lithoclast among the sediment load of the ancestral Tejo river and preserved in sandstones after the dissolution of the clast. The result is the truly extraordinary occurrence of a fossil ammonite in deposits dating to the Late Pliocene, with no Mesozoic outcrops in close vicinity. The ammonite was found in the northwest area of the municipality of Setúbal, in an old sandpit near the town of Quinta do Conde, next to national road 10, in a location with the coordinates 38°4′19.3″ N, 09°01′40.2″ W (Figure 1).

2. Geological Setting

The formation of the Albufeira Syncline, in the Setúbal Peninsula, located on the left bank of the Tejo river south of Lisbon, is related to the Alpine Orogeny evolution of the Iberian Peninsula, especially during the Miocene [10]. During the Betic paroxysm of this orogeny, the strong subsidence that occurred in the Lower Tejo basin controlled the installation of the vestibular part of the Tejo river, with the deposition of thick Pliocene–Quaternary siliciclastic units, especially in the depocentric areas, in a braided-type delta system [11]. Pliocene siliciclastic units crop out in almost the whole Albufeira Syncline, largely corresponding to a single unit, the Santa Marta formation (Figure 2A).
The Santa Marta formation [12], where the ammonite was found, is composed of fine to coarse sandstones, generally arkosic in composition, with frequent cross-bedding. Five main facies were described [11]. The lag of paleochannels (Facies 1, observable in the Almada-Laranjeiro area, 14 km NW to Quinta do Conde) shows clasts of basalt, granite, quartzite, and calciturbidites from the Ramalhão Formation originating in the Lisbon–Sintra region [11]. Further south, the sands are finer, white in color, and have a more homogeneous grain size, and the cross-bedding sets show mainly SW-oriented currents. In the clay fraction, kaolinite and illite predominate [13,14]. Towards the upper part of the formation, there are paleochannels filled with greenish to black pelites (Facies 2 of abandoned channels) with plant remains and large twinned crystals of selenite, lignites, and diatomites (in the central zone of the Albufeira Syncline); locally, there are paleochannels with ostreids (floodplain Facies 3 of Azevêdo [11]). These are evidence of its estuarine nature, located to the west, closer to the present coastline, as would be expected. Facies 4 corresponds to the “Areias de Coina”, white in color (light gray when there are lignite remains and accumulations of heavy minerals), kaolinic, fine- to medium-grained, well-calibrated sandstones organized in small-scale herringbone cross-bedding sets varying between 5 and 10 cm in thickness. The ammonite was found in this facies. Within the sandstones, few cobbles and pebbles of Cretaceous sandstones and flint nodules, originating from the Mesozoic of the Lusitanian Basin, have been found, whose outcrops occur over 20 km to the northwest [11]. Facies 5 occurs just east of Palmela, about 10 km E from the sandpit, and corresponds to coarse and poorly calibrated sandstones, representing a shorter transport of sediment coming from the east [11]; fine, well-calibrated aeolian sands may occur, cross-stratified in thick sets, interspersed in the fluvial-dominated sands. At the top of the formation, there is a microconglomeratic level with mollusk molds, a facies representing the coastal marine environment. The maximum thickness of the Santa Marta formation is 325 m at Pinhal Novo [15] in the zone of maximum subsidence located 9 km ENE from the sandpit, not more than 50 m thick towards the west, in the coastal cliffs of Lagoa de Albufeira, 15 km WSW from the study site. The age of this formation is Late Pliocene (see [11,16] with the references indicated).
The Marco Furado formation overlies the Santa Marta formation cropping out at the Setúbal Peninsula [17]. It is a unit of poorly sorted gravel with a red-colored, sandy-clay matrix and reaches up to 30–40 m in thickness. The angular clasts are predominantly white quartz, but quartzite, jasper, chert, and shale are also represented. Ferricrete paleosols are common, particularly at the top. The clay fraction is composed mostly of illite, together with mica and kaolinite [18], which is indicative of more seasonal precipitation in a Mediterranean-type climate during the Early Pleistocene.
The old sandpit is located just east of the town of Quinta do Conde and shows, in the old quarried front, a 5 m thick succession (Figure 2B and Figure 3A) composed, at the base, of a level of coarse yellowish sandstone (by the presence of iron hydroxides) at least 1 m thick, with sets of “herringbone” cross-stratification, mostly with a few centimeters in thickness (Figure 3B). This layer is followed by a level of quartz sandstone 1 m thick, grayish in color, characterized by the abundance of lignified plant remains of predominantly millimeter size, with scattered quartz clasts measuring 1–2 cm in their longest axis. This sandstone is also organized in sets of cross-bedded lamina, some decimeters in thickness. The last three meters of the succession are composed of white-colored, kaolinic, cross-stratified quartz sandstones, finer-grained than the previous beds, and the presence of kaolinite nodules (Figure 3C), not reaching more than 2 cm in the long axis (Figure 3D). These well-calibrated sands, where sub-rolled grains of stable minerals (mainly quartz) predominate, together with the herringbone cross-bedding in small sets, indicate river sedimentation in braided channels, with provenance from relatively distant sources, in very shallow water environments, where a low hydrodynamic regime and redox conditions may have prevailed.
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Figure 3. Aspects of the sandstone succession at the old sandpit of Quinta do Conde (reprinted from [8]). (A) Former sandpit front showing washout accelerated by runoff waters; (B) level of coarse sandstone with the presence of iron hydroxides, outcropping at the base of the succession; (C) sandstone organized in sets of chevron-shaped cross-stratification, corresponding to the bed where the ammonite fossil was found. Note a small kaolinite nodule near the scale; (D) detail of a kaolinite nodule in the same bed. Ruler is 7 cm in total length.
Figure 3. Aspects of the sandstone succession at the old sandpit of Quinta do Conde (reprinted from [8]). (A) Former sandpit front showing washout accelerated by runoff waters; (B) level of coarse sandstone with the presence of iron hydroxides, outcropping at the base of the succession; (C) sandstone organized in sets of chevron-shaped cross-stratification, corresponding to the bed where the ammonite fossil was found. Note a small kaolinite nodule near the scale; (D) detail of a kaolinite nodule in the same bed. Ruler is 7 cm in total length.
Fossstud 03 00008 g003
An erosional unconformity separates the Santa Marta formation from the overlying Marco Furado formation (Figure 2B). Evidence of ferricretes exist close to the base of the unit in the sandpit, but they were mostly eroded, leaving debris scattered on the surface of the Santa Marta sandstone.

3. Material and Methods

The ammonite was found by one of the authors (M.B.) while prospecting for kaolinite. It is for the moment properly stored in his private collection and available for new studies upon request. The fossil was in a horizontal position in the outcrop formed by the washout of the sandstones by the rainwater runoff. The ammonite was therefore found in situ, approximately 1.5 m below the top of the sandpit succession described in the geological setting, being exposed in a subvertical wall of the creek. However, the position of the fossil at the time it was found was not recorded, although it is assumed that it was found with the ornamentation facing downwards (Figure 4). No other fossils were found in the outcrop despite detailed analysis. Nodules with apparently the same composition were collected from the same bed for geochemical analysis.
Portable X-ray fluorescence (p-XRF) equipment was employed to determine the elemental chemical composition of the cast and a representative nodule, as well as samples of a ferricrete from the overlying Marco Furado formation, for comparison. This analysis took place at the Portuguese National Laboratory of Energy and Geology (LNEG), utilizing a Hitachi X-MET8000 series instrument (Hitachi, Ltd., Tokyo, Japan) on a bench with the mining light elements fundamental parameter method (Mining LE-FP). This analyzer is a portable energy-dispersive device that is equipped with a 50 kV (max.) rhodium (Rh) X-ray generator and a large-area (25 mm2) silicon drift detector. The measurements were acquired for 60 s in two points for each sample. Sample preparation regarded a homogeneous and fine-grained powder inside a sample cup of 32 mm diameter covered with a high-performance XRF film.
Recently, Gil-Delgado et al. [19] have explored the utility of p-XRF non-destructive techniques in identifying the geochemical signature of fossils in museum collections in order to trace their provenance and distinguish fossil sites. Here, we intended to compare the geochemical signature of the mineral composing the cast with nodules that occur in the same unit and are visually similar and therefore reconstitute the diagenetic stages of the development of the ammonite cast and using the Marco Furado formation as a potential source of sulfur.
δ34S‰ (VCDT) with an analytical precision of 1-sigma was measured from a sample of the ammonite cast in the Environmental Isotope Laboratory at the University of Arizona, USA. This isotopic analysis provided important evidence for the environmental origin of the gypsum brine.

4. Description of the Fossil

The ammonite fossil has a good consistency and a non-powdery surface, determining a very pure composition and white color. There are few quartz grains present within the cast, but surfaces are marked by punctuations resulting from quartz grains. The fossil represents about half of an ammonite that originally would have had a maximum diameter of 170 mm. It shows a conch with spiraled whorls. The last two whorls are represented by about half of their total extension and shape, the outermost one consisting of 19 strong ribs in the part that was preserved, with a maximum height of 65 mm. There are no preserved suture lines (Figure 4A). The slightly convex section of the conch at one of the ends, parallel to the height of the loop, shows that the surface that intersected the ammonite, at least in part, is original, certainly corresponding to the contour of the clast where the ammonite would be preserved. Furthermore, nothing remained in the outcrop related to this fossil, although it was its section that revealed its presence in the vertical face of the creek. According to the axis of symmetry, the ammonite is preserved in semi-relief, with one of the faces being completely flat and devoid of morphological aspects (Figure 4B). The preservation of “half-ammonites” is common in the fossil record and results from the partial sedimentary filling of the conch (the lower half of the camerae) with subsequent selective dissolution [4]. This seems to indicate that the ammonite cast originated from a clast preserving an ammonite internal mold. Supporting this interpretation is the existence of a bowl-shaped structure in positive relief, with a long axis length of 55 mm and a height of 27 mm, which appears to intersect the ribs; two pits of different dimensions can be observed in this structure, one smaller and in an apical position and the other larger, in a lateral position (Figure 4A). This bowl-shaped structure could be a fragment of the original matrix that surrounded the ammonite or have a biogenic origin (an epibiont organism that lived on the conch, either just after the death of the ammonite or during its deposition already as a lithoclast). Anyway, it shows the same punctuation texture of the remaining fossil, revealing the same preservation history as the ammonite.

5. Discussion About the Origin and Formation of the Ammonite Cast

5.1. Possible Mesozoic Formations from Where the Ammonite May Have Come from

Certainly, the ammonite had an extrinsic origin to the Pliocene formations where it occurs, having been transported as an extraclast by tributaries of the ancestral Tejo river installed on Mesozoic formations where ammonites are abundant. In the southern bank of the Tejo drainage, in relation to the Arrábida mountains, a possible provenance for the ammonite is the Sesimbra Formation, dating from the Sinemurian-Toarcian, which only outcrops to the east of Sesimbra, and where ammonites were found [20]. The Espichel formation, dating from the upper Kimmeridgian-Tithonian, also provided some ammonites, although it only crops out in the Cape Espichel area [21]. However, the location of these two formations southwards of the Albufeira Lagoon and the axis of the syncline, one of the main drainage axes of the ancestral Tejo river delta, most likely prevented them from being the original provenance of the ammonite, given the position of the Quinta do Conde sandpit a few kilometers to the northeast of the main drainage axis during the Pliocene–Early Pleistocene.
Its most likely origin would be north or northwest of the place where it was found. The ammonite may have been eroded and transported from the Arruda dos Vinhos Mesozoic sub-basin and the Abadia Formation, dated to the Kimmeridgian by the abundant ammonite fauna [22]. The closest outcropping areas occur in Alverca-Alhandra sector, more than 35 km north of the sandpit (see Figure 2A). On the other hand, the occurrence of calciturbidite clasts attributed to the Ramalhão Formation, a unit correlated with the Abadia Formation [23] where ammonites were also identified, together with other igneous lithologies corresponding to the Sintra massif (Figure 2A), in channel lags of the Santa Marta formation present in Laranjeiro (Almada area), shows a proven contribution from the tributaries of the right bank of the ancestral Tejo river with carbonate clasts for the Pliocene sedimentation. Although the majority of the Cretaceous formations around Lisbon mostly represent continental and protected lagoon facies, some ammonite assemblages from the Lower Cretaceous in the Cascais area and from the Cenomanian have been described [24,25].
Although its taxonomic attribution is beyond our reach due to preservation conditions (lack of suture lines), its strongly ribbed ornamentation and the interpreted sub-circular section of the preserved half whorls remind us of some Callovian-Oxfordian peltoceratids that are known to occur in the Lusitanian basin [26] or Cretaceous forms such as Douvilleiceratinae or the Aptian ammonite Colombiceras.

5.2. How the Ammonite Cast May Have Been Formed

Table 1 displays the p-XRF results pertaining to the ammonite composition, along with data from a representative sample of the nodules found within the same bed, and from the ferricrete from the overlying Marco Furado formation. While the elemental characterization of the ammonite points to the surprising presence of gypsum/anhydrite, the nodules are composed mainly of kaolinite (high proportions of Si and Al, with K = 0, being eliminated by hydrolysis). Kaolinite is the main clay present in the sandstones of the Santa Marta formation [11].
Kaolinite is a soft, earthy, usually white mineral produced by the chemical weathering of aluminum silicate minerals such as feldspar and muscovite. When moistened, the tiny, plate-shaped kaolinite crystals acquire a layer of water molecules that cause the crystals to adhere to each other and give the clay its cohesion. The bonds are weak enough to allow the plates to slide past each other when the clay forms a mold but strong enough to hold the plates in place and allow the molded clay to maintain its shape [27]. When the clay dries, most of the water molecules are removed and the slabs bond directly to each other, so the dried clay becomes rigid yet brittle, with the formation of the sandstone cement in the Santa Marta formation, besides kaolinite nodules (Figure 5A,B) and veinlets (Figure 5D). The precipitation of kaolinite in atmospheric conditions requires adequate proportions of its chemical constituents as well as variations in the presence of water. We can see this process taking place nowadays in the old sandpit of Quinta do Conde, through the runoff waters that transport kaolinite and lead to its reprecipitation through evaporation in the temporary ponds formed at the bottom of the sandpit (Figure 5C). The role of alternations between the presence of water and its absence in the formation of kaolinite was observed [28,29]. The existence of a braided system in the ancestral Tejo river delta means that changes in the drainage orientation were frequent, resulting in variations in the water table that allowed kaolinite precipitation. Kaolinite occurs in abundance in soils that are formed from the chemical weathering of silicate rocks, or derived sediments, in hot and humid climates [30]. These are also environments suitable for carbonate leaching.
Fossstud 03 00008 g005
Figure 5. Nodules of kaolinized sandstone found in the same layer as the ammonite and secondary kaolinite resulting from the erosion of the ammonite bed and deposition in ponds (adapted from [8]). (A) Appearance of the common subrounded nodules; (B) detail of the previous nodule, showing the predominance of sand-sized quartz grains over the kaolinite matrix, making these nodules more easily disaggregated. (C) Secondary kaolinite resulting from accumulation in ponds at the base of the sandpit. Subject to evaporation in small pools, the deposition of kaolinite forms plates with high purity and, therefore, is more resistant to disintegration than primary nodules. Note the surface’s punctuated texture resulting from molds of quartz grains, on which the kaolinite was deposited. (D) Millimetric veinlets of pure kaolinite that occur in a sub-horizontal position in the sandstone outcrops, with phreatic origin, after being collected from the outcrop; note that the contacts with the sandstone included numerous quartz grains. Scale bar is 10 mm.
Figure 5. Nodules of kaolinized sandstone found in the same layer as the ammonite and secondary kaolinite resulting from the erosion of the ammonite bed and deposition in ponds (adapted from [8]). (A) Appearance of the common subrounded nodules; (B) detail of the previous nodule, showing the predominance of sand-sized quartz grains over the kaolinite matrix, making these nodules more easily disaggregated. (C) Secondary kaolinite resulting from accumulation in ponds at the base of the sandpit. Subject to evaporation in small pools, the deposition of kaolinite forms plates with high purity and, therefore, is more resistant to disintegration than primary nodules. Note the surface’s punctuated texture resulting from molds of quartz grains, on which the kaolinite was deposited. (D) Millimetric veinlets of pure kaolinite that occur in a sub-horizontal position in the sandstone outcrops, with phreatic origin, after being collected from the outcrop; note that the contacts with the sandstone included numerous quartz grains. Scale bar is 10 mm.
Fossstud 03 00008 g005
In contrast, sedimentary gypsum forms by direct precipitation from evaporating meteoric waters with chemical compositions dependent on the bedrock types and their proportions in the drainage areas [29,30,31]. Its formation is linked to high evaporation rates of zones saturated in calcium sulfate (brines) in lacustrine (continental) or marine environments under arid or semi-arid climates [32]. The diagenetic production of gypsum occurs in sedimentary deposits that allow the migration of water and provide an adequate abundance of Ca, S, and O [33] (see Table 1). Selenite crystals were previously found in the uppermost part of the Santa Marta formation, in facies 2 of abandoned channels of Facies 2 with abundant preserved plant remains and lignite [11,18]. Gypsum may be formed as a by-product of sulfide oxidation when the sulfuric acid generated reacts with calcium carbonate [34]. The replacement of carbonate shells by gypsum can therefore happen as described in Late Pliocene coastal deposits from Lepe (SW Spain) [35,36]. According to these authors, the vertical migration of acidic sulfate-rich waters favored the dissolution of carbonate in the shells and oolites, which in turn favored the nucleation of gypsum crystals.
The acidic character of the Santa Marta formation, corresponding to sandstones made mostly of quartz and feldspars with granitic provenance, together with the subtropical to warm-temperate paleoclimate reconstitution of the area for the Late Pliocene [37], largely prevented the survival of carbonate materials in the upper part of the Santa Marta formation [11]. The paragenesis of clays, with a predominance of kaolinite, also indicates a humid subtropical climate [16].
To this extent, the model we propose for the formation of the ammonite cast (Figure 6) involves the transport across the ancestral Tejo river system of carbonate extraclasts measuring centimeters to decimeters in size (Figure 6C) from the original location in the Lusitanian Basin exposed by Alpine orogeny-related reverse faulting (Figure 6B), most likely from Late Jurassic carbonate formations uplifted by the intrusion of the Sintra Massif or from the north of Lisbon. Under acidic vadose conditions over a water table influenced by marine proximity (high δ34S‰ of +17.7, close to the average isotopic signature of marine waters of +20‰ during the Cenozoic [38,39]), these carbonate clasts would have been dissolved within the already consolidated sandstone by kaolinite (Figure 6D). Thus, a void has been left in the sandstone, preserving the ammonite morphology with its details only made possible by the grain size and the consistency of the rock. In this way, the ammonite cast could have been generated with the vertical migration of sulfate-concentrating waters resulting from the weathering of the overlying Marco Furado Fm. Ferricretes [10,40], with the precipitation of very pure gypsum in the void in the sandstone left by the dissolution of the ammonite internal mold. However, the high content of Sr in the ammonite cast also supports the marine origin of the percolating waters [41], where the high concentration of S could have also come from. The punctuated surface texture of the cast reflects the shape and distribution of the quartz grains that delimited the previously existing void. If the cast had been precipitated directly on the still-existing internal mold, its surface would be smooth.
The timings between the development of the kaolinite nodules and the production of the gypsum cast had to be different, the former being under the direct influence of a warm and humid climate and the cast probably being formed subsequently, still in the Late Pliocene or already during the Pleistocene, during a highstand that favored the progradation of marine facies over the delta plain and the consequent rise of the influence of saltwater in the water table.
The formation of casts in ammonites is not so uncommon. Sometimes, during the diagenetic process and after the lithification of the internal molds, secondary filling by calcite of the space left by the dissolution of the aragonite shell is noted, in the same way that it fills diagenetic fissures that intersect the fossils. This is a process that normally occurs in the initial stages of diagenesis and which may occur over a period of thousands of years [42]. Machalski and Olszewska-Nejbert [43] described the formation of phosphate casts from ammonites from the Upper Albian of Poland. These are unique casts developed from external molds resulting from the process of the growth of oysters on ammonites, into which the conch later came to be dissolved. According to these authors, the absence of suture lines, normally visible in primary internal molds, is indicative of the secondary origin of the casts. Once again, this original form of ammonite fossilization does not demonstrate a broad temporal discontinuity, not at least on a geological scale, between the death of ammonites and the development of the casts.
The case of the ammonite gypsum cast from the old sandpit of Quinta do Conde is different and unique. At least 90 million years or perhaps more than 150 million years have passed between the death of the ammonite and its preservation as a cast, recording the formation of the Lusitanian Basin, the inversion of the relief during the paroxysm of the Alpine Orogeny, and the erosion of the Mesozoic formations by the drainage system related to the ancestral Tejo river, with its transport as sediment load and deposition in the paleo-delta as a limestone extraclast. This is, so far, the only testimony to the enormous temporal discontinuity that occurred during the taphonomic history of an ammonite, with the final preservation in the form of a cast resulting from the in situ dissolution of the limestone cobble with the internal mold of the ammonite by acidic percolating waters and reprecipitation in Ca-rich environment from evaporating waters rich in S, filling the void within the quartz-rich sandstone with massive gypsum, and perhaps the most didactic example of this process of fossilization ever described, in Portugal.

6. Conclusions

The fortuitous finding of a single ammonite in the delta sandstones of the Upper Pliocene Santa Marta formation enabled the development of a taphonomic study to understand the diagenetic history of the fossil cast. For that purpose, p-XRF was applied to obtain the elemental characterization of the cast and compare it with those of nodules found in the same unit. The nodules are made of kaolinite resulting from the weathering of the feldspars constituting the sandstone, whereas the ammonite cast is made of massive gypsum. The cast with gypsum may have occurred after the limestone clast with part of an internal mold of an ammonite dissolved by acidic percolating waters rich in S, more likely resulting from a marine water-influenced water table (δ34S‰ = +17.7 and high Sr value) or from the weathering concentration of S from the Marco Furado ferricretes overlying Santa Marta sandstones. The Ca-rich microenvironment may have triggered the precipitation of gypsum within the sandstones already cemented by kaolinite in the latest Pliocene or during the Early Pleistocene. The time frame between the beginning of the fossil’s diagenesis and the casting of the ammonite may have spanned over 90 to 150 million years if it is coming from the erosion of Jurassic rocks during the Late Pliocene.

Author Contributions

Conceptualization: C.N.d.C.; formal analysis: C.N.d.C., M.B. and S.S.; investigation: C.N.d.C., M.B. and S.S.; methodology: C.N.d.C.; project administration: C.N.d.C.; resources: M.B.; supervision: C.N.d.C.; validation: S.S.; visualization: C.N.d.C. and S.S.; writing—original draft: C.N.d.C.; writing—review and editing; C.N.d.C., M.B. and S.S. All authors have read and agreed to the published version of the manuscript.

Funding

No funding is applied to this work.

Data Availability Statement

All the data was provided in the text.

Acknowledgments

We thank José António Anacleto (Geological Museum LNEG) for research operational support and Pedro Marrecas (Sociedade Portuguesa de Paleontologia) for the photo in Figure 1A. Xiaoyu Zhang from the University of Arizona provided the δ34S result for the ammonite. Three anonymous reviewers helped to improve the manuscript and provided possible taxonomic comparisons for the ammonite under study.

Conflicts of Interest

Author Miguel Barroso owns the company Meteoric Cipher, Lda. All authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. Google Earth@ images locating the old sandpit in the eastern outskirts of the town of Quinta do Conde (close to the city of Setúbal, SW Portugal), where the ammonite was found (the orange symbol marks the precise location): (A) localities indicated in the text (scale bar in the general map of the Iberian Peninsula is 300 km; scale bar in the satellite image is 10 km; (B) location of the sandpit (scale bar is 100 m).
Figure 1. Google Earth@ images locating the old sandpit in the eastern outskirts of the town of Quinta do Conde (close to the city of Setúbal, SW Portugal), where the ammonite was found (the orange symbol marks the precise location): (A) localities indicated in the text (scale bar in the general map of the Iberian Peninsula is 300 km; scale bar in the satellite image is 10 km; (B) location of the sandpit (scale bar is 100 m).
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Figure 2. Geological setting of the studied section. (A) Simplified geological map of the Lower Tejo river region, showing the distribution of the Santa Marta formation in the Albufeira Syncline at the Setubal Peninsula and the location of the main Mesozoic deposits (extract adapted from the geological map of Portugal at a 1:1,000,000 scale edited in 2010 by the Unidade de Geologia, Hidrogeologia e Geologia Costeira, Laboratório Nacional de Energia e Geologia, I.P); the symbol indicates the approximate location of the ammonite outcrop. (B) Synthetic log of the Quinta do Conde sandpit, with the location of the ammonite. Ferricretes of the Marco Furado formation are marked by red arrows.
Figure 2. Geological setting of the studied section. (A) Simplified geological map of the Lower Tejo river region, showing the distribution of the Santa Marta formation in the Albufeira Syncline at the Setubal Peninsula and the location of the main Mesozoic deposits (extract adapted from the geological map of Portugal at a 1:1,000,000 scale edited in 2010 by the Unidade de Geologia, Hidrogeologia e Geologia Costeira, Laboratório Nacional de Energia e Geologia, I.P); the symbol indicates the approximate location of the ammonite outcrop. (B) Synthetic log of the Quinta do Conde sandpit, with the location of the ammonite. Ferricretes of the Marco Furado formation are marked by red arrows.
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Figure 4. Natural gypsum cast of the ammonite from Quinta do Conde sandpit, Portugal. (A) Face that preserved the morphology of the ammonite; note the surface of the fossil with small punctuations corresponding to the casts of rounded quartz grains that compose the surrounding sedimentary matrix; (B) reverse surface that clearly shows the production of the cast in semi-relief, and again the punctuated surface. Scale bar is 10 mm.
Figure 4. Natural gypsum cast of the ammonite from Quinta do Conde sandpit, Portugal. (A) Face that preserved the morphology of the ammonite; note the surface of the fossil with small punctuations corresponding to the casts of rounded quartz grains that compose the surrounding sedimentary matrix; (B) reverse surface that clearly shows the production of the cast in semi-relief, and again the punctuated surface. Scale bar is 10 mm.
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Figure 6. Model for the development of the ammonite cast. (A) Jurassic or Cretaceous deposition of the conch on the Lusitanian basin sea bed and semi-filling by carbonate sediments; (B) alpine orogeny uplifted the limestone beds containing the mold of the ammonite, after Late Miocene; (C) the transport and deposition of the ammonite as a limestone clast in the ancestral Tejo river delta, together with feldspar sands and clasts with other compositions reflecting distinct provenance distances and geological settings; feldspar-rich sands and clasts are replaced by kaolinite, still during the humid climate of the Late Pliocene; (D) the dissolution of the ammonite clast in the vadose zone by percolating, sulfur-rich waters sourcing more likely from marine water-influenced water table (fat arrows) or from the ferricrete paleosols of the Marco Furado fm. (slim arrows), with the precipitation of gypsum by reaction with Ca2+; the cast was produced in a Mediterranean-type climate in the latest Pliocene or already during the Pleistocene (image credits to N. Tamura, Alden Williams, and Kevin Kent, from (A) to (D) respectively).
Figure 6. Model for the development of the ammonite cast. (A) Jurassic or Cretaceous deposition of the conch on the Lusitanian basin sea bed and semi-filling by carbonate sediments; (B) alpine orogeny uplifted the limestone beds containing the mold of the ammonite, after Late Miocene; (C) the transport and deposition of the ammonite as a limestone clast in the ancestral Tejo river delta, together with feldspar sands and clasts with other compositions reflecting distinct provenance distances and geological settings; feldspar-rich sands and clasts are replaced by kaolinite, still during the humid climate of the Late Pliocene; (D) the dissolution of the ammonite clast in the vadose zone by percolating, sulfur-rich waters sourcing more likely from marine water-influenced water table (fat arrows) or from the ferricrete paleosols of the Marco Furado fm. (slim arrows), with the precipitation of gypsum by reaction with Ca2+; the cast was produced in a Mediterranean-type climate in the latest Pliocene or already during the Pleistocene (image credits to N. Tamura, Alden Williams, and Kevin Kent, from (A) to (D) respectively).
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Table 1. p-XRF results of the ammonite composition of selected major and trace elements, along with data from a representative sample of the kaolinite nodules found within the same bed and the ferricrete from the overlying Marco Furado fm. (MF) for comparison.
Table 1. p-XRF results of the ammonite composition of selected major and trace elements, along with data from a representative sample of the kaolinite nodules found within the same bed and the ferricrete from the overlying Marco Furado fm. (MF) for comparison.
NameS+/-Al+/-Si+/-Fe+/-K+/-Ca+/-Sr+/-Fe2O3SiO2P2O5Al2O3CaO
Nodule_10017.83460.060826.14860.00450.7430.007000.11860.0013001.062655.93190.159233.68950.166
Nodule_20017.2660.063626.66770.00480.89390.0081000.12280.0014001.278357.04230.144732.61560.1718
Ammonite_136.72520.02440.16560.02740.779200.18660.00580036.98370.02320.10830.00080.26681.666800.312851.7402
Ammonite_237.08990.02450.11710.02810.612600.20590.00620037.46410.02350.14260.00090.29441.310400.221252.4123
MF_12.56330.00743.68510.031229.86550.035910.81910.0251.58710.00421.02640.00290.00370.000215.471463.88220.03316.96111.4359
MF_23.3940.00741.67830.022924.83460.0316.15690.02850.59190.00271.51420.00320.00390.000223.104353.12120.11613.17042.1183
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Neto de Carvalho, C.; Barroso, M.; Soares, S. An Ammonite Preserved in the Upper Pliocene Lower Tejo River Deposits of Quinta Do Conde (Southwest Portugal). Foss. Stud. 2025, 3, 8. https://doi.org/10.3390/fossils3020008

AMA Style

Neto de Carvalho C, Barroso M, Soares S. An Ammonite Preserved in the Upper Pliocene Lower Tejo River Deposits of Quinta Do Conde (Southwest Portugal). Fossil Studies. 2025; 3(2):8. https://doi.org/10.3390/fossils3020008

Chicago/Turabian Style

Neto de Carvalho, Carlos, Miguel Barroso, and Sofia Soares. 2025. "An Ammonite Preserved in the Upper Pliocene Lower Tejo River Deposits of Quinta Do Conde (Southwest Portugal)" Fossil Studies 3, no. 2: 8. https://doi.org/10.3390/fossils3020008

APA Style

Neto de Carvalho, C., Barroso, M., & Soares, S. (2025). An Ammonite Preserved in the Upper Pliocene Lower Tejo River Deposits of Quinta Do Conde (Southwest Portugal). Fossil Studies, 3(2), 8. https://doi.org/10.3390/fossils3020008

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