Cephalopoda
Cephalopods in the form of ammonites, belemnites and teuthoids occur frequently in the Solnhofen Limestone and are sometimes exceptionally well preserved (Fuchs et al. 2015). Many have been reported with aspects of their soft tissues preserved, including impressions of tentacles with hooklets in belemnites and teuthoids (Klug et al. 2016), the musculature of the mantle in teuthoids (Klug et al. 2015) and the siphuncle and pellicula in ammonites (Keupp 2007). Although ammonites make up a large portion of the fossils from the Solnhofen plattenkalks, they are often poorly preserved due to the aragonitic composition of the shell which is readily dissolved during diagenesis (Seilacher et al. 1976). This dissolution leaves behind an external or composite mould in the matrix, occasionally with the original outline and the calcitic aptychi in the body chamber (Keupp 2007).
Although rarely preserved, the original shell can be observed once replaced by calcite in the phragmacone (Fig. 4) and the body chamber (Arratia et al. 2015). The body chamber is present within the halo of the dissolved shell and can be enhanced under UV and LSF (Fig. 6). The siphuncle, pellicula and other non-mineralised elements are often phosphatised in Solnhofen ammonites and these may fluoresce more intensely than the remaining shell (Keupp 2007). UV fluorescence displays colour differences on these ammonites (Fig. 6C), but the contrast between non-mineralised parts and the surrounding shell is lacking, when compared with the LSF image of specimen LB 4 (Fig. 6D). It appears that the raised darker areas on isolated aptychi (LB 1) (Fig. 3) are the thick spongy layer on the inside of the aptychus underlying the thinner crenulated outer layer, rather than soft tissues following Lehmann (1976).
Lumbricaria Goldfuss, 1831, a coprolite attributed to ammonites (Janicke 1970) lies on the same slab as an ammonite (Fig. 6) and appears to contain aptychi of a smaller ammonite. Plesioteuthis prisca (Rüppell, 1829), a squid from the Solnhofen Limestone with a central rachis that is revealed in its entirety and fluoresces at two distinct levels under LSF (Fig. 7D). This rachis runs along the centre of the visible body outline, although no soft tissues of the mantle are present (Fig. 7).
Decapoda
Decapod crustacea occur frequently in the Solnhofen plattenkalks and are often well preserved as fully articulated individuals, even in the larval stage (Haug et al. 2008; Winkler 2014). The fossils are valuable for understanding arthropod ontogeny, with specimens that display growth cycles only known from a few other Lagerstätten (J. T. Haug et al. 2009). Numerous studies have been carried out using UV fluorescence, revealing unseen details to allow distinctions between taxa or increasing the specimen-matrix contrast (Haug et al. 2011; Winkler 2012; Audo et al. 2014). The decapods lend themselves to techniques using fluorescence as they hold autofluorescent compounds within their exoskeleton (Haug et al. 2011; Glenn et al. 2013).
The preservation of the fossils studied (LB 6–9), vary from isolated appendages to complete articulated examples. A single walking leg attributed to Aeger tipularius von Schlotheim, 1822 (Fig. 8) is near complete with only a few setae missing from the exposed surface. UV fluorescence enhances the specimen-matrix contrast but fails to fluoresce below the matrix where the complete setae are revealed through LSF (Fig. 8).
Anatomical details of complete specimens of Cycleryon propinquus von Schlotheim, 1822 (Fig. 9) and Antrimpos speciosus Münster, 1839 (Figs 10, 11) are enhanced through fluorescent techniques as previous studies demonstrate (Schweigert & Garassino 2004). However, it must be reiterated that previous studies on Solnhofen material have only used ultraviolet as a means of fluorescence (Keupp 2007; Schweigert 2011; Tischlinger & Arratia 2013). A distal portion of the left first pereiopod on LB 7 is unseen under UV light but is revealed, albeit faintly, under LSF (Fig. 8). In Antrimpos speciosus (LB 8) (Figs 10–11), more detail of the body outline is exposed by LSF under a green 532 nm laser compared with UV and this is emphasised in the magnified portions (Figs 9, 10 B-C, E-F).
As the figures of Solnhofen decapods herein show, the detail revealed under Laser-Stimulated Fluorescence, often surpasses that revealed by ultraviolet fluorescence, with the outline of the entire animal outline being revealed, along with small elements such as antennae and swimmerets (Figs 10, 11). The veneer of the matrix on LB 8 in Figs 10–11 may have obstructed the UV fluorescence and could be prepared further to assist the use of this method. Ultraviolet fluorescence on a specimen of Alcmonacaris winkleri Polz, 2008 (LB 9) reveals a faint outline of the animal while recording colour patterning (Fig. 12). Under LSF, green laser light, this colour information is lost but the animal is revealed in its entirety (Fig. 12D). Techniques to rectify this loss of information have been developed using multiple wavelengths (Kaye et al. 2015). As Figs 9–12 show, the preserved exoskeleton and the body outline fluoresce at different levels because of the autofluorescent compounds within the arthropod exoskeleton.
Vertebrata
The Solnhofen Limestone has achieved much of its fame as a fossil Konservat-Lagerstätten because of the exceptional preservation of its vertebrate fossils, especially those of volant animals such as the earliest unequivocal fossil bird Archaeopteryx and a diverse assemblage of pterosaurs (Beardmore et al. 2017; Schwarz et al. 2019; Longrich et al. 2020) where parts of the flight surfaces are preserved, including wing membranes and feathers (Frey et al. 2003; Jäger et al. 2018; Benton et al. 2019; Kaye et al. 2019a; Foth et al. 2020; Wilkin 2020). Some of this exceptional preservation can be seen in the examples below (Figs 13–16). The hollow bones of pterosaurs are rarely preserved and through LSF the contrast between preserved bone and the imprints of the skeleton in the matrix is exemplified. The dwarf crocodilyform Alligatorellus (Fig. 16) under the blue laser displays soft tissues around the entire body as well as a brighter section at the base of the tail that may represent partial preservation of the caudofemoralis muscle.
However, the exceptional preservation is not restricted to the Tetrapoda, with many fishes also exceptionally well preserved with full articulation and soft part preservation (Konwert 2016; Arratia et al. 2019; Ebert & Lane 2019) as well as reptiles (Tennant & Mannion, 2014) (see Fig. 17). Specimens LB 10–13 (Figs 17–20) vary in completeness from articulated individuals to isolated sources of phosphate, resulting in the high level of fluorescence under UV and LSF (Fig. 12).
The specimens studied vary between right and left lateral views, seen in specimens LB 10 and 11 (Figs 17 and 18 respectively). The fluorescence of these specimens (LB 10–13), results from the vertebrate endoskeleton containing high levels of phosphorus (Tischlinger & Arratia 2013). The fossils in white light contrast little with the surrounding sediment as in many other Solnhofen fossils. Indeed, it may be difficult to see some fossils, especially when weathered, without using fluorescence.
Many Solnhofen fossil fish possess a fully ossified skull, fins, neural and haemal spines made apparent through fluorescence along with thin autocentra (Arratia & Schultze 2013). The autocentra surrounding the dark chordocentra are thin and almost translucent and fluoresce under both UV and LSF. Species of the teleost family Orthogonikleithridae are the most common vertebrates in the Solnhofen Limestones (Konwert 2016). The ossification patterns observed on a specimen of Orthogonikleithrus hoelli Arratia, 1997 (LB 12) (Fig. 19), correspond with previous studies using UV for fluorescence (Tischlinger & Arratia 2013).
Fish fluoresce well under both UV and LSF and although the difference is often minimal, clearly in Fig. 18 LSF surpasses the UV photograph with an increased fluorescence of both the skull and vertebral column completing the structures seen and this results from the higher flux and subsurface illumination.