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Lomax, D. R. and Massare, J. A. 2018. A forefin of Leptonectes solei from the Lower Jurassic (Pliensbachian) of Dorset, UK. Proceedings of the Geologists’ Association. A forefin of Leptonectes solei from the Lower Jurassic (Pliensbachian) of Dorset, UK

DEAN R. LOMAX1* and JUDY A. MASSARE2

1School of Earth and Environmental Sciences, The University of Manchester, Oxford Road,

Manchester, M13 9PL, U.K., [email protected]

2Department of Earth Sciences, State University of New York, College at Brockport,

Brockport, NY, 14420, U.S.A., [email protected]

* corresponding author

Abstract

______

An incomplete forefin in the collections of the National Museum of Wales, Cardiff, is herein assigned to the rare leptonectid species, Leptonectes solei, known only from the west Dorset coast. It is only the third specimen of the species and is smaller than both the holotype and referred specimen. The new specimen is from the Lower Jurassic (lower Pliensbachian,

Charmouth Mudstone Formation, Tragophylloceras ibex Ammonite Zone,

Acanthopleuroceras valdani Ammonite Subzone) of Seatown, Dorset, UK. It confirms that all three species of Leptonectes were present in the Pliensbachian and expands the known diversity of ichthyosaurs for that time period. We show that isolated forefins of Leptonectes can be assigned to a species with a high degree of confidence.

Key words: Ichthyosauria; Leptonectes solei; Pliensbachian; Early Jurassic; Leptonectidae. 1. Introduction

The genus Leptonectes McGowan 1996, is one of the common ichthyosaurian taxa from the

Lower Jurassic of the UK and is known from many complete and partial skeletons.

Leptonectes has a wide geographic distribution, with specimens recorded from various counties in the UK (Somerset, Dorset, Leicestershire, Gloucestershire, Warwickshire and possibly Nottinghamshire) (McGowan and Motani, 2003; Smith and Radley, 2007; Lomax and Gibson, 2015) and several occurrences from Europe, including specimens from

Switzerland (Maisch and Reisdorf, 2006), Belgium (Godefroit, 1992) and Germany (Maisch,

1999). The genus also has a long stratigraphic range with specimens recorded from the Upper

Triassic (Rhaetian) to the Lower Jurassic (upper Pliensbachian) (Storrs, 1994; Maisch and

Reisdorf, 2006; Lomax et al., 2018). However, specimens recorded from the Rhaetian of the

UK are from historical collections and lack precise stratigraphic information and some might actually be from the Lower Jurassic (Hettangian).

Leptonectes belongs to the family Leptonectidae Maisch, 1998a, which also includes

Eurhinosaurus, Excalibosaurus and Wahlisaurus (Abel, 1909; McGowan, 1986; Lomax,

2016). There are three species of Leptonectes. The type species, Leptonectes tenuirostris, is a moderately sized taxon (<4 m) recorded from the UK and Europe and has a long stratigraphic range from the (?)Rhaetian to Pliensbachian (McGowan, 1996; McGowan and Motani, 2003;

Maisch and Reisdorf, 2006). One specimen, probably referable to this species, has been found with embryos (Lomax and Massare, 2012). As noted by previous workers, considering the amount of variation in specimens currently identified as L. tenuirostris, the species requires a detailed revision that is beyond the scope of this study (Maisch and Reisdorf, 2006;

Lomax, 2016; Lomax et al., 2018). The second species, L. solei, is a large taxon (>7 m) known from two specimens from the Sinemurian of Dorset, UK (McGowan, 1993). The third species, L. moorei, is known from a single anterior skeleton, including a fairly complete skull, from the Pliensbachian of Dorset (McGowan and Milner, 1999).

Here, we report on a large, incomplete isolated forefin (NMW 91.296.2.2) from the

Pliensbachian (Lower Jurassic) of Seatown, Dorset. We assign the specimen to Leptonectes solei, only the third specimen of that species.

1.1 Institutional Abbreviations

BRSMG, Bristol City Museum and Art Gallery, Bristol, UK; MHN, Museon, The Hague,

Netherlands; NHMUK, The Natural History Museum, London, UK; NMW, National

Museum of Wales, Cardiff, UK.

2. Materials and geological setting

NMW 91.296.2.2 is a large, isolated and incomplete forefin lying mostly in matrix, but it can be viewed on both the dorsal and ventral sides (Fig. 1). It is impossible to determine whether the fin is a left or right because the humerus has been restored and might not belong with the rest of the fin (see below). One side of the fin has been fully prepared and the bones coated with a preservative, which clearly distinguishes them from the matrix (Fig. 1A). The reverse side has not been prepared, although the humerus has been treated with preservative (Fig.

1B). This side exposes several of the same elements as the prepared side, but some are covered by matrix. Most are damaged, and the cracks have been filled.

The entire proximal region of the humerus is made of plaster, although most of the shaft and the distal end are original bone (Fig. 1). The humerus has been attached to the rest of the fin with glue or epoxy, which is clearly visible (Fig. 1). Although, the distal end of the humerus is expanded anteroposteriorly as is typical of the genus, it is not as wide as the radius and ulna, which are significantly wider anteroposteriorly than their respective facets on the humerus. The distal facets of the humerus are thus too small for the radius and ulna. This strongly suggests that the humerus does not belong with the specimen and thus it is a composite. Ichthyosaur composites are common in historic collections (McGowan, 1990;

Maisch, 1998b; Massare and Lomax, 2014, 2016b) and even recently collected material can be composites (e.g. Lomax and Sachs, 2017). The remainder of the forefin is authentic.

The specimen was collected in 1990 from Seatown, Dorset, and acquired by the

NMW in 1991 from The Old Forge Fossil Company, Dorset. It is from the Lower Jurassic, recorded as “1 foot below the belemnite stone”. The term ‘belemnite stone’ refers to the top of the Stonebarrow Marl Member (‘Belemnite Marls’) of the Charmouth Mudstone

Formation. It is lower Pliensbachian, from the Tragophylloceras ibex Ammonite Zone,

Acanthopleuroceras valdani Ammonite Subzone according to museum records. The single specimen of Leptonectes moorei, also from Seatown, was from a horizon less than a metre below this.

3. Description of specimen

The humerus is not included in the description or species assignment because it probably does not belong with the rest of the forefin. See previous section for details.

Systematic Palaeontology

Order Ichthyosauria de Blainville, 1835

Family Leptonectidae Maisch, 1998a

Genus Leptonectes McGowan, 1996

Leptonectes solei (McGowan, 1993) Emended diagnosis: As in McGowan (1993) with the addition of: forefin with three digits

(II, III, IV); carpals, metacarpals, and phalanges rounded and widely spaced.

Specimen number: NMW 91.296.2.2.

Element: Incomplete forefin.

Locality: Seatown, Dorset, England, UK.

Stratigraphy: Lower Jurassic (lower Pliensbachian), Acanthopleuroceras valdani Ammonite

Subzone, Tragophylloceras ibex Ammonite Zone, Charmouth Mudstone Formation.

Description:

NMW 91.296.2.2 has three primary digits (II, III and IV; Motani, 1999) preserved up to metacarpal two of digit II, the first phalanx of digit III, and metacarpal four of digit IV (Fig.

1). Excluding the humerus, it has a preserved length of at least 24 cm and maximum width of

17 cm. Digit II has the largest elements of the three digits. Only the radius, ulna, and proximal carpals could have possibly interlocked, although even these are not in contact with one another as preserved in this specimen and in the referred specimen of L. solei

(McGowan, 1993).

The radius is large, rounded and lacks a notch on the leading edge, as in Leptonectes solei and L. moorei (McGowan, 1993; McGowan and Milner, 1999), but unlike L. tenuirostris, which always has a notched radius, even in embryos (McGowan, 1993;

McGowan and Milner, 1999; McGowan and Motani, 2003; Lomax and Massare, 2012). No elements are notched in NMW 91.296.2.2. Notching of other elements of the leading edge is common in specimens of L. tenuirostris. Only a single element is notched in the holotype of

L. solei, but its position in the fin is unknown. The referred specimen (MHN 96270), which has a more complete forefin, has no notched elements. L. moorei also lacks notching.

The ulna of NMW 91.296.2.2 is sub-rectangular and proximodistally slightly shorter than the radius, but anteroposteriorly approximately equal to it. There is no foramen between the radius and ulna. Many specimens of L. tenuirostris (McGowan and Motani, 2003) have a circular foramen between the radius and ulna (Fig. 2), although other specimens do not. L. moorei might have a very small, narrow, elliptical foramen between the radius and ulna

(McGowan and Milner, 1999), but L. solei lacks a foramen (McGowan, 1993).

The radiale is anteroposteriorly elongate, but round; it is noticeably larger than the intermedium and ulnare. Similarly, the intermedium is rounded and is larger than the ulnare.

The ulnare is small and circular. The distal carpals, metacarpals, and the one preserved phalanx are oval, with the long axis oriented anteroposteriorly.

Leptonectes tenuirostris has a notched radius and a broad notch on the tibia as well, and so NMW 91.296.2.2 cannot be assigned to that species, the most common member of the genus. The forefin elements of NMW 91.296.2.2, and particularly the carpals, are not polygonal nor closely packed like those of L. moorei (McGowan and Milner, 1999, fig. 4), but are rounded and widely spaced, similar to those of the only referred specimen of L. solei

(Fig. 2; McGowan, 1993, fig. 5C,D). The forefin of the holotype of L. solei (BRSMG Ce

9856) is disarticulated and incomplete (McGowan, 1993, fig. 3A), but the elements distal to the radius and ulna are oval, not polygonal. In addition to the spacing and shape of elements, the elements of digit II in L. solei (i.e., MHN 96270) are larger than those in digit IV, as in

NMW 91.296.2.2, whereas L. moorei shows little size difference among the digits (Fig. 2).

The studied specimen has three digits as in L. solei, whereas L. moorei has four digits.

However, the fourth digit is either digit V or a posterior accessory digit. We suggest it is most likely the latter because the element contacts the ulnare posteriorly rather than distally.

Therefore, NMW 91.296.2.2 is assigned to L. solei. It is much smaller than the holotype but only slightly smaller than MHN 96270, based on the size of the radius. Note that the hindfin of L. solei might have only two digits (McGowan, 1993, fig. 5A), so the described specimen is not a hindfin. Remarks:

The rounded, widely spaced carpals and metacarpals of NMW 91.296.2.2 are distinctly different from the polygonal, closely packed elements in other Lower Jurassic genera (e.g.,

Ichthyosaurus, Protoichthyosaurus, Temnodontosaurus, Stenopterygius, Hauffiopteryx,

Suevoleviathan) (Motani, 1999; McGowan and Motani, 2003; Lomax et al., 2017).

Furthermore, many of the Lower Jurassic taxa have a notch on the anterior edge of the radius, carpal two, and/or metacarpal two (e.g., Temnodontosaurus, Stenopterygius, Hauffiopteryx)

(Motani, 1999; McGowan and Motani, 2003; Maisch, 2008). Among the Leptonectidae,

Eurhinosaurus and Excalibosaurus have closely packed, polygonal elements in the carpus

(McGowan, 2003; McGowan and Motani, 2003). Both known specimens of Excalibosaurus have a notched radius (McGowan, 2003). Eurhinosaurus usually has notching in at least one element of the carpus (McGowan and Motani, 2003). The forefin of Wahlisaurus, another leptonectid, is unknown, but the tibia is notched in the hindfin (Lomax, 2016), suggesting that the forefin might also have notched elements. Similar arguments can be made for hindfins of these Lower Jurassic taxa. Thus NMW 91.296.2.2 can be assigned with confidence to

Leptonectes, as above.

4. Discussion

Johnson (1977) demonstrated that juveniles of Stenopterygius have rounder, more widely spaced fin elements than adults but this might not be the case for all genera (Lomax et al., 2017). The differences between Leptonectes solei and L. moorei, however, are not ontogenetic: the holotype and only specimen of L. moorei is much smaller than previously described specimens of L. solei (McGowan, 1993; McGowan and Milner, 1999), yet its fin elements are more tightly packed and polygonal. Thus the shape, relative size, and spacing of fin elements seem to be valid characters for distinguishing the two species. Pliensbachian ichthyosaurs are rare. Only a few substantially complete or well- preserved specimens have been reported. Many ichthyosaurs in historic collections lack detailed stratigraphic information, so some of those could be from the Pliensbachian.

Nevertheless, ichthyosaurs previously recorded from the Pliensbachian include: Leptonectes moorei, Ichthyosaurus anningae, and I. conybeari from Dorset, UK (McGowan and Milner,

1999; Lomax and Massare, 2015; Massare and Lomax, 2016a), and Leptonectes tenuirostris from Switzerland (Maisch and Reisdorf, 2006). Hungerbühler and Sachs (1996) described an incomplete skull of Temnodontosaurus from Bielefeld, Germany. Maisch and Hungerbühler

(1997) recognized Temnodontosaurus nuertingensis (von Huene, 1931) from Nürtingen,

Germany, although the validity of the species has been questioned (McGowan and Motani,

2003). Additionally, ‘Ichthyosaurus’ numismalis (Fraas, 1892), possibly referable to the genus Leptonectes (Maisch, 2010), has also been reported from the lower Pliensbachian of

Germany. Thus, the studied specimen, herein identified as Leptonectes solei, adds another species to the Pliensbachian diversity. Of particular note, this specimen demonstrates that all three species of Leptonectes were present in the Pliensbachian.

Acknowledgements

We would like to thank Cindy Howells and Caroline Buttler at NMW for providing access to study the specimen. We also thank Michael Maisch and Benjamin Moon for their helpful reviews.

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Figures

Figure 1. NMW 91.296.2.2: An isolated forefin herein assigned to Leptonectes solei. A. Best preserved side, anterior to the right. B. Poorly preserved and unprepared underside, anterior to the left. The humerus probably does not belong with the rest of the fin and it is impossible to determine whether this is a left or right forefin, and thus which is ventral or dorsal view.

Abbreviations: 2, distal carpal two; 3, distal carpal three; 4, distal carpal four; I, intermedium; ii, metacarpal two; iii, metacarpal three; iv, metacarpal four; p1, first phalange of digit two;

R, radius; Ra, radiale; U, ulna; Ul, ulnare. Dashed line indicates proximal portion of humerus that is made from plaster. Scale measures 10 cm.

Planned for two-column width. Colour online only. Figure 2. Comparative illustrations of the forefins discussed in the text. A. NMW 91.296.2.2:

Forefin specimen shown in Figure 1, reversed to show the same orientation as B and C. B.

MHN 96270: Left forefin in dorsal view of referred specimen of Leptonectes solei, drawn from McGowan (1993, fig. 5). C. NHMUK R14370: Left forefin in dorsal view of only specimen of Leptonectes moorei. D. BRLSI M3575: Left forefin in dorsal view of a typical

Leptonectes tenuirostris forefin. The posterior digit is either digit V or an accessory digit.

Anterior to the left for all specimens. Abbreviations: 2, distal carpal two; 3, distal carpal three; 4, distal carpal four; I, intermedium; ii, metacarpal two; iii, metacarpal three; iv, metacarpal four; p, phalanx; R, radius; Ra, radiale; U, ulna; Ul, ulnare; v, metacarpal five.

Planned for two-column width. Colour online only.