The genus Platypterygius was the first (and until recently, sole) ichthyosaur known from the Cretaceous, and is a geographically widespread and geologically long-lived genus. Its status as a wastebasket taxon has been noted by many authors [11]. Previously, it was thought that all members of this group may eventually be synonymized into fewer species [14], or that they may belong to a single supraspecific taxon [5]—this is in contrast to the more recent theory that the genus may eventually be split into additional genera, supported by the morphological disparity noted by Fischer [15]. Indeed, the genus as it currently stands may be polyphyletic [9,10,11]. Pending further revision, eight species of Platypterygius are discussed here: five previously recognized by McGowan and Motani [2], Platypterygius hauthali, Platypterygius platydactylus, Platypterygius australis, Platypterygius americanus, and Platypterygius campylodon; Platypterygius hercynicus, synonymized with P. campylodon by McGowan and Motani [2], but considered a separate valid species by Kolb and Sander [16]; an additional species from South America not considered in McGowan and Motani’s [2] extensive review, Platypterygius sachicarum; and Platypterygius ochevi, a species erected subsequent to McGowan and Motani’s [2] review by Arkhangelsky et al. [17].

Platypterygius hauthali—Originally described as Myobradypterygius hauthali, von Huene [18] erected this South American ichthyosaur on forelimb material from the Barremian [19] of Argentina (Figure 1)—McGowan [13] referred the specimen to the genus Platypterygius. McGowan and Motani [2] retained the species, but thought it might prove synonymous with the contemporaneous Platypterygius platydactylus. However, Fernández and Aguirre-Urreta [19] undertook a revision of the holotype material, and considered it a valid species based on its carpal pattern. Hauterivian-Barremian occurrences of this species have been reported in southern-most Chile [20,21], but descriptions are currently limited to a single forefin considered Platypterygius sp [22]. Description of this relatively complete material may aid in distinguishing the two contemporaneous South American species, the holotypes of which do not share comparable elements.

Platypterygius platydactylus—Platypterygius platydactylus was first described by Broili [7] from the Aptian of Germany (Figure 1), and is the type species for the genus Platypterygius. Unfortunately, the holotype was destroyed during World War II [2], and additional material for this species has not yet been reported.

Platypterygius australis—Australia’s species of Platypterygius, Platypterygius australis, include the best preserved, most abundant, and most complete remains of this genus [23]. Originally described in the nineteenth century from the Albian of Queensland [24,25], the erection of this species on non-diagnostic material has led to confusion surrounding the taxonomic identity of Australian specimens (as summarized—and resolved—by Zammit [26])—Platypterygius marathonensis [27] and Platypterygius longmani [28] have been synonymized with this taxon [2,13,29]. Remains attributable to this species have also been reported from the Aptian–Albian of South Australia and the Northern Territory [29] (Figure 1).

Platypterygius campylodon—Originally referred to Ichthyosaurus, Platypterygius campylodon was first described in 1846 from Cambridge, UK [31]. The brief type description was followed by a slightly longer report later in the same year [32]. This species is now known from the Albian-Cenomanian of England [33], France [6], and Russia [2,34,35], and the Cenomanian of Germany [36] (Figure 1). However, the taxonomy of several Russian species is uncertain—Storrs et al. [35] provisionally retained Platypterygius bedengensis, Platypterygius birjukovi, Platypterygius bannovkensis, and Platypterygius kiprijanoffi, while McGowan and Motani [2] considered all four taxon nomen dubium and synonymized the latter two taxa with P. campylodon. The revision of McGowan and Motani [2] is followed here as (a) Storrs et al. [35] were uncertain regarding the validity of P. bannovkensis, P. bedengensis, and P. birjukovi holotypes for taxonomic diagnosis, and (b) McGowan and Motani [2] demonstrated that the characteristic features of P. kiprijanoffi are not diagnostic.

Platypterygius americanus—Platypterygius americanus was first described from the upper Albian [23] of Wyoming [37]—originally described as a new genus, Myopterygius, it was later referred to Platypterygius [13]. It also occurs in the Lower Cenomanian of Wyoming [23,38] (Figure 1).

Platypterygius hercynicus—Though Platypterygius hercynicus was originally described in 1946 [39], it was synonymized with P. platydactylus [2] before being resurrected by Kolb and Sander [16]. Resurrection of the species was further supported by Fischer [15], based on the cranial characters of a referred specimen. The species occurs in the Aptian and upper Albian sediments of Lower Saxony, Germany, and north-western France, respectively [15] (Figure 1).

Platypterygius sachicarum—Platypterygius sachicarum is the second species of Platypterygius known from South America. The holotype, consisting of a nearly complete skull, was identified as being derived from the Barremian-Aptian Paja Formation of Colombo [40] (Figure 1). Cranial features suggested a closer affinity with the North American species than the Australian or Eurasian forms [40], but this is yet to be tested in a phylogenetic analysis. Currently, the holotypes of the two South American Platypterygius do not share comparable elements, and thus, may eventually be synonymized—however, Fernández and Aguirre-Urreta [19] point out that there is a conspicuous difference in size.

Platypterygius ochevi—Described by Arkhangelsky et al. [17], Platypterygius ochevi was erected on features of the forelimb, specifically the large size of the humeral facet for the extra zeugopodial element anterior to the radius—however, the taxonomic utility of this character requires assessment (an opinion shared by Fischer et al. [11]). It is provisionally retained here pending re-examination of the material. The remains of this species of Platypterygius occur in the Albian-Cenomanian strata of the Voronezh Region, Russia [17] (Figure 1).

Additional Platypterygius remains—Several other occurrences attributed to Platypterygius also need to be considered. This includes Lydekker’s [41] Ichthyosaurus indicus, which was erected on fifteen associated vertebrae from the Cretaceous of India. This material was considered indeterminate by McGowan and Motani [2]—despite this, Underwood et al. [42] tentatively referred teeth to Lydekker’s [41] species, placing the material in Platypterygius indicus. The teeth were placed in this genus based on similarity to Platypterygius, and the proximity to Lydekker’s [41] locality. However, the tooth form in other Cretaceous ichthyosaurs, such as Brachypterygius, is similar to Platypterygius [8]—thus, a more detailed comparison is required to conclusively show the teeth can be referred to Platypterygius.

Specimens from Italy [43], New Zealand [44], and Bavaria [45] have also been referred to Platypterygius. Bardet et al. [45] considered the robustness of the Bavarian remains (and the inferred large size) and tooth morphology consistent with Platypterygius sp. However, the fragmentary specimens from Italy and New Zealand were assigned to Platypterygius in part because no other Cretaceous genus was known at the time of discovery. Further examination may provide additional support for referral to this genus, particularly for the largest, well-preserved rostral fragment figured by Sirotti and Papazzoni [43].

Each species of Platypterygius appears to be endemic to a given region (Figure 1), though this may in part be due to geographic criteria being used in taxonomic assignment, e.g., [44]. However, Platypterygius remains have been described from both North America and Australia that appear inconsistent with the endemic species. The specimens described by Maxwell and Caldwell [46] and Adams and Fiorillo [47] appear to show more similarity with the European or Australian forms of this genus—however, while the forefin figured by Adams and Fiorillo [47] is consistent with the forelimbs of P. hercynicus and P. australis, the atlas-axis complex lacks the suture at the base of the neural canal exhibited by these two species [48,49]. Choo [50] described a fragmentary humerus that was more comparable with European rather than Australian or North American forms. Due to the incomplete nature of the material, the affinities of this humerus are uncertain—however, if this interpretation is upheld, the potential implication of this find is the possibility of two ichthyosaur dispersal events into Australia, as mentioned by Kear [51].

Maiaspondylus consists of a single species, Maiaspondylus lindoei, recovered from the Albian of the Northwest Territories, Canada [8] (Figure 1). The holotype of M. lindoei was found associated with ichthyosaur remains referred to Platypterygius [46], indicating that the two genera occurred together.

Athabascasaurus consists of a single species, Athabascasaurus bitumineus, described from the earliest Albian of Alberta, Canada [9] (Figure 1). The holotype consists of a nearly complete skull and partial postcranial skeleton that was collected in 2000 [9]. Platypterygius remains have been reported from the same formation containing Athabascasaurus [9].

Described from the upper Barremian of western Russia (Figure 1), the genus Sveltonectes contains a single species, Sveltonectes insolitus, erected on a nearly complete skeleton [10]. As a result, the authors have been able to identify many unique features, some of which relate to diet and swimming style that were also used to establish the generic name (see Etymology of Fischer et al. [10]).

The most recently described Cretaceous genus, Acamptonectes, is monotypic, only containing the species Acamptonectes densus. It occurs in the basal Hauterivian—upper Hauterivian of north Yorkshire, UK, and Lower Saxony, Germany [11] (Figure 1). Specimens attributable to the genus have also been identified from the Albian-Cenomanian of Cambridge, UK, but show features that differ from the already described species [11]. The preserved section of the snout is slender, exhibiting one of the lowest snout depth ratios in all known ophthalmosaurid ichthyosaurs, though the relative length of the rostrum is unknown [11]. Together with the tooth morphology described by Fischer et al. [11], this may indicate a different diet and thus lifestyle to previously known Cretaceous ichthyosaurs.

Ophthalmosaurus—Ophthalmosaurus is a well-known Jurassic ichthyosaur. The extension of this genus into the Cretaceous has been suggested by several authors [2,52,53], but a recent study [11] considers these reports to be ambiguous as the elements may belong to other genera now known from the Cretaceous. However, the same study refers a basioccipital and humerus from the Berriasian of the UK (Figure 1) to cf. Ophthalmosaurus, indicating survival across the Jurassic-Cretaceous boundary.

Caypullisaurus—Previously thought to be limited to the Upper Jurassic [1], the monotypic genus Caypullisaurus has since been shown to extend into the Berriasian (Lower Cretaceous) of Argentina [54] (in Reference [19]) (Figure 1). Additional material that may be attributed to Caypullisaurus has been recovered from the Hauterivian-Barremian of Chilean Patagonia, and occurs with P. hauthali [20,21].

Aegirosaurus—the anterior part of a rostrum is the only record of Aegirosaurus from the Cretaceous. Found in late Valanginian strata of southeastern France (Figure 1), the snout was referred to this genus based on four characters: anterior-posterior order in which the cranial bones emerged; slenderness of the snout; tooth morphology; and tooth density [55]. This species was previously only known from the upper Jurassic [2]. Like P. platydactylus, holotype material was destroyed during World War II (though syntype material, thin sections of the teeth, survived, [56]), but comparable material was referred to the genus at the beginning of the 21st Century [57].

Additional Genera—In addition to Ophthalmosaurus, Caypullisaurus, and Aegirosaurus, Brachypterygius may also have crossed the Jurassic–Cretaceous boundary. Ensom et al. [58] redescribed and dated an indeterminate ichthyosaur specimen that had affinities with Brachypterygius. Pending comparison with additional Cretaceous genera (such as Maiaspondylus), this material indicates that another Jurassic ichthyosaur may have survived into the Cretaceous.