Les Muricidae forment une vaste famille d'environ 2500 espèces fossiles et actuelles. La comparaison des espèces les plus anciennes (Maastrichtien–Danien) avec celles de niveaux géologiques plus récents montre fréquemment une augmentation du nombre des éléments sculpturaux et de leur précocité [10] and [13], qui est bien attestée chez les formes tropicales, comme les Muricinae modernes et les Muricopsinae Radwin & D'Attilio, 1971 (exemple : Favartia, Muricopsis et Murexsul) [10] and [12]. Les espèces du nouveau genre Eofavartia, jusqu'à présent confondues avec les Favartia [1], [2] and [18], font partie de ces Muricidae tropicaux très ornés, dont les patterns sculpturaux se sont avérés superficiellement définis par la méthode descriptive traditionnelle [7], [10], [11] and [14]. Cette note propose un examen approfondi des patterns sculpturaux des Eofavartia et des Favartia, fondé sur l'identification d'homologies structurales, testées par une analyse cladistique. Elle souligne l'importance de la méthode analytique employée pour aborder l'étude de la radiation des Muricidae en optimisant l'utilisation des coquilles et donc des fossiles.

The muricids represent a large family of about 2500 fossil and living species. When the oldest species from the Maastrichtian and the Palaeocene are compared to younger forms, an increase in the number of the sculptural elements and their precocity is frequently observed in the geologically younger species [10] and [13]. This is particularly true for numerous tropical taxa, such as the modern Muricinae (e.g.: Hexaplex Perry, 1811, Murex Linnaeus, 1758 and Siratus Jousseaume, 1880) and the Muricopsinae Radwin & D'Attilio, 1971 (e.g.: Favartia Jousseaume, 1880, Muricopsis Bucquoy & Dautzenberg, 1882, Murexsul Iredale, 1915) [10] and [12]. The species of Eofavartia, previously assigned to Favartia (s.s.) (Fig. 1A) or Favartia (Murexiella) Clench & Pérez Farfante, 1945 (Fig. 1B) [1], [2] and [18], belongs to these tropical sculptured muricids. However, the significant aspects of their sculptural patterns appear too superficially defined by the traditional descriptive method [7], [10], [11] and [14]. This paper examines muricid morphology more closely by researching structural homologies, which are tested by a cladistic analysis to delineate the sculptural patterns in the Favartia/Eofavartia complex. It emphasises the importance of this method for a better understanding of muricid radiation, based on fossil shells.

A difficulty encountered when describing muricids shells is how to accurately characterise their diverse sculptural elements, which are usually counted without being precisely identified [7] and [10]. In addressing this problem of comparative morphology, structural homologies have been proposed referring to the construction of the characters [14] or to their ontogenetic (homologous order of appearance) and topological correspondences [10] and [11]. For the cords and the denticles of outer lip, these structural homologies are codified by text-conventions, giving a precise presentation of the characters for the whole family. They have already been presented elsewhere [6], [10], [11], [12] and [13] and are briefly summarised below.

Repository: MNHN–LP: Muséum national d'histoire naturelle, Paris, laboratoire de paléontologie; MNHN-BIMM: Muséum national d'histoire naturelle, Paris, laboratoire de biologie des Invertébrés marins et malacologie; DLE: Daniel Ledon sample; JPO: Jacques Pons sample. Text-conventions [11] and [12]: P (capital letter) = primary cords (cord appearing in first order); s (small letter) = secondary cords (cord appearing in second order); D (capital letter) = denticles of the outer lip.

Class Gastropoda Cuvier, 1797

Section Neogastropoda, Thiele, 1929

Family Muricidae Rafinesque, 1815

Subfamily Muricopsinae Radwin & d'Attilio, 1971

Genus Eofavartia nov. genus (Fig. 1D–F)

Etymology. From the Greek Eos, dawn (i.e., early), combined with Favartia.

Type-species. Murex frondosus Lamarck, 1803.

Species included. Eofavartia frondosa, Lutetian, France (Fig. 1D); Eofavartia sp. (= Murexsul sp., Vokes, 1994: 81, pl. 8, fig. 1) [10] and [18], Ypresian, Alabama, USA; E. mantelli (Conr., 1834), Bartonian, Alabama (Fig. 1E); E. marchandi (Coss., 1903), Bartonian, France (Fig. 1F); E. subrudis (d'Orb., 1850) [10], Bartonian to Priabonian, France, England and Germany.

Diagnosis. Biconic shell of 30 to 40 mm in height, with six or seven teleoconch whorls. Cords P1 to P6 on first whorl. P1 to P3 visible on spire. Last whorl with IP on infrasutural ramp, P1 to P6 on convex part of the whorl, ADP, MP and ABP on siphonal canal. IP and ABP poorly developed, P6 well marked. Surface of primary cords with deep spiral grooves. Axial sculpture with five to nine varices. Adapertural face of the varices with imbricated growth laminae. Subcircular aperture with closed, rounded shoulder sinus. Outer lip with ID, D2 to D6 (generally split) and D1 (atrophied or absent). Erect, smooth columella with inductura. Siphonal canal open. Aragonitic microstructure.

Comparisons. Eofavartia may be compared with Favartia (Muricopsinae) and Early Neogene species [H. heptagonata (Bronn, 1831), Europe; H. crispangula (Heilp., 1886), North-America] of Homalocantha Mörch, 1852, a genus attributed to the Muricinae [5] or to the Muricopsinae [18]. Homalocantha has an atypical muricine radula, also differing from the muricopsine radula [5]. A well-marked P6 and a strong development of the siphonal cords ADP and MP are features of Eofavartia, which are also widespread in modern Muricinae [10] and [12] and in Early Neogene Homalocantha. These Homalocantha have a rounded and closed shoulder sinus as in Eofavartia, but differ by P6 on the siphonal canal, by poorly marked growth laminae, by varices partially covering the preceding whorl and, by lacking D6 and spiral grooves on the cords. The Muricopsinae have an atrophied P6 on the siphonal canal and lack D6 [10] and [11]. These features do not occur in Eofavartia. Among the Muricopsinae, Eofavartia seems most similar to Favartia (s.s.), F. (Murexiella), F. (Caribiella) Perilliat, 1972 (Fig. 1A–C), Pygmaepterys Vokes, 1978 and Maxwellia Baily, 1950 by having varices with imbricated growth laminae and primary cords with spiral grooves. The closest similarities are with Favartia and Maxwellia, both of which possess a closed and rounded shoulder sinus. Eofavartia differs from Maxwellia by the microstructure lacking a calcitic layer, and from Pygmaepterys by the absence of columellar ornamentation.

Subfamilial discussion and cladistic analysis. Although Eofavartia shows closest similarities with Favartia, it shares some character states with modern Muricinae (well marked P6 on the base of the convex part of the whorl and presence of D6) [10], [11] and [12] and early Homalocantha (well marked P6, Fig. 2), lacking in Favartia (Fig. 1) and other Muricopsinae [10] and [11]. Consequently, the subfamilial assignment of Eofavartia needed to be examined closely. A short cladistic analysis using 11 consistent characters and 11 taxa including two Early Neogene Homalocantha, three modern Muricinae and three Muricopsinae was helpful in exploring this problem. Paziella dyscrita (Coss., 1889) [10] and [13] and P. argillacea (Wrig., 1930), from the European Ypresian, are chosen as outgroups. The matrix (Fig. 3) was processed by the computer programs Hennig86 (opt.: i.e., unordered) [3] and Parsimony Jackknifer (opt.: cut 50; 10 000 reps) [4]. Hennig86 generated a single cladogram (ci: 0.87; ri: 0.91) having a basal clade (clade 18) supported by two apomorphies (developed ADP and MP, reduced D1) (Fig. 4A). This is divided into two clades, the modern muricine clade 16, congruent with anatomical studies [15] and supported by two apomorphies (growth laminae parallel to the growth direction and anal sulcus), and the clade 17. Clade 17, supported by a single apomorphy (closed and rounded shoulder sinus) and less supported by the parsimony jackknifing treatment (Fig. 4B), includes Eofavartia and the clade 15, supported by two apomorphies (P6 on the siphonal canal and the loss of D6). The clade 15 is divided in the Homalocantha and the Muricopsinae clades, both congruent with the radular characters [16]. Eofavartia is regarded as a basal taxon, because it lacks autapomorphies of the Muricopsinae and the Homalocantha clades. Finally, the analysis highlights the important differences between Eofavartia and Favartia, but suggests that Eofavartia and Homalocantha are more closely related to the Recent Muricopsinae than the modern Muricinae, representing potential sister groups. According the obtained hypothesis, these two genera may be provisionally attributed to the Muricopsinae.

One important consequence of the individualisation of Eofavartia presented here is a reconsideration of the stratigraphic range of Favartia. The oldest authenticated representative of Favartia is F. vaughani (MacNeil & Dockery, 1984) [9] and [10] from the North American Rupelian. Considering that the oldest Muricopsis and Murexsul are known, respectively, from the Rupelian (Europe) and from the Priabonian (Europe and Australia), the radiation of the Recent Muricopsinae (clade 13) occurred at least in the Late Palaeogene [10]. These results are also consistent with the cladistic analysis, suggesting that the Recent Muricopsinae and Homalocantha are more derived than Eofavartia. On the other hand, the loss of D6, a character state regarded as an apomorphy of the Recent Muricopsinae, indicates a simplification of the aperture. The adaptive significance of this change, if any, remains unexplained, but the Muricopsinae live in tropical and warm temperate shallow waters, where apertural changes due to predation pressure have already been reported in the Muricidae and in other gastropod families [17].

A convergent shell evolution has often been proposed to explain taxonomic problems in the Muricidae family [8], but the superficiality of the traditional descriptive method may also have been a major cause of confusions [6], [10] and [14]. This view is confirmed in the case of Eofavartia. The description of this genus and the associated analysis of the shell morphology are based on a clarified presentation of the structural homologies, thus highlighting its distinction from Favartia. It is clear, therefore, that a reconsideration of the shell morphology using this more rigorous descriptive method is essential for a better understanding of the systematic and the evolutionary history of muricid radiation.