Leedsichthys was a big fish.
For those of you who are kindly supporting me on Patreon, you will have noticed that I have been working on a rather unusual skeletal diagram over the past few months, that of Leedsichthys problematicus, a large Jurassic pelagic filter-feeding fish.
Since I had never created a fish skeletal diagram and had limited knowledge of fish anatomy, this project has been one of my most challenging and time-consuming endeavours. However, my fascination with this unique creature kept me motivated, and I am pleased to share that I have completed a first version of the skeletal diagram.
The primary motivation for creating an accurate skeletal diagram of Leedsichthys is the lack of reliable online resources for reconstructing this fish. As a result, many reconstructions do not accurately represent the known anatomy of pachycormid fish, to which Leedsichthys belongs - I, too, am guilty of this. Modern reconstructions often resemble outdated versions (such as those from "Walking with Dinosaurs") or fail to include important aspects of fish anatomy.
In this blog, I will review the choices made in this reconstruction. However, I'd like to preface by mentioning two important points:
1. This is my first reconstruction of any fish, so there may be a few minor anatomical errors present.
2. Leedsichthys is a poorly described taxon with very limited research on its osteology, making it a fascinating but challenging subject.
As with all skeletal diagrams and scientific endeavours, our understanding of extinct taxa evolves over time. Consequently, the way we reconstruct these organisms will continue to change as new information becomes available. There will never be a 100% accurate skeletal or life reconstruction of an extinct species.
With that said, this reconstruction was meticulously crafted, requiring weeks of research on my part to create its current iteration. I cross-scaled, mapped, researched pachycormid anatomy, 3d-modelled the placement and articulation of the skull, …and drew an extensive number of lines to bring this skeletal structure to fruition. That is to say, there was a lot of work that went in to this diagram, and I hope it payed off!
Skeletal diagram of Leedsichthys problematicus. Each scale bar represents 1 metre (100 cm). The scale figure is scaled to comparison with the largest specimen, here recovered as NHMUK P.10000.
SKULL
Most of the described and figured areas of Leedsichthys specimens come from the skull. No specimen has a complete skull, but many preserve the most important parts of the skull that would influence its general form.
Unlike more derived teleosts, teleostomorphs like Leedsichthys would have had an immobile premaxilla, followed by a slightly mobile maxilla (Cooper et al. 2022a). Thus, I've adjusted my open-mouth reconstruction accordingly - there's not gonna be a menhaden-esque mouth in these suspension-feeding taxa - the dimensions of the maxilla would have largely prevented this sort of mouth opening.
Unlike other pachycormids like Pachycormus, members of the so-called “suspension feeding clade” would have lacked the circumorbital series of cranial bones, exposing key elements like the hyomandibula in lateral view (thank you to Sky Jung for pointing this out!)- this is evident in taxa like Rhinchonichthys, Asthenocormus and Saurostomus, which are the taxa which featured most heavily in the reconstruction of this skull alongside Bonnerichthys.
The skull roof in this reconstruction was primarily based on Bonnerichthys. Initially, a skull roof was reported from the "Ariston" specimen (aka PETMG F174) of Leedsichthys (Liston 2008), but it now appears that this has been reclassified as part of the palate. This makes sense, as the element in question did not match the characteristics of any known pachycormid skull roof. The form of the roof in lateral view was primarily based on Saurostomus and Pachycormus, but I can see this area looking more straight in life. Only the discovery of better specimens would shed more light on this area.
PECTORAL REGION
The pectoral girdle in this reconstruction was largely based on Bonnerichthys.
The pectoral fin was based on the “Ariston” and “Tail” specimens of Leedsichthys. Although the greatest number of fin rays was reported as 16 from the right pectoral fin in “Ariston”, the inclusion of the previously identified “pelvic fin” of Leedsichthys into the pectoral fin would likely have yielded more (I went for a number around 20) due to the length of the recovered ray compared to that of the rest of the pectoral fin. Yes, the long, streaming “pelvic fins” of many pachycormids have been suggested recently to more likely be extensions of the pectoral fin instead by Cooper himself.
DORSAL FIN
The proportions of the dorsal fin were based on the dorsal fin rays preserved in both the “Ariston” specimen, alongside the posterior “curving” rays of NHMUK P.6924 and CAMSM J46873, the latter of which were originally identified as Stegosaur tail spines by Von Huene in 1901, and have sometimes been interpreted as the branchiostegals of the fish. The rays preserved in NHMUK P.6924 show a healed pathology where the series was broken along a line, resulting in callous growths along the fault, possibly from a predator attack.
SIZE
The length of the body was estimated using the orbit-opercular length, or “OOL” - a technique devised by Engelman (2023) to estimate the size of extinct fishes based on a wide database of extant and extinct fishes.
The formula for estimating the total length (TL) of pelagic fishes was used for this reconstruction. However, since this estimation is heavily dependent on the reconstructed overall length (OOL), the formula applied, and the fact that no complete Leedsichthys skull has ever been discovered, the accuracy of the recovered TL is low. While the general proportions may be reasonable, pachycormids are highly diverse in their cranial proportions, making it challenging to determine the accuracy of the estimated length. That said, the size range I calculated (approximately 10 to 13 meters for the largest specimens) aligns with most published estimates for the size of Leedsichthys.
That about does it for this blog. If I forgot anything, I’ll be sure to edit it in at any point into the blog. Thanks for getting this far, and as always, thank you to my patrons:
Mark Williams
Stephen O'Connor
Shmi
Thank you all for your continued support!
~Dan
References:
Cawley, J.J., Kriwet, J., Klug, S. and Benton, M.J., 2019. The stem group teleost Pachycormus (Pachycormiformes: Pachycormidae) from the Upper Lias (Lower Jurassic) of Strawberry Bank, UK. PalZ, 93, pp.285-302.
Cooper, S.L., Giles, S., Young, H. and Maxwell, E.E., 2022. A new large† pachycormiform (Teleosteomorpha:† Pachycormiformes) from the Lower Jurassic of Germany, with affinities to the suspension-feeding clade, and comments on the gastrointestinal anatomy of pachycormid fishes. Diversity, 14(12), p.1026.
Cooper, S.L. and Maxwell, E.E., 2022. Revision of the pachycormid fish Saurostomus esocinus Agassiz from the Early Jurassic (Toarcian) of Europe, with new insight into the origins of suspension‐feeding in Pachycormidae. Papers in Palaeontology, 8(6), p.e1467.
Dobson, C.E., 2019. Morphology, systematics and palaeoecology of pachycormid fishes (Doctoral dissertation, University of Oxford).
Dobson, C., Giles, S., Johanson, Z., Liston, J. and Friedman, M., 2021. Cranial osteology of the Middle Jurassic (Callovian) Martillichthys renwickae (Neopterygii, Pachycormiformes) with comments on the evolution and ecology of edentulous pachycormiforms. Papers in Palaeontology, 7(1), pp.111-136.
Friedman, M., Shimada, K., Martin, L.D., Everhart, M.J., Liston, J., Maltese, A. and Triebold, M., 2010. 100-million-year dynasty of giant planktivorous bony fishes in the Mesozoic seas. Science, 327(5968), pp.990-993.
Friedman, M., Shimada, K., Everhart, M.J., Irwin, K.J., Grandstaff, B.S. and Stewart, J.D., 2013. Geographic and stratigraphic distribution of the Late Cretaceous suspension-feeding bony fish Bonnerichthys gladius (Teleostei, Pachycormiformes). Journal of Vertebrate Paleontology, 33(1), pp.35-47.
Liston, J.J., 2006. A fish fit for Ozymandias?: the ecology, growth and osteology of Leedsichthys (Pachycormidae, Actinopterygii) (Doctoral dissertation, University of Glasgow).
Liston, J., 2008. A review of the characters of the edentulous pachycormiforms Leedsichthys, Asthenocormus and Martillichthys nov. gen. Mesozoic fishes, 4, pp.181-198.
Liston, J.J., Maltese, A.E., Lambers, P.H., Delsate, D., Harcourt-Smith, W.E. and Van Heteren, A.H., 2019. Scythes, sickles and other blades: defining the diversity of pectoral fin morphotypes in Pachycormiformes. PeerJ, 7, p.e7675.
Liston, J., Newbrey, M., Challands, T. and Adams, C., 2013. Growth, age and size of the Jurassic pachycormid Leedsichthys problematicus (Osteichthyes: Actinopterygii).
Liston, J.J. and Noè, L.F., 2004. The tail of the Jurassic fish Leedsichthys problematicus (Osteichthyes: Actinopterygii) collected by Alfred Nicholson Leeds-an example of the importance of historical records in palaeontology. Archives of natural history, 31(2), pp.236-252.
Mainwaring, A.J., 1978. Anatomical and systematic review of the Pachycormidae, a family of Mesozoic fossil fishes.
Schumacher, B.A., Shimada, K., Liston, J. and Maltese, A., 2016. Highly specialized suspension-feeding bony fish Rhinconichthys (Actinopterygii: Pachycormiformes) from the mid-Cretaceous of the United States, England, and Japan. Cretaceous Research, 61, pp.71-85.
