Laffonia (Ziegler, 1991)

Laffonia as a holocephalan egg capsule

Laffonia is here identified as having a fusiform ribbed capsule accompanied by two narrow striated flanges laterally, resembling a chondrichthyan egg capsule, as already noted by Heer (1877). At least ten capsule morphotypes are recognized among extant and extinct chondrichthyans, relating to the shape and ornamentation of the capsule, the number and morphology of the anterior and posterior appendages (beaks, horns, pedicles etc.) and the size and arrangement of the flange (lateral, spiral etc.; Fischer et al., 2014; Mancusi et al., 2021). Laffonia lacks the spirally twisted flanges typical of Palaeoxyris and Fayolia , two morphotypes (named after the respective genera) known only from the fossil record, with their producers most likely being extinct hybodontiform and xenacanthiform sharks, respectively ( Fischer & Kogan, 2008; Fischer et al., 2011, 2014). Te lack of spiral flanges also differs from extant heterodontid shark capsules ( Fischer et al., 2014). Te single elongated pedicle of Laffonia greatly differs from the pair of curved horns exhibited by egg capsules from extant and extinct batoids (rays), orectolobiformes (carpet sharks), and scyliorhinids (catsharks), and these three groups often exhibit extremely reduced flanges that are much narrower than that of Laffonia . ( Caruso & Bor, 2007; Concha et al., 2010; Fischer et al., 2014; Kiel et al., 2011; Mancusi et al., 2021; Steininger, 1966; Treloar et al., 2006). Te morphology of Laffonia is therefore inconsistent with all currently recognised elasmobranch capsule morphotypes (Ziegler, 1991) and the specimen is thus unlikely to have been produced by an elasmobranch.

In contrast to elasmobranch capsules, holocephalan capsule morphotypes exhibit relatively constrained morphologies from both extant and extinct taxa from across all three modern chimaeroid families ( Fig. 6 View Fig ; Dean, 1906; Didier, 1995; Fischer et al., 2014). Tis morphotype is typically characterised by a fusiform capsule with a beak and pedicle that is all surrounded by laterally striated flanges that often vary in size between taxa ( Fig. 6 View Fig ; Dean, 1906; Fischer et al., 2014). Comparable features between Laffonia , Pseudocaudina and the recent holocephalan morphotype include a fusiform capsule, pedicle and lateral flanges, although the incomplete preservation of the studied specimen prevents comparisons of a potential beak. More specifically, Laffonia exhibits several similarities to extant chimaerid capsules such as narrow lateral flanges, ribbed capsule surface and fine oblique lines like that of Hydrolagus View in CoL ( Fig. 6C–E View Fig ; Dean, 1903). However, the flanges of Laffonia appear to be of equal size along the entire length of the capsule edges and possess a prominent longitudinal ridge. In contrast, the flanges of extant chimaerid capsules are relatively broad along the edges of the anterior and posterior capsule and reduce at the central body sides, with no obvious ridge ( Fig. 6C–E View Fig ; Dean, 1906; Mancusi et al., 2021). In addition, the at least seven longitudinal ribs that Laffonia exhibits on its inferred dorsal and lateral surfaces are not observed in recent holocephalan capsules ( Fig. 6 View Fig ; Berio et al., 2024; Dean, 1906; Mancusi et al., 2021). Tis body ornamentation is more similar to that of the two Lower Pennsylvanian capsules, Vetacapsula and Crookallia, both of which are phylogenetically recovered as most closely related to chimaerid holocephalan morphotype ( Fischer et al., 2014). Both taxa have ribbed capsules that are suggested to possess two narrow lateral flanges, although such flanges have not yet been documented in Vetacapsula ( Fig. 2 View Fig ; Fischer & Kogan, 2008; Mottequin et al., 2022). However, the number of longitudinal ribs on the capsule surface is different among these three taxa, with over twenty ribs suggested in Vetacapsula ( Crookall, 1928; Fischer & Kogan, 2008) and between eight and twenty in Crookallia (Mottequin et al., 2022). Laffonia and Crookallia lack a prominent middle ridge (dorsal keel) that is obvious in Vetacapsula and extant chimaerid capsules ( Figs. 2 View Fig , 6C– E View Fig ; Fischer et al., 2014). A pedicle length exceeding body length is known from Vetacapsula, Crookallia and recent holocephalan capsules, but the incomplete preservation of the Laffonia pedicle prevents us from determining whether the Jurassic fossil shared this trait. In contrast to the Pennsylvanian fossil and modern chimaerid capsules, Laffonia possesses a larger central body and its body length–width ratio is closer to that of recent rhinochimaerid capsules than to any other holocephalan group ( Table 1).

Te morphological similarities between Laffonia , the Pennsylvanian fossil capsules and modern chimaerid capsules are supported by our phylogenetic analysis ( Fig. 5 View Fig ). Te unresolved polytomy relationships are likely due to the low number of characters and the incomplete preservation of the Laffonia capsule. Discovering additional material in the future may help to further clarify the phylogenetic position of Laffonia .

Evolutionary significance

Crookallia and Vetacapsula from the Carboniferous (Pennsylvanian) supposedly represent the oldest records of capsules that were produced by holocephalans ( Fischer et al., 2014; Mottequin et al., 2022), although there is a possibility that fossil capsules from the Middle or Late Devonian ( Carr & Jackson, 2018; Chaloner et al., 1980) that have been regarded as placoderms could actually be from holocephalans ( Fischer et al., 2014). However, the absence of transitional forms in both morphological and stratigraphical contexts makes these assignments tentative. Te most definite holocephalan capsules are known from the Late Triassic of New Zealand (Gottfried & Fordyce, 2015) and Yakutia, Russia (Vozin, 1968), while most fossil capsules stem from the Jurassic and Cretaceous and few remains from the Paleogene ( Fig. 7 View Fig ; Brown, 1946; Warren, 1948; Obruchev, 1967; Harrison et al., 2021; Duffin et al., 2022; Kiel et al., 2024; Johns et al., in press). Nevertheless, all currently known Mesozoic fossil capsules exhibit fusiform bodies with broad lateral flanges that greatly resemble either modern callorhinchid or rhinochimaerid capsules ( Brown, 1946; Duffin et al., 2022; Gottfried & Fordyce, 2015; Harrison et al., 2021; Obruchev, 1967; Stahl, 1999; Vozin, 1968; Warren, 1948). Until now, 12 recognised species have been assigned to the ichnogenus Vaillantoonia Meunier, 1891 , previously used under the generic name Chimaerotheca Brown, 1946 (Kiel et al., 2024), given that the convergence of capsule morphologies and the uncertainty of producers make it difficult to confidently assign them into specific extant genera, although such attempts were proposed in several studies (Obruchev, 1967; Stahl, 1999; Vozin, 1968). In contrast, no fossil capsule that possesses morphological features similar to either chimaerid capsules or Carboniferous taxa has been found in the Mesozoic, resulting in a long and uncertain ghost lineage ( Fischer et al., 2014), although potential chimaerid body fossils go back at least to the Cretaceous ( Duffin, 2001).

Laffonia and Pseudocaudina represent the first example of a chimaerid-like capsule from the Mesozoic, increasing the morphological diversity of known Mesozoic egg capsule morphotypes ( Fig. 7 View Fig ). Te reassignment of Laffonia not only supports the hypothesis that Vetacapsula and Crookallia were produced by holocephalans instead of elasmobranchs ( Fischer et al., 2014), but that Laffonia most likely represents a transitional form between known Carboniferous morphotypes and extant chimaerid capsules. Te capsule surfaces in Laffonia, Crookallia and Vetacapsula are all ornamented with several longitudinal ribs, indicating a ribbed capsule surface that likely represents an ancestral state of the total-group chimaerid egg capsules. Compared to the Carboniferous taxa, the number of longitudinal ribs has been reduced in Jurassic Laffonia , and only one prominent dorsal middle ridge is retained in extant chimaerid capsules. However, considering a similar middle ridge is possibly present in Vetacapsula , it is uncertain whether the prominent middle ridge is a plesiomorphic or a derived feature from the longitudinal ribs until more chimaerid fossil capsules are revealed. A narrow lateral flange is present both in Crookallia and Laffonia . However, the flange surface is likely smooth in Crookallia while it is striated in Laffonia . Tey are different from the ribbed flange in extant chimaerid capsules, indicating that the modern narrow flange may have appeared even later. More broadly, the morphological differences between fossil and recent capsules also indicate changes in holocephalan body size evolution even in the absence of body fossils. For example, it is reported in modern holocephalans that total capsule length is around one fifth to one quarter the body length of the producer ( Dean, 1904, 1905). Assuming similar relationships for extinct taxa, it is reasonable to suggest that Laffonia was produced by a larger individual than the producers of the Carboniferous capsules, indicating an increase in body size of chimaerid-like holocephalans between the Carboniferous and Jurassic. However, the possibility that the Carboniferous capsules were produced by sexually mature, but still skeletally immature individuals cannot be ruled out, nor can the possibility of preservation bias in the Carboniferous in favour of smaller capsules.