The occurrence of early Pleistocene marine fish remains from the Gulf Coast of Mobile County, Alabama, USA (original) (raw)

1. Introduction

The marine record of fossil fishes in Alabama is among the most diverse and stratigraphically extensive in the United States. Fossil sharks and other marine fishes have been described in Alabama from strata ranging from the Paleozoic to the Cenozoic, with published reports of taxa from the Carboniferous (Ciampagalio et al. 2011), Late Cretaceous (Ikejiri et al. 2013; Ciampagalio et al. 2013; Cicimurri & Ebersole 2014), Paleocene (Ehret & Ebersole 2014; Cicimurri & Ebersole 2015a), Eocene (Clayton et al. 2013; Cicimurri & Ebersole 2015b; Maisch et al. 2014, 2016; Cappetta & Case 2016), and Oligocene (Whetstone & Martin 1978). Absent from this fossil record, however, are any reports of marine vertebrates from Pliocene or Pleistocene strata in Alabama. Although the Pleistocene vertebrate record has been thoroughly documented in the state, these reports have been limited to terrestrial and fluvial species (see Ebersole & Ebersole 2011; Jacquemin et al. 2016).

The purpose of this study is to report and describe the first early Pleistocene marine fish remains from Alabama. Reported herein are 91 specimens collected as beach wash on the Gulf of Mexico barrier islands. Dauphin Island and nearby Sand Island and Pelican Island (now referred to as the Sand/Pelican Island Complex), located in southern Mobile County, Alabama, USA. Although verbal reports have surfaced regarding the occurrence of fossil shark teeth on Dauphin Island beaches (D. Ehret, pers. comm. 2015), these reports have heretofore gone unverified. Herein, we also comment on the age of these fossils, provide discussions on their potential lithostratigraphic units of origin, and discuss the possible geological processes that led to them being deposited on the beaches.

2. Material and methods

The 91 specimens examined as part of this study were collected by Robert Dixon of the DISL, predominately on the western beaches of Dauphin Island, Alabama, USA. A few additional specimens were collected by R. Dixon on the nearby beaches of Sand/Pelican Island Complex, located just southeast of the eastern end of Dauphin Island. Species identifications and tooth positions within the mouth were determined primarily by direct comparison to isolated teeth and articulated jaws of Recent species housed at McWane Science Center, Birmingham, Alabama, USA and the South Carolina State Museum, Columbia, USA. The higher taxonomic rankings presented herein follow that of Nelson (2016), and any deviations from this source are discussed within the text. Within the Systematic Paleontology section, extinct genera and species are highlighted by the symbol “†”. The geographic and stratigraphic ranges are not all inclusive and are instead intended to provide a broad scope of the range of each taxon. Although cited ranges were derived from peer reviewed studies, identifications of the taxa reported herein were not personally confirmed as part of the current study.

Fig. 1.

Study area for this paper. (A) The southernmost part of the mainland of coastal Alabama. (B) A detailed view of Dauphin Island, including areas of fossil collection.

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All examined specimens were photographed with a Nikon D80 camera with Tamron macro lens and all photographs were rendered in Adobe Photoshop CC 2015.5 software as part of the production of the presented figures. All specimens are accessioned in the collections at the DISL located on Dauphin Island, Alabama, USA.

The specimens examined in this study were predominately collected on the western beaches on Dauphin Island, Alabama, USA, but a few were collected from the beaches of the adjacent Sand/Pelican Island Complex (Fig. 1). These specimens were collected over the course of a 25-year period and were generally recovered after storm events (R. Dixon, pers. comm. 2015). Dauphin Island is a 23-km-long microtidal barrier island located approximately 8.0 km offshore from the mainland edge of southwestern Alabama in Mobile County. Dauphin Island is situated between Mississippi Sound to the north and the Gulf of Mexico to the south. Immediately northeast of the island is Mobile Bay, the ebb-tidal delta of which separates Dauphin Island and Fort Morgan Peninsula, located 4.3 km (3.0 mi) to the east. Just to the southeast of Dauphin Island, siliciclastic sediments have formed two small islands, Pelican Island and Sand Island, which have recently joined to form the Sand/Pelican Island Complex (Froede 2008). The Sand/Pelican Island Complex has slowly moved in a northwesterly direction and is now merged with the southeastern shore of Dauphin Island.

The surface geology and sediment of Dauphin Island is chiefly Holocene beach sand and sand dunes, and the eastern end of the island has a Pleistocene core of Citronelle Formation (Hummell & Parker 1995; Hummell 1996; Hummell & Smith 1996; Morton 2007), an iron-stained silty-clay paleosol (Fig. 2). This Citronelle core, however, is not present on the western side of the island. The source of the Holocene barrier island sand is mainly from westward long-shore currents traveling from the Florida panhandle, across Fort Morgan Peninsula (Douglass & Haubner 1992). Sand and silt from Mobile Bay also contribute to Dauphin Island, the Sand/Pelican Island Complex, and the associated shoals (Douglass & Haubner 1992; Morton 2007). Some of the beaches on Dauphin Island also contain dredged material from underwater berms. Dredged sand from within the Mobile Ship Channel was used in 1987 to create an underwater berm on the ebbtidal delta shield (Douglass & Haubner 1992); this sand later migrated to Sand Island shoal complex (Douglass & Haubner 1992; Hands 1991). Sand dredged in 1980 from Fort Gaines Channel was deposited along the shoreline in front of Fort Gaines (Douglass & Haubner 1992) and was also eroded by the same westerly currents that have been moving sediments to the Sand/Pelican Island Complex. In 1991, as part of beach restoration efforts at the request of the Dauphin Island Park and Beach Board, Mobil Exploration and Producing U.S., Inc. dredged a section of Aloe Bay and transported an estimated 14,000 cubic meters of the material to the public beach area of the fishing pier on the eastern end of the island (Douglass & Haubner 1992). Over time, this material has been carried westerly along the shore with the normal currents (Douglass & Haubner 1992), and storm wave action (Morton 2007) may have also contributed to erosion and deposition of this material. As reported by Mobil, the average shell content of the dredged material was 5% (Douglass & Haubner 1992). The first engineered beach nourishment project in this area took place in Spring of 2016, utilizing material that was mined 7.2 km (4.5 mi) south of the eastern end of the island (S. Douglass, pers. comm. 2016).

The specimens examined in this study were collected from two areas, the Gulf side of western Dauphin Island along a hurricane break (Fig. 1B), and on the beaches of the Sand/Pelican Island Complex (Fig. 1B). The specimens we examined were found as beach wash and not collected in situ, and their precise stratigraphic origin is therefore not known. However, all specimens, except for one, are fossilized, indicating they were derived from preHolocene strata. The fossilized specimens are all blackish-blue and brown in color, suggesting a similar method of preservation and place of origin. The lone exception, DISL 2015.1.15 (Fig. 3), is whitish in color and not as dense as the other specimens, suggesting it may be Recent in origin. Nevertheless, the discovery of fossilized marine vertebrate remains on the beaches of Dauphin Island and adjacent Gulf Shores and Orange Beach, AL is extremely uncommon (JAE and SME, personal observation), and no occurrences of such remains have been previously documented in the literature.

Although they were not collected in situ, these fossils were likely derived from the shallow subsurface pre-Holocene units of Dauphin Island and/or the pre-Holocene sediment dredged from nearby areas (Fig 2). Cores taken from Mississippi Sound and Dauphin Island show Pleistocene (Cronin 2001; Hummel 1996; McBride et al. 1991) facies below the present Holocene material. The pre-Holocene units include shoal to eolian sands of the Gulfport Formation (Otvos 2001), the alluvial Prairie Formation, and the underlying Biloxi Formation (Hummel 1996; Otvos 1972, 1991, 1997). The sand of the Gulfport Formation sometimes overlies the Prairie Formation, and sometimes the Biloxi, but has not been well identified (or understood) in the immediate Dauphin Island area. The Biloxi Formation is a compositionally fossiliferous, gray, or greenish-gray, to brown sand and clayey sand that was interpreted by Otvos (2001) as representing estuarine to open-shelf environments with transgressive and regressive cycles. Samples from USGS-Belle Fontaine No. 1 Core indicate an age as old as late Pleistocene based on the ostracode assemblage (Cronin 2001), and calcareous nannofossils suggest an age as old as the Calabrian (Gohn et al. 2001). Similarly, in a Mississippi Sound core (core S-2, drillhole #25; Otvos 1981), large forms of the calcareous nannofossils Gephyrocapsa oceanica and Gephyrocapsa caribbeanica were identified in the Biloxi Formation and interpreted as being as old as early Calabrian by Cita et al. (2012). Interpretation of depth to Biloxi and other pre-Holocene units on and around Dauphin Island varies with the subsurface lithology samples (Hummell 1996), and ranges from 70 feet deep to at or near the surface (e.g., Otvos DI-2, Gsa 1007, Otvos DI-9 of P'-P” in Hummell 1996).

It is plausible that some of the fossils collected on western Dauphin Island may have originated from material reworked by heavy storm and wave action. Examples of the intensity of reworking include island breaks (Fig. 1B) as wide as 2.4 km and even larger areas undergoing washovers and deep scouring from high velocity flow during Hurricanes Frederick (1979), Georges (1998), Ivan (2004), and Katrina (2005) (Morton 2007). These events of severe erosion and overwash deposits may have allowed erosion and deposition of material from older underlying units onto the current beach. If the fossil specimens indeed originated from lower Dauphin Island strata, storm reworking may be one possible mechanism of displacement. An example of a recent break on Dauphin Island was formed by Hurricane Katrina in 2005. Known locally as the “Katrina Cut”, the hurricane incised a shallow, several meter-wide break through the western end of Dauphin Island. Over the succeeding five years, continued ebb and flow erosion had widened this gap to nearly 2.4 km, making the far western side of the island inaccessible to beachgoers. Most of the specimens examined in this study were collected along the edges of the “Katrina Cut” (Fig. 1B), suggesting their origin might be the result of the aforementioned mechanism.

It is also possible that some of the specimens, especially those collected on the Sand/Pelican Island Complex, were derived from dredged sediment from either the Mobile Bay Shipping Channel or Fort Gaines Channel, which was carried by currents to the Sand/Pelican Island Complex. Dredged material from Aloe Bay (Douglass & Hauber 1992), may also account for fossils found west of the pier. Strong tropical storms passing over or through the Dauphin Island area, depending on surge and wind intensity, can naturally dredge large quantities of sand and other material and transport it to varying depths and distances, thus providing another possible mechanism of transport of the fossils.

3. Systematic paleontology

Chondrichthyes Huxley, 1880

(Cartilaginous fishes, rays, and sharks)
Not figured

Euselachii Hay, 1902

Elasmobranchii Bonaparte, 1838b
Selachii Cope, 1871
Galeomorphii Compagno, 1973
Odontaspididae Müller & Henle, 1839
Carcharias Rafinesque, 1810
Carcharias taurus Rafinesque, 1810
(Sand Tiger Shark)
Pl. 1, Figs. 3–5

Lamnidae Müller & Henle, 1838

Carcharodon Smith in Müller & Henle, 1838
Carcharodon carcharias (Linnaeus, 1758)
(Great White Shark)
Pl. 1, Fig. 7

†Cosmopolitodus Glikman, 1964

†Cosmopolitodus hastalis (Agassiz, 1843)
(Extinct mako shark)
Pl. 1, Fig. 8

Carcharhiniformes Compagno, 1984

Carcharhinidae Jordan & Evermann, 1896
Carcharhinus De Blainville, 1816
Carcharhinus leucas (Valenciennes in
Müller & Henee, 1839) (Bull Shark)
Pl. 1, Fig. 6

Carcharhinus longimanus (Poey, 1861)

(Oceanic Whitetip Shark)
Pl. 1, Fig. 9

Carcharhinus sp. cf. C. obscurus (LeSueur, 1818)

(Dusky Shark)
Pl. 1 Fig. 10

Carcharhinus sp. cf. C. plumbeus (Nardo, 1827)

(Sandbar Shark)
Pl. 1, Fig. 11

Carcharhinus sp. (Requiem sharks)

Pl. 1, Figs. 12–13

Negaprion Whitley, 1940

Negaprion brevirostris (Poey, 1868) (Lemon Shark)
Pl. 1, Fig. 14

Galeocerdidae Herman et al., 2010

Galeocerdo Müller & Henle, 1837
Galeocerdo cuvier (Péron & Lesueur in Lesueur, 1822)
(Tiger Shark)
Pl. 1, Figs. 15–16

Hemigaleidae Hasse, 1879

Hemipristis Agassiz, 1843
†Hemipristis serra Agassiz, 1843 (Snaggletooth Shark)
Pl. 1, Fig. 17

Fig. 2.

Stratigraphy of the Dauphin Island and Mississippi Sound areas. GSSP, Global Boundary Stratotype Section and Point. Ma, millions of years ago. N, foraminiferan zone of Blow (1969). NN, Neogene calcareous nannofossil zonations of Martini (1971). CN, calcareous nannofossil zonations of Okada & Bukry (1980). Coccoliths Neogene Lithostratigraphic ages based in part on nannoplankton from Belle Fontaine core (Gohn et al. 2001) and Mississippi Sound core S-2, drillhole #25 (Otvos 1981, 2001) and estimated stratigraphic position.

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Batomorphi Cappetta, 1980

Myliobatiformes Compagno, 1973
Suborder Myliobatoidei Compagno, 1973
Myliobatidae Bonaparte, 1838a
Aetobatus De Blainville, 1816
Aetobatus sp. (Eagle ray)
Pl. 1, Fig. 18

Osteichthyes Huxley, 1880

Actinopterygii Klein, 1885
Holostei Müller, 1846
Teleosteomorpha Arratia et al., 2004
Teleostei Müller, 1846
Teleocephala de Pinna, 1996
Clupeocephala Patterson & Rosen, 1977
Otocephala Johnson & Patterson, 1996
Acanthopterygii Greenwood et al., 1966
Percomorpha Rosen, 1973
Ovalentaria Smith and Near in Wainwright et al., 2012
Tetradontiformes Berg, 1940
Tetradontoidei Nelson et al., 2016
Diodontidae Bonaparte, 1835
(Porcupinefishes and burrfishes)
Pl. 1, Fig. 19

4. Discussion

Most of the 91 specimens described in this study were found washed up on the beach on the western end of Dauphin Island in south Alabama (with a few others collected from the adjacent Sand/Pelican Island Complex). Among these specimens, 12 unequivocal taxa representing six families, eight genera, and nine species were identified. Of these 12 unequivocal taxa, two are extinct (Hemipristis serra and Cosmopolitodus hastalis), eight are extant and still have ranges in the Gulf of Mexico today (Carcharias taurus, Carcharodon carcharias, Carcharhinus leucas, Carcharhinus sp. cf. C. longimanus, Carcharhinus obscurus, Carcharhinus sp. cf. C. plumbeus, Negaprion brevirostris, and Galeocerdo cuvier) and two (Aetobatus sp. and Diodontidae) could not be speciated, but each have representatives that currently reside in the Gulf of Mexico. With the exception of the described Aetobatus sp. and Diodontidae specimens, the remaining unequivocal taxa each represent first fossil records for Alabama.

As noted previously, the specimens examined in this study were not recovered in situ but likely eroded from subsurface pre-Holocene units present at or around Dauphin Island and Sand/Pelican Island Complex and subsequently re-deposited on the surface. Three pre-Holocene lithologic units have been identified within the vicinity of these islands, including the upper Pliocene to lower Pleistocene (upper Piacenzian to Gelasian) Citronelle Formation, the lower Pleistocene (Gelasian to Calabrian) Biloxi Formation, and the middle Pleistocene (Ionian) Prairie Formation (Fig. 2). Of these three formations, circumstantial evidence suggests the fish remains in our sample were likely derived from the Biloxi Formation.

Previous analyses of microfossils from Biloxi Formation cores suggest a temporal age for this unit that ranges from the Gelasian to Calabrian of the early Pleistocene (Cita et al. 2012; Otvos 1981; Fig. 2). The possibility that the teeth in our sample could be as young as the early Pleistocene is supported by an analysis of the published stratigraphic distributions for the taxa identified (Table 1). The temporal ranges presented were compiled through a search of the peer-reviewed literature for localities that reported the occurrence of any of the unequivocal taxa identified within our sample. Recent literature was examined on a global scale (see references cited herein), and with the recent 2008 ratification of the Pliocene/Pleistocene boundary from 1.8 to 2.6 Ma (Gibbard & Head 2010; see Fig. 2), the geology of any locality reporting a late Pliocene age was reexamined to determine the amended age for the reported assemblage.

Table 1.

Published stratigraphic ranges for fossil taxa represented in the Dauphin Island assemblage. Shaded area represents the range of stratigraphic overlap of all recovered taxa.

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The results of this analysis (see Table 1) suggest that nearly all the taxa we identified were present, or even originated, in the Miocene. The lone exception in our sample is the occurrence of Carcharodon carcharias. Although occurrences of Carcharodon have been reported from the Miocene (i.e., Visaggi & Godfrey 2010; Cione et al. 2011; Sharma & Patnaik 2014), Cappetta (2012) suggests that any reports of pre-Pliocene C. carcharias should be treated with skepticism because they likely represent an ancestral form of the modern taxon. This was supported by Ehret et al. (2012), who proposed the origin of C. carcharias to have occurred within the middle Pliocene, possibly around 4.0 Ma (D. J. Ehret, pers. comm. 2016). This in turn suggests the Dauphin Island assemblage can be no older than middle Pliocene in age.

Our sample also includes two extinct taxa, Cosmopolitodus hastalis and Hemipristis serra. Although recent studies suggest these two species became extinct at the end of the Pliocene, Scudder et al. (1995) documented the recovery of C. hastalis and H. serra teeth from the lower Pleistocene component of the Belmont Formation at the Leisley Shell Pits in Hillsboro County, Florida. Utilizing several lines of evidence, Morgan & Hulbert (1995) elucidated an estimated age range of 1.07 to 1.55 ma (Calabrian) for the Pleistocene exposures at the site. Although an unconformity exists at the locality where the Pleistocene strata directly overlie those of Miocene age, Scudder et al. (1995) proposed that, due to their pristine condition, the teeth had not been reworked and were indeed early Pleistocene in age. These teeth represent the youngest occurrences of both C. hastalis and H. serra in the fossil record, indicating that populations of these taxa survived in the Gulf of Mexico at least into the Calabrian. Furthermore, when combined with the occurrence of Carcharodon carcharias in our sample, the presence of C. hastalis and H. serra suggest a bracketed age for our assemblage that ranges from the middle Pliocene to early Pleistocene (see Table 1). Because C. hastalis and H. serra are two taxa that are not known to persist into the middle Pleistocene, this makes it unlikely that the teeth in our sample were derived from the Ionian Prairie Formation.

Although the eastern end of Dauphin Island is known to have a Citronelle Formation core, this formation is absent within the western part island (Hummell & Parker 1995; Hummell 1996; Hummell & Smith 1996; Morton 2007), the location where most of the teeth in our sample were collected. The absence of the Citronelle Formation at the main area of collection leads us to believe the specimens were derived from shallow underlying deposits of the Biloxi Formation. The Biloxi Formation depositional setting was interpreted by Otvos (2001) to be an estuarine to open-shelf environment (Otvos 2001). Similarly, the recovered fossil assemblage consists of species that today prefer warm, shallow, near-shore habitats with a water depth of 100 m or less (see Table 2 and references cited herein). Although we cannot definitively rule out that these specimens could have come from the Citronelle Formation, the aforementioned lines of circumstantial evidence lead us to believe they were more than likely derived from the lower Pleistocene Biloxi Formation and may be as young as Calabrian in age.

Table 2.

Water depth preferences for extant taxa represented in the Dauphin Island fossil assemblage. *Hemipristis elongata is used as a modern analogue for Hemipristis serra. **Aetobatus narinari, an extant member of the genus known to reside in the Gulf of Mexico today, is used in place of the unspeciated Aetobatus sp. Shaded area represents the overlap of depth preferences for the assemblage.

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5. Conclusions

A total of 12 unequivocal fish taxa were identified within our sample of Dauphin Island fossils: Carcharias taurus, Carcharodon carcharias, Cosmopolitodus hastalis., Carcharhinus leucas, Carcharhinus sp. cf. C. longimanus, Carcharhinus obscurus, Carcharhinus sp. cf. C. plumbeus, Negaprion brevirostris, Galeocerdo cuvier, Hemipristis serra, Aetobatus sp., and Diodontidae. Although these specimens were not collected in situ, circumstantial evidence suggests these specimens originated from the lower Pleistocene (Gelasian to Calabrian) Biloxi Formation. If this stratigraphic context is correct, the 91 specimens described herein represent the first Quaternary marine vertebrates of any kind reported from Alabama, and aside from the remains of Aetobatus sp., and Diodontidae, the first fossil occurrences in the state for each of the remaining unequivocal taxa. Two extinct taxa, Cosmopolitodus hastalis and Hemipristis serra, were identified within our sample. In conjunction with the specimens reported by Scudder et al. (1995) from the Leisley Shell Pits in Hillsboro County, Florida, the Dauphin Island C. hastalis and H. serra specimens are likely among the youngest stratigraphic occurrences of each of these species. When compared to modern representatives, the represented fossil taxa likely preferred a warm, shallow, nearshore habitat with a water depth of 100 m or less.

Although these specimens were all found as beach wash, it is plausible that they were derived from the shallow subsurface pre-Holocene units present at, or around. Dauphin Island and subsequently re-deposited on shore through beach restoration projects and/or storm wash. This latter hypothesis is supported by the fact that a large majority of the teeth were collected adjacent to the “Katrina Cut”, a hurricane-induced break that once divided the western side of Dauphin Island into two parts. This break undoubtedly incised into the shallow subsurface pre-Holocene deposits that make up the island (likely the Biloxi Formation), with ebb and flow tides more than likely uncovering and redepositing the specimens onshore.

Plate 1

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