Inferring ‘weak spots’ in phylogenetic trees: application to mosasauroid nomenclature

Preferred phylogenetic definition

The most inclusive clade containing Mosasaurus hoffmannii Mantell, 1829 and Aigialosaurus dalmaticus Kramberger, 1892, but not Dolichosaurus longicollis Owen, 1850, Adriosaurus suessi Seeley, 1881, or Pontosaurus lesinensis Kornhuber, 1873. This definition is branch-based.

Remarks

Mosasauroidea traditionally includes mosasaurids and ‘aigialosaurs’ (e.g., Bell, 1997; Bell & Polcyn, 2005; Conrad, 2008). Proper delimitation of the extent of the name Mosasauroidea, however, requires adequate knowledge of the early evolution of Mosasauria and reappraisal of the phylogenetic positions of potential non-mosasauroid mosasaurs (e.g., the species belonging to Adriosaurus, Pontosaurus, Dolichosaurus). These taxa, or their subset, have been hypothesized to be either more closely related to snakes (see e.g., Palci & Caldwell, 2007; Caldwell & Palci, 2010; Palci & Caldwell, 2010) or to mosasaurids (e.g., Reeder et al., 2015). Considering that (1) the ‘dolichosaurs’ are traditionally regarded as non-mosasauroids, and (2) ‘aigialosaurs’ and mosasaurids are frequently inferred more closely related to each other than either is to the ‘dolichosaurs’, we propose a new definition that seems to adhere to the traditional use of Mosasauroidea (i.e., ‘aigialosaurs’ plus mosasaurids, but not ‘dolichosaurs’) and reflects the uncertainties surrounding the phylogenetic placements of near-mosasaurids and early mosasaurids as inferred, among others, in the present study (see Figs. 1–7).

Preferred phylogenetic definition

The most inclusive clade containing Aigialosaurus dalmaticus Kramberger, 1892 and Opetiosaurus bucchichi Kornhuber, 1901 but not Dolichosaurus longicollis Owen, 1850, Adriosaurus suessi Seeley, 1881, Pontosaurus lesinensis Kornhuber, 1873, or the clade originating with the most recent common ancestor of Halisaurus platyspondylus Marsh, 1869, Mosasaurus hoffmannii Mantell, 1829, and Tylosaurus proriger (Cope, 1869). This definition is branch-based.

Remarks

Aigialosauridae has a long and problematic history. The last thorough review of the interrelationships of early Mosasauria, i.e., those species associated with the evolutionary transition to aquatic lifestyle, was published by Dutchak (2005) who concluded that “redescriptions of the key taxa (Aigialosaurus dalmaticus, Opetiosaurus bucchichi and ‘the Trieste aigialosaur’) are essential to further investigations into re-testing the most recent hypotheses” (p. 228). Although A. dalmaticus and O. bucchichi have since been redescribed (Dutchak & Caldwell, 2006; Dutchak & Caldwell, 2009; respectively), and ‘the Trieste aigialosaur’ was assessed and given the name Komensaurus carrolli (Caldwell & Palci, 2007), the status of Aigialosauridae did not change. Indeed, Dutchak & Caldwell (2009) argued that O. bucchichi should be assigned to Aigialosaurus (as A. bucchichi), suggesting close relationships of the two taxa. Still, their analysis does not necessarily support this conclusion (see Dutchak & Caldwell, 2009: Fig. 4).

While it is certainly possible that A. dalmaticus and O. bucchichi are more closely related to one another than either is to other mosasauroids, such a result is currently not strongly supported statistically. The ‘full’ parsimony analyses (with all ‘dolichosaurs’ included and A. suessi selected as outgroup) reconstruct the taxa in a basal polytomy with other mosasauroid subclades (Fig. 1) or as successively more closely related to mosasaurids, with A. dalmaticus being the more basal of the two (Fig. 2). The Bayesian inference, majority of the weighted parsimony analyses (except for Figs. 3D and 3F), and parsimony analyses using different ‘dolichosaurs’ as outgroups, nevertheless, reconstruct a clade formed by both these species (Figs. 3–5), though their position on the mosasauroid tree is unstable.

Considering the problematic nature of mosasauroid origins, we admit that Aigialosauridae might be of use in the future. In this case, however, we strongly encourage using a complex self-destructive phylogenetic definition to reflect the history of the name as well as its unstable contents (see ICPN: Art. 11.9). The self-destructive branch-based definition that is proposed here keeps Aigialosauridae in use only if A. dalmaticus and O. bucchichi are more closely related to each other than either is to ‘dolichosaurs’ or Mosasauridae sensu Madzia & Conrad (in press). Also, it does not allow the use of the name in the cases when A. dalmaticus and O. bucchichi are reconstructed within Mosasauridae.

Preferred phylogenetic definition

The least inclusive clade containing Mosasaurus hoffmannii Mantell, 1829, Halisaurus platyspondylus Marsh, 1869, and Tylosaurus proriger. This definition is node-based.

Remarks

The history of the name Mosasauridae, its approximate synonyms, and its application were discussed by Madzia & Conrad (in press) who also provided the phylogenetic definition for the clade name as will be recognized by the ICPN.

The Bayesian analysis and parsimony analyses using different ‘dolichosaurs’ as the outgroup maintain the monophyly of mosasaurines, plioplatecarpines, tylosaurines, tethysaurines, yaguarasaurines, and the two halisaurine species. The ‘unweighted-ordered’ parsimony analysis, however, reconstructs tethysaurines and yaguarasaurines outside Mosasauridae, with Romeosaurus being inferred as the sister taxon to Komensaurus carrolli + mosasaurids, outside tethysaurines + a clade formed by Yaguarasaurus and Russellosaurus (Fig. 2). Thus, it makes Yaguarasaurinae polyphyletic.

The mutual relationships of particular mosasaurid clades are unsettled and highly dependent on the tree-search strategies used (Figs. 1–7). Still, even though the hypotheses of mosasaurid interrelationships are differing, the definition proposed by Madzia & Conrad (in press) does not require modifications. It covers all ‘traditional’ mosasaurid taxa, including the plioplatecarpines. Though not represented in the phylogenetic definition, Plioplatecarpus and its kin are kept within Mosasauridae under all inferred topologies.

Preferred phylogenetic definition

The most inclusive clade containing Halisaurus platyspondylus Marsh, 1869, but not Mosasaurus hoffmannii Mantell, 1829, Tylosaurus proriger (Cope, 1869), Tethysaurus nopcsai Bardet, Suberbiola & Jalil, 2003, or Yaguarasaurus columbianus Páramo, 1994. This definition is branch-based.

Remarks

Bardet et al. (2005) defined Halisaurinae as “Mosasauridae more closely related to Halisaurus than to Mosasaurus” (p. 464). Later, Conrad (2008) used equivalent branch-based definition with type species as specifiers: “All taxa sharing a more recent common ancestor with Halisaurus platyspondylus than Mosasaurus hoffmannii” (p. 127). Because the position of the species for which the name Halisaurinae was proposed is not very stable within Mosasauroidea (see the results of the present analysis and the Natantia paragraph below), we consider the proposed branch-based definition including additional external specifiers, representing other inferred clades, to be the most appropriate one.

Nevertheless, the current data set is not fully suitable for testing the phylogenetic position of Halisaurinae within Mosasauridae as the clade is represented by only two taxa (H. platyspondylus and Eonatator sternbergii).

Preferred phylogenetic definition

The most inclusive clade containing Mosasaurus hoffmannii Mantell, 1829, Tylosaurus proriger (Cope, 1869), and Plioplatecarpus marshii Dollo, 1882, but not Halisaurus platyspondylus Marsh, 1869. This definition is branch-based.

Remarks

Bell (1997) resurrected the name Natantia from the mid-nineteenth century (Owen, 1851). It was used to unite Bell’s (1997) ‘Russellosaurinae’ (see the Russellosaurina paragraph) and Mosasaurinae, exclusive of the Halisaurus species and the ‘aigialosaurs.’ Conrad (2008: 128) proposed the following branch-based definition: “All taxa sharing a more recent common ancestor with Mosasaurus hoffmanni, Tylosaurus proriger, and Plioplatecarpus marshi than with Halisaurus platyspondylus”. When applied on some recent phylogenetic hypotheses based on the data set initially published by Bell & Polcyn (2005), that infer halisaurines to be nested within the smallest clade containing Mosasaurus, Tylosaurus, and Plioplatecarpus, Natantia self-destructs.

Our analyses do not support the concept of Natantia either (Figs. 1–7). In the ‘unweighted-ordered’ parsimony analysis (Fig. 2), some weighted parsimony analyses (Figs. 3D and 3F), parsimony analysis with Pontosaurus as the outgroup (Fig. 4C), and Bayesian analysis (Fig. 5), halisaurines form the sister taxon to mosasaurines. When Adriosaurus is used as outgroup, and other ‘dolichosaurs’ are excluded, and under some weighted parsimony analyses, halisaurines are more closely related to the clade formed by tethysaurines, yaguarasaurines, tylosaurines, and plioplatecarpines than to mosasaurines (Figs. 3A– 3C, 3E and 4A).

It is worth noting that Boas (1880) used the name Natantia for a subgroup of decapod crustaceans. Although Owen’s (1851) Natantia was published earlier, the priority issue is problematic. The ICZN (1999) does not govern the names above the family group, and Natantia, approximately corresponding to the concept of Owen (1851), had not been in use until Bell (1997). Similarly, the use of Boas (1880) is outdated (WoRMS, 2015), though it was of importance in the past (see, for example, the discussion in Felgenhauser & Abele, 1983).

We refrain from providing a lengthy discussion of the nomenclatural issue or a solution to it but since the name Natantia Owen (1851) was published earlier, we provisionally keep it as the name for the potential grouping as discussed above.

Preferred phylogenetic definition

The most inclusive clade containing Mosasaurus hoffmannii (Mantell, 1829), but not Tylosaurus proriger (Cope, 1869), Plioplatecarpus marshii Dollo, 1882, Halisaurus platyspondylus Marsh, 1869, Tethysaurus nopcsai Bardet, Suberbiola & Jalil, 2003, or Yaguarasaurus columbianus Páramo, 1994. This definition is branch-based.

Remarks

Mosasaurinae is traditionally considered to represent a species-rich clade with substantial morphological and ecological diversity (e.g., Bell, 1997; Bell & Polcyn, 2005; Bardet et al., 2015).

The first published phylogenetic definition is the following: “All taxa sharing a more recent common ancestor with Mosasaurus hoffmanni than with Tylosaurus proriger or Plioplatecarpus marshi” (Conrad, 2008: 128). This branch-based definition keeps the traditional contents of Mosasaurinae intact when applied to the majority of recent analyses. We added additional external specifiers, Halisaurus platyspondylus, Tethysaurus nopcsai, and Yaguarasaurus columbianus, to reflect the traditional contents of Mosasaurinae and the inferred overall instability in the mosasaurid interrelationships. The monophyly of mosasaurines, however, is inferred by all our analyses (Figs. 1–7).

Preferred phylogenetic definition

The most inclusive clade containing Mosasaurus hoffmannii Mantell, 1829, but not Globidens alabamaensis Gilmore, 1912. This definition is branch-based.

Remarks

Bell (1997: 322) abandoned Mosasaurini on the basis of the supposed paraphyly of Mosasaurus and “expanded [Plotosaurini] to include basic taxa previously referred to Mosasaurus”. Both taxon names, Mosasaurini and Plotosaurini, were introduced in the same publication (Russell, 1967). However, it seems that the former has gained more attention (e.g., Leblanc, Caldwell & Bardet, 2012; Fanti, Cau & Negri, 2014). Leblanc, Caldwell & Bardet (2012: 101) argued to replace Plotosaurini with Mosasaurini which they used for “the group consisting of (Eremiasaurus (Mosasaurus + Plotosaurus))”. Although the close connection of these taxa is generally supported by recent phylogenetic studies (e.g., Grigoriev, 2013; Palci, Caldwell & Papazzoni, 2013; Fanti, Cau & Negri, 2014; Jiménez-Huidobro & Caldwell, 2016), analyses using multiple tree-search strategies show conflicting results (Simões et al., 2017). The grouping is maintained in the ‘unweighted-unordered’ parsimony analysis, under one ‘weighted-unordered’ parsimony analysis (Fig. 3E), and when only one of the ‘dolichosaur’ taxa is included (Fig. 4). Still, ‘unweighted-ordered’ parsimony, other weighted parsimony analyses, and the Bayesian inference fail to support such topology.

Preferred phylogenetic definition

The most inclusive clade containing Globidens alabamaensis (Gilmore, 1912), but not Mosasaurus hoffmannii Mantell, 1829. This definition is branch-based.

Remarks

Bell (1997) used Russell’s (1967) Globidensini to unite Globidens, Prognathodon, and Plesiotylosaurus. Although such close connection of these taxa is not necessarily supported by current studies (e.g., Palci, Caldwell & Papazzoni, 2013; Fanti, Cau & Negri, 2014; Jiménez-Huidobro & Caldwell, 2016), there is indeed a tendency to keep them together under the name Globidensini (e.g., Schulp et al., 2008; Leblanc, Caldwell & Bardet, 2012). Nevertheless, forcing Prognathodon solvayi, the type species of Prognathodon, to be a globidensin (by selecting it as an internal specifier), would be potentially ineffective considering the likely para- or even polyphyletic nature of the taxa attributed to Prognathodon.

All our analyses fail to reconstruct Globidensini with more than only the two species of Globidens included (Figs. 1–7). Nevertheless, the clade name may still be useful for discussions related to mosasaurid ecology (due to the specialized dentition of Globidens and Carinodens, its potential close relative (Schulp, Jagt & Fonken, 2004)).

Preferred phylogenetic definition

The least inclusive clade containing Russellosaurus coheni Polcyn & Bell, 2005, Tylosaurus proriger (Cope, 1869), and Plioplatecarpus marshii Dollo, 1882, but not Mosasaurus hoffmannii Mantell, 1829. This definition is node-based.

Remarks

Due to its problematic history, the name Russellosaurina is discussed here in detail. In his PhD thesis, Bell (1993) proposed a new name, Russellosaurinae, to link tylosaurines and plioplatecarpines together, and provided the following node-based definition: “The most recent common ancestor of Tylosaurus, Ectenosaurus and Plioplatecarpus and all of its descendants” (p. 183). He noted that Russellosaurinae consists of “Tylosaurus and Plioplatecarpini” (p. viii), which matched his definition. Bell’s PhD thesis was published four years later (Bell, 1997). Until that time, ‘Russellosaurinae’ was in use in an informal sense as a node-based name for a clade consisting of ‘tylosaurines’ and ‘plioplatecarpines’ (Caldwell, 1996). Because the paper by Bell (1997) was originally intended to simply be the published version of his PhD thesis, Bell (1997) again introduced ‘Russellosaurinae’ as a new taxon name. However, its extent seems to be different as the name was introduced “in anticipation of formally designating the taxon and describing a new taxon, Russellosaurus, from new Turonian material from Texas” (p. 322). Although there was no explicit information about how closely related Russellosaurus was to ‘russellosaurines’ (sensu Bell, 1993), and in the ‘Summary’ paragraph of Bell (1997: 324) ‘Russellosaurinae’ is again listed as consisting of “Tylosaurus and Plioplatecarpini” only, it is clear that Bell (1997) intended to anchor ‘Russellosaurinae’ on the taxon Russellosaurus. Until Polcyn & Bell (2005), where ‘Russellosaurinae’ was officially replaced with Russellosaurina, authors used the name in the traditional informal way, and always as a node-based name for a clade containing Tylosaurus and Plioplatecarpini (Christiansen & Bonde, 2002) or Plioplatecarpinae (Bardet et al., 2005); the latter two names referring to the same content.

Polcyn & Bell (2005) introduced the name Russellosaurina “to give identity to the monophyletic grouping of Tylosaurinae plus Plioplatecarpinae and closely related forms” (Polcyn & Bell, 2005: 323). What the “closely related forms” are is clear from the ‘Systematic palaeontology’ paragraph (p. 322), according to which the only non-mosasaurine mosasaurid taxa listed there as Russellosaurina are “[t]he subfamilies Tylosasaurinae [sic] and Plioplatecarpinae and their sister-clade containing the genera Tethysaurus, Russellosaurus and Yaguarasaurus”. Unfortunately, the composition of Russellosaurina is not that transparent in other parts of that paper. According to the abstract, Russellosaurina “includes Plioplatecarpinae, Tylosaurinae, their [most recent] common ancestor and all [of its] descendants” (p. 321), and according to the phylogenetic definition, Russellosaurina consists of “[a]ll mosasaurs more closely related to Tylosaurinae and Plioplatecarpinae, the genus Tethysaurus, their common ancestor and all descendants than to Mosasaurinae” (p. 322). This definition is clearly branch-based with “Tylosaurinae and Plioplatecarpinae, the genus Tethysaurus, their common ancestor and all descendants” being a node-based clade and an internal specifier of the definition. This wording is therefore inconsistent with all previously cited statements.

When Polcyn & Bell (2005) established the name, they gave it the rank of ‘parafamily,’ a term introduced by Olshevsky (1991) for ‘paraphyletic family’ (the prefix ‘para-’ indicates ‘paraphyly’), and not recognized by the ICZN. Therefore, it is of the same level as ‘family’. However, the suffix ‘-ina’ typically indicates a subtribe in zoological nomenclature, so when assigning the name Russellosaurina a rank, the taxon should be contained within a tribe and a subfamily. Here, Russellosaurina is considered an unranked clade name with the node-based definition provided above. In our definition, M. hoffmannii is used as a qualifying clause (ICPN: Art. 11.9). The suggested compilation is preferred for various reasons. First, it should “[supersede] previous references to ‘Russellosaurinae”’ (Polcyn & Bell, 2005: 323), thus applying to the clade originating with the most recent common ancestor of Tylosaurinae, Plioplatecarpinae, and R. coheni. Further, Russellosaurina has always been understood as a node-based name. Although Conrad (2008) “tentatively” followed the original branch-based definition, he simultaneously noted that “the definition Polcyn & Bell (2005) intended for Russellosaurina is frustratingly ambiguous” (Conrad, 2008: 129). Since R. coheni was omitted from the specifiers, the original definition violated the ICPN (Art. 11.7).

According to the new definition, Russellosaurina contains the species R. coheni, Y. columbianus, T. nopcsai, the clade Plioplatecarpinae, and the clade Tylosaurinae (as inferred, e.g., in Bell & Polcyn, 2005; Dutchak & Caldwell, 2006; Cuthbertson et al., 2007). It may also contain Halisaurinae as reconstructed in Caldwell & Palci (2007), or self-destruct under the hypothesis from Bardet et al. (2005). Russellosaurina may also contain only Plioplatecarpinae and Tylosaurinae, if R. coheni and Y. columbianus are basal members of Plioplatecarpinae as it was suggested by Polcyn & Bell (2005: 332) and inferred in Dutchak & Caldwell (2009: Fig. 5). Russellosaurina self-destructs if R. coheni, Y. columbianus and T. nopcsai form the sister taxon to the least inclusive clade including M. hoffmannii and T. proriger as reconstructed in Dutchak & Caldwell (2009: Fig. 4).

The ‘unweighted-unordered’ parsimony analysis (Fig. 1), some weighted parsimony analyses (Figs. 3A–3C and 3E), parsimony analyses with Adriosaurus and Pontosaurus used as outgroups (Figs. 4A and 4C), and Bayesian analysis (Fig. 5) support Russellosaurina. Under all other topologies, Russellosaurina self-destructs (Figs. 2, 3D, 3F and 4B).

Preferred phylogenetic definition

The least inclusive clade containing Tethysaurus nopcsai Bardet, Suberbiola & Jalil, 2003 and Pannoniasaurus inexpectatus Makádi, Caldwell & Ösi, 2012, but not Halisaurus platyspondylus Marsh, 1869, Mosasaurus hoffmannii (Mantell, 1829), Tylosaurus proriger (Cope, 1869), Plioplatecarpus marshii Dollo, 1882, or Yaguarasaurus columbianus Páramo, 1994. This definition is node-based.

Remarks

Makádi, Caldwell & Ösi (2012) introduced the name Tethysaurinae for “[t]he most recent common ancestor of Pannoniasaurus inexpectatus and Russellosaurus coheni Polcyn & Bell, 2005 […] and all its descendants”. Following the results of their phylogenetic analysis, the clade Tethysaurinae was formed by P. inexpectatus, R. coheni, Tethysaurus nopcsai, and Yaguarasaurus columbianus. However, by omitting T. nopcsai from the internal specifiers, the phylogenetic definition violates the ICPN (Art. 11.7). Later, Palci, Caldwell & Papazzoni (2013) introduced the name Yaguarasaurinae and defined it as “[t]he most recent common ancestor of Romeosaurus, gen. nov., Russellosaurus, and Yaguarasaurus, and all of its descendants”. Tethysaurinae was kept only for Pannoniasaurus and Tethysaurus, that formed the sister clade to the Yaguarasaurinae (see below for comments on this name).

We follow the node-based concept of Tethysaurinae as delimited by Palci, Caldwell & Papazzoni (2013) but considering the unstable position of the two tethysaurines on the mosasauroid tree (see Figs. 1–7), we added five external specifiers to maintain the ‘traditional’ contents.

All our analyses reconstruct monophyletic tethysaurines (Figs. 1–7).

Preferred phylogenetic definition

The least inclusive clade containing Yaguarasaurus columbianus Páramo, 1994, Russellosaurus coheni Polcyn & Bell, 2005, and Romeosaurus fumanensis Palci, Caldwell & Papazzoni, 2013, but not Tethysaurus nopcsai Bardet, Suberbiola & Jalil, 2003, Halisaurus platyspondylus Marsh, 1869, Tylosaurus proriger (Cope, 1869), Plioplatecarpus marshii Dollo, 1882, or Mosasaurus hoffmannii Mantell, 1829. This definition is node-based.

Remarks

As noted above, Yaguarasaurinae was introduced by Palci, Caldwell & Papazzoni (2013) who defined it as “[t]he most recent common ancestor of Romeosaurus, gen. nov., Russellosaurus, and Yaguarasaurus, and all of its descendants”. We follow such definition but considering the weak support for the connection of Yaguarasaurinae and Tethysaurinae (Figs. 1, 2, 5 and 7), we added five external specifiers to prevent the name to cover an unintended clade.

The Bayesian analysis and majority of the parsimony analyses support the monophyly of the yaguarasaurines as delimited by Palci, Caldwell & Papazzoni (2013). Only under the topology resulting from the ‘unweighted-ordered’ parsimony analysis and two ‘weighted-ordered’ parsimony analyses Yaguarasaurinae self-destructs (Figs. 2, 3D and 3F).

Preferred phylogenetic definition

The most inclusive clade containing Plioplatecarpus marshii Dollo, 1882, but not Mosasaurus hoffmannii Mantell, 1829, Tylosaurus proriger (Cope, 1869), Tethysaurus nopcsai Bardet, Suberbiola & Jalil, 2003, or Yaguarasaurus columbianus Páramo, 1994. This definition is branch-based.

Remarks

Conrad (2008: 130) defined Plioplatecarpinae as “[a]ll taxa sharing a more recent common ancestor with Plioplatecarpus marshi[i ] than with Tylosaurus proriger or Mosasaurus hoffmannii”. Such definition matches the published hypotheses; Plioplatecarpinae as sister taxon to Tylosaurinae or to Mosasaurinae (e.g., Bell, 1997; Bardet et al., 2005; Bell & Polcyn, 2005; Leblanc, Caldwell & Bardet, 2012; Palci, Caldwell & Papazzoni, 2013; Jiménez-Huidobro & Caldwell, 2016), but does not reflect the possible close connection of plioplatecarpines with yaguarasaurines (as suggested by Polcyn & Bell [2005: 332] and then inferred, together with Tethysaurus, by Dutchak & Caldwell [2009: Fig. 5]). Thus, we included two additional external specifiers, Tethysaurus nopcsai and Yaguarasaurus columbianus, that assure the adherence of the name Plioplatecarpinae to the traditional contents under alternative hypotheses.

The topologies inferred through our parsimony and Bayesian analyses support the monophyly of the traditional plioplatecarpines as delimited by Konishi & Caldwell (2011) (Figs. 1–7).

Preferred phylogenetic definition

The most inclusive clade containing Tylosaurus proriger (Cope, 1869), but not Plioplatecarpus marshii Dollo, 1882, or Mosasaurus hoffmannii Mantell, 1829. This definition is branch-based.

Remarks

The tylosaurine interrelationships have been intensively studied during the past decade (e.g., Bullard, 2006; Martin & Fernández, 2007; Caldwell et al., 2008; Bullard & Caldwell, 2010; Jiménez-Huidobro & Caldwell, 2016; Otero et al., 2017), resulting, among others, in numerous changes in binomial nomenclature. The monophyly of Tylosaurinae, nevertheless, has not been put into question.

Conrad (2008: 130) defined Tylosaurinae as “[a]ll taxa sharing a more recent common ancestor with Tylosaurus proriger than with Mosasaurus hoffmannii or Plioplatecarpus marshi[i ]”. This definition adheres to the traditional contents of Tylosaurinae under all current topologies, including these inferred by our parsimony and Bayesian analyses (Figs. 1–7).

Outgroup selection

In the initial version of the data set introduced by Bell (1993) and Bell (1997), the outgroup was constructed following the algorithm described by Maddison, Donoghue & Maddison (1984). The final outgroup OTU was based on the characters present in eight modern squamates (Aspidoscelis sexlineata, Crotaphytus collaris, Dipsosaurus dorsalis, Gekko gecko, Gerrhonotus liocephalus, Plestiodon laticeps, Shinisaurus crocodilurus, and Varanus niloticus) and two extinct squamates (Estesia mongoliensis and Gilmoreteius chulsanensis). Such ‘composite’ operational taxonomic unit was used by most later authors (e.g., Bell & Polcyn, 2005; Caldwell & Palci, 2007; Leblanc, Caldwell & Bardet, 2012). More recently, however, some studies preferred to use only the character states present in Varanus as the outgroup (e.g., Palci, Caldwell & Papazzoni, 2013; Jiménez-Huidobro & Caldwell, 2016) “because both taxa [i.e., Mosasauroidea and Varanus] are large-bodied anguimorphs that share a number of symplesiomorphic features” (Palci, Caldwell & Papazzoni, 2013: 608).

The outgroup sampling is known to have a great effect on the structure of phylogenetic trees (e.g., Graham, Olmstead & Barrett, 2002; Spaulding, O’Leary & Gatesy, 2009; Kirchberger et al., 2014; Wilberg, 2015). Given the alternative placements of Mosasauroidea among different phylogenies published (e.g., Conrad, 2008; Gauthier et al., 2012; Reeder et al., 2015), it is not universally agreed which squamates may represent the closest sister group of mosasauroids. Therefore, outgroup selection among extant squamates may be biased by preference among the alternative placement of Mosasauroidea.

The problems with the use of the ‘composite’ OTU, then, was already commented on by Palci, Caldwell & Papazzoni (2013: 608) who noted that the “outgroup is problematic for several reasons: (1) it does not reflect the character state composition of a real organism; (2) it can produce paradoxical combinations of character states where a feature coded as absent in one character is further defined in a second character […]; and (3) lack of repeatability of the process that produced such codings”, noting that Bell (1997) “was not very explicit on how he obtained the character states for his outgroup”. The third point (lack of repeatability of the process), however, does not seem to be entirely fair. Even though Palci, Caldwell & Papazzoni (2013) are certainly correct that Bell (1997) was not particularly specific regarding the scores of his ‘composite’ OTU, that paper was supposed be the published version of his PhD thesis (Bell, 1993), which is explicitly referred to by Bell (1997: 294) and includes information on where the scores come from (Bell, 1993: 9–16, 251, 265–268).

To solve the issues with outgroup selection, Simões et al. (2017) expanded the data set by adding three ‘dolichosaur-grade’ taxa: Adriosaurus suessi Seeley, 1881, Dolichosaurus longicollis Owen, 1850, and Pontosaurus kornhuberi Caldwell, 2006, and designed A. suessi as the basalmost outgroup. Even though A. suessi constitutes a much better outgroup than the ‘composite’ OTU and Varanus, because its age and morphology more closely reflect those of the last common ancestor of all mosasauroids, such approach forces Dolichosaurus and Pontosaurus to be inferred more closely to mosasaurids than to Adriosaurus. This outgroup setting may thus lead to the construction of an artificial ‘dolichosaur grade’ as the basalmost mosasauroid condition (i.e., due to the outgroup setting in TNT used by Simões et al., 2017, ‘dolichosaurs’ are constrained to form a paraphyletic series leading to Mosasauroidea), which may lead to spurious relationships among the ingroup taxa merely based on squamate symplesiomorphies that are absent among the ‘dolichosaur’ taxa. As Simões et al. (2017) noted, some studies reconstruct these ‘dolichosaurs’ to represent snake-branch pythonomorphs (see e.g., Palci & Caldwell, 2007; Caldwell & Palci, 2010; Palci & Caldwell, 2010). Thus, all these three OTUs may be ‘equally’ distantly related to Mosasauridae. It is noteworthy that the latter hypothesis is supported by the Bayesian analysis using the FBDSA model, which reconstructed all ‘dolichosaur’ taxa as forming a clade excluding all other OTUs.

To avoid any bias due to a priori assumptions on character state transformation (because of the alternative extant squamate outgroup used and potentially incorrect outgroup/basal ingroup designation), we suggest to perform analyses using different outgroup selection or to consider the use of a ‘remote outgroup’. Perhaps, the well preserved Early Cretaceous (Aptian) squamate Huehuecuetzpalli mixtecus Reynoso, 1998, might serve as the root in a separate analysis. That taxon is universally recognized as more basal than any alternative mosasauroid outgroup used previously (Conrad, 2008; Gauthier et al., 2012), and may represent the ancestral squamate morphology regardless of the preferred closest relatives of mosasauroids. However, see also Graham, Olmstead & Barrett (2002) and Kirchberger et al. (2014) for independent tests regarding the effects of the use of phylogenetically distant outgroups in molecular studies.

Taxon sampling

As discussed above, the outgroup selection has a substantial impact on the structure of the inferred tree topology, including the statistical support of the basal branching near the root of Mosasauroidea. Still, the resolution of the rootward mosasauroids might not necessarily improve without an increased number of early mosasaurids and near-mosasaurids analyzed. The most recent version of the data set was expanded with the addition of Adriosaurus suessi, Dolichosaurus longicollis, and Pontosaurus kornhuberi, and separation of Opetiosaurus bucchichi from the Aigialosaurus OTU (even if it is assigned to Aigialosaurus as A. bucchichi; Dutchak & Caldwell, 2009; Simões et al., 2017). Still, it could also benefit, for instance, from addition of Acteosaurus tommasinii (Palci & Caldwell, 2010), Adriosaurus microbrachis (Palci & Caldwell, 2007), Adriosaurus skrbinensis (Caldwell & Palci, 2010), Aphanizocnemus libanensis (Dal Sasso & Pinna, 1997), Carsosaurus marchesettii (e.g., Caldwell, Carroll & Kaiser, 1995; Caldwell & Palci, 2007), Coniasaurus crassidens (Caldwell & Cooper, 1999), Eidolosaurus trauthi (Nopcsa, 1923), and Pontosaurus lesinensis (Pierce & Caldwell, 2004). The fact that some or most of these taxa can be more closely related to snakes than to mosasaurids (see e.g., Palci & Caldwell, 2007; Caldwell & Palci, 2010; Palci & Caldwell, 2010), is not a problem as their morphology approximates to that of the mosasaurid ancestor, and therefore supplements the knowledge of early pythonomorph evolution.

The data set of Simões et al. (2017) contains members of all well-recognized mosasauroid subclades; the taxa traditionally contained within Halisaurinae, Mosasaurinae, Plioplatecarpinae, and Tylosaurinae. It also contains all tethysaurines and yaguarasaurines (except Romeosaurus sorbinii Palci, Caldwell & Papazzoni, 2013), as these two clades were inferred in studies using recent versions of the data set (Makádi, Caldwell & Ösi, 2012; Palci, Caldwell & Papazzoni, 2013; respectively). Still, some of the clades are substantially underrepresented even though detailed descriptions of their members have been published and some of those taxa have been scored for characters in older versions of the same data set. For example, the current version of the data set includes only two halisaurine OTUs (Halisaurus platyspondylus and Eonatator sternbergii; with the latter being labeled as ‘Halisaurus sternbergi’) even though detailed studies have also been published, for example, for Halisaurus arambourgi (Bardet et al., 2005; Polcyn et al., 2012) or Phosphorosaurus ortliebi (Lingham-Soliar, 1996; Holmes & Sues, 2000; Bardet et al., 2005). Likewise, the data set could be supplemented by recently described Eonatator coellensis (Páramo-Fonseca, 2013) and Phosphorosaurus ponpetelegans (Konishi et al., 2016). Such sampling could test some of the implied relationships (the connection of E. coellensis to E. sternbergii, H. arambrourgi to H. platyspondylus, P. ponpetelegans to P. ortliebi). A phylogenetic analysis of Halisaurinae was recently published by Konishi et al. (2016). The analysis did not reconstruct monophyletic Halisaurus nor Eonatator but inferred sister-taxon relationships between P. ortliebi and P. ponpetelegans, a taxon described by these authors. However, the analysis was based on only 21 cranial characters and rooted on Platecarpus tympaniticus, a derived plioplatecarpine that might not serve best as the outgroup for such analysis due to its placement and age. Considering the unsettled relationships within Halisaurinae, and the differing position of the clade within Mosasauridae, an expansion of the data set by using more halisaurines (and modification of the characters to better reflect their morphology) might result in improving the resolution of the mosasauroid tree topology.

New reappraisals of certain tylosaurine species have also been published recently. For example, Hainosaurus pembinensis and H. bernardi, the latter being the type species of Hainosaurus, have been assigned to Tylosaurus (Bullard & Caldwell, 2010; Jiménez-Huidobro & Caldwell, 2016, respectively), and Tylosaurus kansasensis was proposed to be a juvenile of T. nepaeolicus, and thus removed from the data set (Jiménez-Huidobro, Simões & Caldwell, 2016). However, T. pembinensis is not included in the recent version of the data set, which does not enable to further test the newly proposed hypotheses. Interestingly, the ordered-unweighted parsimony analysis and the Bayesian analysis do not support the monophyly of Tylosaurus (represented by T. proriger, T. bernardi, and T. nepaeolicus) exclusive of Taniwhasaurus (Figs. 2 and 5). When only one ‘dolichosaur’ is in the data set and used as the outgroup, regardless of which one it is, Tylosaurus is monophyletic (Fig. 4). The resolution might improve with a more appropriate outgroup selection and addition of T. pembinensis and, possibly, ‘Hainosaurus’ neumilleri (Martin, 2007). Additionally, Tylosaurus ‘saskatchewanensis’ (Bullard, 2006) and ‘Hainosaurus’ ‘kenbrowni’ (Thompson, 2005; Thompson, 2011) can also be considered pending their formal descriptions.

The understanding of the plioplatecarpines, in turn, may improve by separation of the Plioplatecarpus OTU into several terminal units. Such sampling could test the monophyly of Plioplatecarpus (a taxon consisting of a few species, including P. marshii, P. houzeaui, P. primaevus, and the recently described P. peckensis; Cuthbertson & Holmes, 2015), estimate the support for the tree topology obtained by Konishi & Caldwell (2011) and Cuthbertson & Holmes (2015), test the connection of ‘Latoplatecarpus’ nichollsae and L. willistoni, or provide additional support for the separation of Plesioplatecarpus planifrons (labeled as ‘Platecarpus planifrons’ in the data set of Simões et al., 2017) from Platecarpus tympaniticus (Konishi & Caldwell, 2011).

Mosasaurines are problematic, as is apparent from differing and often poorly resolved tree topologies. The inference of the structure of the mosasaurine phylogenetic tree appears to be difficult especially due to the unstable positions of the taxa attributed to Prognathodon (e.g., Leblanc, Caldwell & Bardet, 2012; Simões et al., 2017; our study). Nevertheless, numerous derived mosasaurines are currently under revision as is apparent from Street & Caldwell (2017) that provided detailed reappraisal of Mosasaurus hoffmannii, preliminary discussion of some other taxa traditionally assigned to Mosasaurus, and reported on an ongoing research. Together with reconsideration of some species traditionally attributed to Prognathodon, the resolution of the mosasaurines might benefit from addition of some presumably rootward mosasaurine taxa that have not been included in previous ‘complete’ versions of the Bell’s data set (i.e., when the aim was to assess the interrelationships within all major clades of mosasauroids). These include, for example, Kourisodon puntledgensis (Nicholls & Meckert, 2002). This taxon, which has previously been used as an outgroup in some analyses (Konishi & Caldwell, 2011; Cuthbertson & Holmes, 2015), originates from the upper Santonian of British Columbia, Canada, and is one of the oldest known mosasaurines. Its inclusion might have an impact on the resolution of Mosasaurinae.

Character sampling

We suggest that character statements are redefined from those used in recent versions of Bell’s (1997) data set, following the recommendations in Sereno (2007) and Brazeau (2011). In particular, compound characters are suggested to be atomized; i.e., neomorphic and transformational features should be considered as distinct characters and not as alternative states of a single character. Therefore, when not resulting in loss of information, characters are suggested to be defined as binary. When multistate character statements are included, and the states form unambiguous morphoclines that describe a nested set of alternative states (e.g., marginal tooth numbers, vertebral numbers, phalangeal formulas), the corresponding character statements should be set as ordered, to avoid a priori exclusion of potential synapomorphies represented by the subset of states representing a derived condition (e.g., Wilkinson, 1992; Sereno, 2007; Brazeau, 2011). Such states, however, should be formulated to avoid marked polymorphism. For example, the current version of the data set (Simões et al., 2017) includes a six-state character dealing with the dentary tooth count: “(53) Dentary tooth number: 20–24 (0); 17–19 (1); 15–16 (2); 14 (3); 13 (4); 12 (5)”. Yet such defined states insufficiently reflect differences in taxa where the dentary tooth count is one of the few distinguishing characters. Furthermore, once set as ordered to reflect the homology among nested state-transitions, the character defined this way leads to inflating the phylogenetic importance of a feature that may be merely size-related and individually variable among the same taxon. For instance, Mosasaurus hoffmannii is often reported as having 14 dentary teeth (e.g., Street & Caldwell, 2017). However, some specimens have 15 dentary teeth (e.g., CAMSM F22228, IRSNB R 0303; D Madzia, pers. obs., 2017; Mulder, Cornelissen & Verding, 2004), or only 13 (NHMM 009002; Everhart et al., 2016). Thus, M. hoffmannii can be scored for states 2, 3, and 4. At the same time, Mosasaurus lemonnieri, which is currently considered to be distinct from M. hoffmannii (Street & Caldwell, 2017; D Madzia, 2017, unpublished data), has always 16 dentary teeth. Still, it would be covered under the same state (2).

This example demonstrates that character definitions and among-state transition settings may significantly influence relationships, and must be discussed prior to phylogenetic analyses.

‘Data handling’

As we have expressed above, we consider the current versions of the Bell’s (1997) data set to be insufficient for accurate inferences of mosasauroid phylogenetic relationships. We suggest to (1) reconsider the outgroup selection, (2) increase the number of analyzed taxa, and named some of those that we think might improve the resolution of the mosasauroid phylogenetic tree, and (3) revise the morphological characters and their states. Naturally, it is essential to note that the steps should be undertaken after careful considerations and simultaneously. Specifically, increasing the number of analyzed taxa could have an entirely opposite effect and cause more instability if the additions do not sufficiently reflect the differing morphologies of the proposed OTUs and their character evolution. Also, we suggest to consider even those taxa that might be regarded as too incomplete to be included in the data matrix (see e.g., Wiens, 2003a; Wiens, 2003b; Wiens & Morrill, 2011). The relevance of all additions might be tested, for example, following the principle of safe taxonomic reduction (Wilkinson, 1995), using TAXEQ3 (Wilkinson, 2001), or through ‘concatabominations’ (Siu-Ting et al., 2015). However, it has also been argued that “there is no justification—either a priori or a posteriori—to definitively exclude unstable taxa from the data matrix as this involves the deletion of phylogenetic information that can be relevant (or even critical) for understanding the relationships of the entire group” (Pol & Escapa, 2009: 13). Therefore, Pol & Escapa (2009) offered to use a TNT script, IterPCR, that provides a list of characters related to the instability of each unstable taxon. This script has already been implemented in TNT (Goloboff & Szumik, 2015).