LEPTONECTIDAE Maisch, 1998
|
publication ID |
https://doi.org/ 10.26879/1205 |
|
persistent identifier |
https://treatment.plazi.org/id/3D56879C-DA64-4D09-FC54-FA7935A9FE04 |
|
treatment provided by |
Felipe |
|
scientific name |
LEPTONECTIDAE Maisch, 1998 |
| status |
|
Family LEPTONECTIDAE Maisch, 1998 Leptonectidae indet.
Referred material. MNHNL LM266, from the lower Pliensbachian (Valdani to Luridum Ammonite subzones of the Ibex Ammonite Zone) of the Cloche d’Or locality, central Luxembourg.
Snout
Two contiguous fragments of a rostrum are preserved over 51 mm, containing articulated premaxillae, dentaries, splenials, and teeth ( Figure 5 View FIGURE 5 ). The premaxilla and the dentary are slender, being as dorsoventrally high as the apicobasal height of the corresponding tooth crowns. The premaxillary and dentary fossae are present, in the form of a series of elongated foramina that are 13 mm long and 3 mm high for the premaxillary fossa and slightly longer for the dentary fossa. The anterior part of the splenial is preserved in articulation with the dentary and has a slit-like ventral exposure. The splenial thus possesses a long ventral exposure, which is similar to that of Leptonectes moorei (V.F. pers. obs. holotype specimen NHMUK PV R 14370) and differs from the condition seen in the rather short-snouted Hauffiopteryx spp. , where the nasal rapidly protrude from the dorsal margin of the rostrum, extending anteriorly as far (or further) than the splenial ( Marek et al., 2015; Maxwell and Cortés, 2020). In MNHNL LM266, the nasal is not even present internally, indicating its dorsal exposure is de facto set more posteriorly than that of the splenial.
The tooth crowns are slightly recurved and markedly elongated: the apicobasal height / basal diameter ratio is 11 mm / 4 mm = 2.75 ( Figures 5 View FIGURE 5 , 6 View FIGURE 6 ), very similar to some species of Stenopterygius (Maxwell, Fernández, et al., 2012), Hauffiopteryx ( Maxwell and Cortés, 2020) , and in leptonectids such as most species of Leptonectes (Fraas, 1892; Huene, 1922; McGowan, 1989, 1993) (with the exception of Leptonectes moorei [ McGowan and Milner, 1999]), Eurhinosaurus longirostris ( Reisdorf et al., 2011) , and Wahlisaurus massarae (Lomax, 2016) ( Figure 6 View FIGURE 6 ). This condition is clearly distinct from the stouter teeth seen in Temnodontosaurus spp. (Fraas, 1891; McGowan, 1974; Godefroit, 1993), Suevoleviathan spp. ( Maxwell, 2018), Protoichthyosaurus ( Lomax and Massare, 2018b; Lomax et al., 2019), and some species of Ichthyosaurus (Fraas, 1891; Maisch, 1997; Maisch et al., 2008). Another similarity with Stenopterygius , Hauffiopteryx , and leptonectids is the reduction of longitudinal striations along the crown ( Maisch, 1998; Maxwell, 2012; Lomax, 2016; Fernández et al., 2018) ( Figures 5 View FIGURE 5 , 6 View FIGURE 6 ). Yet, sparse striations are present in MNHNL LM266 but do not reach the apical quarter of the crown (as in Leptonectes spp. ; Figure 6 View FIGURE 6 ), and many crowns exhibit a single basal ring, as in Wahlisaurus massarae (Lomax, 2016) , Leptonectes spp. ( Figure 6 View FIGURE 6 ), and Eurhinosaurus longirostris (in which more than one basal ring is usually present (e.g., Godefroit, 1994; Fischer et al., 2011)). The acellular cementum ring is not ridged, unlike in Temnodontosaurus ( Godefroit, 1993; Maxwell, Caldwell, et al., 2012), Ichthyosaurus ( McGowan, 1973; Vincent et al., 2014), and Protoichthyosaurus ( Lomax et al., 2019) . The root bears fine striations, but only in the basal half, as in some leptonectids (Fraas, 1892; Godefroit, 1994). This differs from the condition of Leptonectes moorei ( Figure 6 View FIGURE 6 ), Leptonectes solei ( Figure 6 View FIGURE 6 ), Stenopterygius ( Godefroit, 1994) , and Hauffiopteryx ( Maxwell and Cortés, 2020) , where the root ridges reach the base of the acellular cementum ring. The root is slightly expanded mesiodistally and slightly compressed labiolingually, another usual feature that is often present in leptonectids (Fraas, 1891; Reisdorf et al., 2011; Lomax, 2016).
The small size of these rostral fragments can be interpreted in two ways: (i) it belongs to a small individual, distinct from the other remains from the site or (ii) it represents the anterior extremity of a long snouted ichthyosaurian, where the tip of the rostrum can be extremely small compared to the size of the animal ( McGowan, 1993, 2003). The presence of a discontinuous premaxillary fossa and, to a lesser extent, the absence of both a nasal and a maxilla (even internally) suggests that this fragment was located from the anterior quarter of the rostrum. The exposure of the splenial is less indicative of a position within the rostrum, because at least one leptonectid ( Leptonectes moorei ) possesses a very long ventral splenial exposure (V.F. pers. obs. holotype specimen NHMUK PV R 14370). In the absence of other evidence and given the taphonomy, stratigraphy, and the controlled excavations, we regard all the ichthyosaur fragments described here as likely belonging to a single individual. Even so, we detail the taxonomic information present in all fragments in isolation ( Table 1), and the conclusions of the paper do not rely on this association .
Possible Surangular
Two contiguous fragments are interpreted here as the posterior part of a large right surangular ( Figures 4 View FIGURE 4 , Supplementary Information Figure 1 View FIGURE 1 ). The lateral surface is convex and forms a shallow anteroposterior ridge at mid-height. The medial surface is concave and forms a wide prominent ridge located within the ventral half of the medial surface. The ventral surface is rounded while the bone tapers dorsally to a thin, saddle-shaped ridge, as might be seen close to the coronoid process in neoichthyosaurians ( Sollas, 1916; McGowan, 1973). The perfectly straight and parallel bone fibres on the concave side ( Figures 4 View FIGURE 4 , Supplementary Information Figure 1 View FIGURE 1 ) are features of ichthyosaurian surangulars (V.F. pers. obs. on material from MNHNL, MHNH). We do not derive taxonomic information from these fragments, because we consider their identification too tentative.
Centra
Two centra fragments are present. One ( Figures 4 View FIGURE 4 , 5 View FIGURE 5 ) is a large dorsal centrum with an anteroposterior length of 52 mm and a diameter certainly above 100 mm (as the preserved portion is 100 mm wide), and probably between 120 and 130 mm. One apophysis is visible; the fact that the dorsal part of the centrum is missing suggests that this apophysis is the parapophysis. This apophysis is> 20 mm long anteroposteriorly and 15 mm high dorsoventrally. It is teardrop-shaped, with anterior ridge connecting to the anterior margin of the centrum, as is typically of parvipelvians and clearly distinct from the dorsoventrally elongated diapophyses and often minuscule parapophyses seen in shastasaurids ( Merriam, 1902; Sander, 1997; Fischer et al., 2014). Ventrally, the centrum forms a slightly concave surface. We interpret this as a parapophysis, present on the lateral surface of the centrum at mid-height. Accordingly, this centrum is regarded as an anterior dorsal centrum. The large size and the aspect ratio match with the morphology of Temnodontosaurus spp. ( McGowan, 1974; Godefroit, 1993; Martin et al., 2012; Swaby and Lomax, 2021) and Leptonectes solei ( McGowan, 1993) . However, most other leptonectids have smaller centra, with diameters ranging from 25 to 65 mm ( Huene, 1951; McGowan and Milner, 1999; McGowan, 2003; Lomax, 2016).
A second fragmentary bone solely consists of the margin of the articular surface of a small centrum. The lateral or ventral surface is concave and lacks chevron facets or apophyses. It is interpreted as a posterior caudal centrum, given the small size.
Ribs and Gastralia
Several tens of rib fragments are preserved ( Figures 5 View FIGURE 5 , Supplementary Information Figure 1 View FIGURE 1 ). Most have wide anterior and posterior grooves, giving the rib an “8” shape in cross-section, as is usually the case in neoichthyosaurians. However, the dorsal ribs of Leptonectes tenuirostris , Temnodontosaurus azerguensis , and some species of Ichthyosaurus seem to lack the grooves, giving their ribs a rounded cross-section ( Martin et al., 2012; Lomax and Massare, 2016). In one fragment, one of the grooves can be observed vanishing medially; the largest fragments have a single groove, and the opposite side is gently convex ( Figure 5 View FIGURE 5 ). A series of other, small fragments are straight and exhibit a rugose texture; we interpret these as fragmentary gastralia.
| NHMUK |
Natural History Museum, London |
No known copyright restrictions apply. See Agosti, D., Egloff, W., 2009. Taxonomic information exchange and copyright: the Plazi approach. BMC Research Notes 2009, 2:53 for further explanation.