In 1834, Kaup and Scholl mentioned a not otherwise specified tooth of an extinct sea cow from the early Oligocene of Germany near Flonheim (Mainz Basin; Fig. 1) as Pugmeodon schinzii. As Domning (1996) already recognised, both, the genus and species name, have to be considered as nomina nuda, because Kaup and Scholl (1834) neither provided a description nor a figure of the respective tooth. However, a description and figure of another premolar (HLMD-WT Az 48) under the same species name was given by Kaup (1838), who subsequently transferred it to the genus Halitherium, which seemingly is the correct generic name for the species as indicated by later studies (e.g., Kaup 1855; Lepsius 1882; Sickenberg 1934).

Figure 1. 

Geographic setting and palaeogeography of the Mainz Basin (black star) in western Germany during the early Oligocene (~ 32 Ma). White areas indicate land, grey areas represent marine conditions (modified after Probst 1986).

Therefore, Domning (1987) opened the nomenclatural case no. 2569 to apply for a proper designation of the holotype of Pugmeodon schinzii. By Opinion 1535 of the Bulletin of Zoological Nomenclature Volume 46 (ICZN 1999), all previous designations of type species for the nominal genus Halitherium Kaup, 1838 are set aside and Pugmeodon schinzii Kaup, 1838 is designated as type species under the plenary powers. Furthermore, the names “Halitherium” and “schinzii” are ruled to be placed on the official lists and indexes of names and works in Zoology by the International Commission on Zoological Nomenclature (2001).

In the subsequent decades, early Oligocene sirenian finds from the Mainz Basin comprising cranial and postcranial elements, and occasionally partial skeletons with nearly complete skulls and mandibles, were generally assigned to H. schinzii. Thus, this taxon has become firmly established in the literature as the only sea cow species that had evolved in this specific region to that time (e.g., Krauss 1858, Barthel 1962; Bahlo and Tobien 1982). Apart from its type locality, H. schinzii is also known from the Lower Rhine area in western Germany (Voss 2008) and from the Bay of Leipzig in eastern Germany (Fischer and Krumbiegel 1982). Sirenian finds from the Paris Basin (Bizzarini 1995; Bizzarini and Reggiani 2010), the Antwerp and East Flanders Provinces in North Belgium (Sickenberg 1934), Switzerland (Pilleri 1987) and Hungary (pers. obs.) that are referred to H. schinzii indicate the widespread distribution of this taxon in Central Europe.

However, considerable intraspecific morphological variations have been postulated for H. schinzii since the middle of the 19th century (e.g., Kaup 1855; Lepsius 1882; Sickenberg 1934; Fischer and Krumbiegel 1982). Attempted revisions of H. schinzii consequently resulted in the description of numerous specimens forming the basis of new species, as are, respectively, the skull roofs of H. kaupi and H. bronni (for both taxa see Krauss 1858). However, those are considered to be not valid and synonymous with H. schinzii by Domning (1996). Hartlaub (1886) described another species, Manatherium delheidi, based on cranial remains from the lower Oligocene of Antwerp (North Belgium) in comparison with H. schinzii from the Mainz Basin (Germany). However, Sickenberg (1934) revised Hartlaub’s (1886) species, focussing on aspects of juvenile stages and intraspecific variability. Sickenberg (1934), who already mentioned that a revision of Halitherium and H. schinzii was necessary, provided a comprehensive and critical review of the Belgian material, but without considering a separation on species level. According to his investigations, all known Belgian records belong exclusively to smaller animals. Therefore, Sickenberg (1934: 271) defined separate morphological groups, “forma typica” for the German and “forma delheidi” for the Belgian one, and assigned both to H. schinzii (see also Domning 1996). The most recent new combination for H. schinzii refers to skeletal material from the lower Oligocene of France known as H. schinzii lareolensis (Pilleri 1987; Domning 1996).

This review documents that the attempts of different authors to identify morphological distinctions on species level remain ambiguous until today. The debate on splitting and lumping of species currently referred to H. schinzii was recently revived by Voss (2012). This author stated that morphological variations within German and Belgian early Oligocene Sirenia might be related to the species level and postulated the presence of an additional species.

The hypothesis of two morphospecies necessitates the examination of the holotype of H. schinzii as a target for a revision of this type species. However, the preservation and nature of the holotype material pose major problems. Voss (2010) already postulated that no significant taxonomic definition for H. schinzii can be deduced from the premolar HLMD-WT Az 48 and, consequently, this species and the thereon based genus lack a diagnosis.

The present paper aims to clarify this issue by elucidating the morphology and taxonomic value of the premolar and provides perspectives for new taxonomic and systematic approaches.

BSPG – Bayerische Staatssammlung für Paläontologie und Geologie München (Germany); HLMD – Hessisches Landesmuseum Darmstadt (Germany); M3 – upper molar 3; P2–4 – upper premolar 2–4.

The hitherto only cladistic approach with the attempt to consider the entire order Sirenia is represented by Domning’s phylogeny published in 1994. Domning’s (1994, 1996) suprageneric classification maintains the traditional taxonomic concept of Simpson (1945). Accordingly, four sirenian families are distinguished (Fig. 3): the Prorastomidae Cope, 1889, the Protosirenidae Sickenberg, 1934, the Dugongidae Gray, 1821 (including the three subfamilies Halitheriinae, Hydrodamalinae and Dugonginae) and the Trichechidae Gill, 1872 (1821) (comprising the two subfamilies Miosireninae and Trichechinae).

Figure 3. 

Phylogeny of the Sirenia including 36 species and subspecies based on 62 informative cranial and dental characters (modified after Domning 1994). TL = 162, CI = 0.76, RI = 0.91.

The Prorastomidae and Protosirenidae, both representing semiaquatic quadrupeds, are confined to the Eocene and considered paraphyletic (Domning 1994; 2001a, b). The Dugongidae and Trichechidae represent fully aquatic animals and also include the few extant members belonging to the genera Dugong and Trichechus (Domning 2001a). According to Domning (1994), trichechids are monophyletic and nested within the Dugongidae. Consequently, the Dugongidae are paraphyletic. Within dugongids only two monophyletic groups are distinguished, the Dugonginae and Hydrodamalinae (Fig. 3). The most diverse dugongid subfamily and largest group within sirenians at all is represented by the Halitheriinae comprising the genera Caribosiren Reinhart, 1959, Eosiren Andrews, 1902, Eotheroides Palmer, 1899, Halitherium Kaup, 1838, Metaxytherium De Christol, 1840, and Prototherium De Zigno, 1887. However, this exclusively extinct group is paraphyletic indicating that higher-level phylogeny and systematics of the whole order Sirenia is paraphyly-based. Moreover, Domning’s phylogenetic analysis (1994) points out that “halitheriines” itself mainly comprise genera that are invariably paraphyletic and in addition the type genus of this subfamily, “Halitherium”, is also not monophyletic (Fig. 3).

Domning (1994: 187) stated that his phylogenetic analysis is a preliminary work and in need of systematic revisions in many parts. Nevertheless, the traditional classification concept of four sirenian families is unrevised subject of systematic studies up to now (e.g., Velez-Juarbe et al. 2012). This causes many systematic problems due to missing diagnoses and hampers a reliable taxonomic assignment of new sirenian finds. According to Domning (1994), the most controversial groups refer to the Eocene dugongids and Eocene sirenian taxa in general, the incompletely resolved clade of the dugongines and the inter- and intrageneric relationships of Metaxytherium. The need of revision of these groups is beyond doubt. Additionally, the present study emphasises that the “Halitheriinae”, comprising amongst others the Eocene dugongids and the genus Metaxytherium, constitute one of the most questionable sirenian groups, which is the focus of the following discussion.

According to article 11.7.1.1 (ICZN 1999) a family or subfamily, respectively, should be based on a valid genus. However, as outlined above this does not apply here, because the genus “Halitherium” is considered a nomen dubium and therefore is rejected. Since the “Halitheriinae” comprise a number of genera this problem could be solved by the designation of a new type genus. This taxonomic practise would be reasonable if the group in question represents a clade. However, the subfamily “Halitheriinae” has never been shown to form a monophyletic group in phylogenetic analyses considering Sirenia as a whole (Domning 1994; Fig. 3) as well as on subsets of the order (e.g., Bajpai and Domning 1997; Domning and Pervesler 2001; Domning and Aguilera 2008; Velez-Juarbe et al. 2012). Doubts on the “halitheriine” monophyly are reinforced by the most up to date phylogenetic analysis on Sirenia recently performed by Voss (2013) in her doctoral thesis, which will be published at a later date.

In fact, Simpson (1945) and Domning (1996) demonstrate that the “Halitheriinae” rather represent a historically grown assemblage of taxa characterised by morphological features that neither substantiate a monophyletic grouping nor distinguish it from other subfamilies. The latter is amongst others corroborated by the overlap of characters such as “cheek teeth enamelled, closed roots” that were provided by Simpson (1932: 423–424) to define the “Halitheriinae”, Miosireninae and Rytiodontinae. Consequently, the designation of a new type genus is not possible.

Likewise, the type genus “Halitherium” is traditionally considered to be monophyletic, because this has never been substantiated with sufficient evidence from any phylogenetic analysis up to now. The “Halitherium”-species complex regarded as valid within the European Palaeogene includes “H. schinzii” Kaup, 1838 (early Oligocene of Europe), “H.” cristolii Fitzinger, 1842 (late Oligocene of Upper Austria) and “H.” taulannense Sagne, 2001 (late Eocene of France). As indicated above, none of these taxa form a clade. Instead, each species represents individual branches in phylogenetic studies (e.g., Domning 1994; Domning and Aguilera 2008).

This paraphyly of “Halitherium” is the main reason for the conclusion drawn herein to question the current use of the subfamily “Halitheriinae”. Based on the fact that “Halitherium” is the type genus of the “Halitheriinae”, this subfamily is to be reduced to a single species, the type species “H. schinzii”, when all hitherto known phylogenies (e.g., Domning 1994; Domning and Aguilera 2008) are considered. Even if a new substitute name would be established for the genus “Halitherium” and the respective taxa of the former “Halitherium”-species complex receive a new designation, the taxonomic content of this subfamily will still comprise a single species. Therefore, the use of the “Halitheriinae” is not considered essential, because it does not contribute to a better understanding of sirenian diversity, but would rather lead to confusion. Instead, the rejection of the “Halitheriinae” is considered as a target-oriented solution into a more rigid taxonomic framework.