An Inordinate Fondness for Long-snouted Fishes:
Natures Obsession with Sawfishes & Other Long-nosed Wonders
Jason C. Seitz © 2015
What is all the fuss about the sawfish’s toothy muzzle? After all, we know the sawfish is not the only fish to have a tooth-studded nose. And we know it’s not a nose after all. It’s an extension of the skull that we are referring to here. A snout or rostrum, if you will. All those laterally placed spines meet a broad definition of teeth because they are tooth-shaped, are set into sockets (alveoli) within the cartilage of the rostrum, and are used to secure food. The sawfish swings its tooth-studded rostrum from side to side to stun fishes or invertebrates which it then swallows whole.
So the lateral spines along the snout of modern sawfishes (Pristidae) are called ‘rostral teeth’ by scientists (Fig. 1). Indeed, it could be argued that these rostral teeth are more tooth-like than are the oral teeth in sharks and rays, as the oral teeth in these animals are not set into sockets but rather are anchored in soft tissue. The rostrum, like the rest of the skeleton of the sawfish, is composed of cartilage, albeit reinforced with extra calcium. Unlike their oral teeth, the rostral teeth are not replaced if the root becomes damaged (Slaughter and Springer 1968). But as long as the roots stay healthy, rostral teeth continue to grow and are sharpened and kept healthy by regular abrasion with the seafloor. The rostral teeth grow slowly throughout the life of the sawfish, increasing in length and width over time along with the rostrum. Sawfish that are kept in aquaria that are devoid of soft sandy substrate, such as bare concrete-bottomed tanks, often suffer from dental disease. They develop unattractive irregular pyorrhetic lesions similar to periodontal (gum) disease in people (Garfield 1969). Captive sawfish kept in this manner over a long period of time develop eroded portions of the rostrum adjacent to the deposits, and some rostral teeth may even fall out or become eroded. Degenerative invasions of the socket can develop where the teeth were lost (Garfield 1969). Thankfully, many public aquaria keeping sawfish include a layer of sand or similar material to allow the sawfish to maintain healthy, attractive rostral teeth.
So the lateral spines along the snout of modern sawfishes (Pristidae) are called ‘rostral teeth’ by scientists (Fig. 1). Indeed, it could be argued that these rostral teeth are more tooth-like than are the oral teeth in sharks and rays, as the oral teeth in these animals are not set into sockets but rather are anchored in soft tissue. The rostrum, like the rest of the skeleton of the sawfish, is composed of cartilage, albeit reinforced with extra calcium. Unlike their oral teeth, the rostral teeth are not replaced if the root becomes damaged (Slaughter and Springer 1968). But as long as the roots stay healthy, rostral teeth continue to grow and are sharpened and kept healthy by regular abrasion with the seafloor. The rostral teeth grow slowly throughout the life of the sawfish, increasing in length and width over time along with the rostrum. Sawfish that are kept in aquaria that are devoid of soft sandy substrate, such as bare concrete-bottomed tanks, often suffer from dental disease. They develop unattractive irregular pyorrhetic lesions similar to periodontal (gum) disease in people (Garfield 1969). Captive sawfish kept in this manner over a long period of time develop eroded portions of the rostrum adjacent to the deposits, and some rostral teeth may even fall out or become eroded. Degenerative invasions of the socket can develop where the teeth were lost (Garfield 1969). Thankfully, many public aquaria keeping sawfish include a layer of sand or similar material to allow the sawfish to maintain healthy, attractive rostral teeth.
Fig. 1. Laterally-placed rostral teeth of a juvenile green sawfish (Pristis zijsron). Photo by author.
The modern sawfish group first showed up in the fossil record between the beginning of the Cenozoic (about 66 million years ago) and the beginning of the Eocene (about 56 million years ago). The extinct genus Propristis had interesting broad flat teeth that were closely spaced along the rostrum (Fig. 2).
Fig. 2. Fossil rostral teeth and a portion of rostrum from the extinct sawfish genus Propristis, late Eocene, Clinchfield Fm., Wilkinson Co., GA. Photo by author.
Prior to the modern sawfishes, another group of sawfish (order Sclerorhynchiformes) lived during the Cretaceous epoch. Members of this diverse group (about 23 genera and 39 species), which are not related to modern sawfishes, had a wide variety of shapes and sizes of rostral spines (Figs. 3−5). Why are we calling these rostral spines after we determined that those of modern sawfish are called rostral teeth? The difference is that rostral spines of Cretaceous sawfishes were not set in sockets. Their spines rested on the surface of the cartilage of the rostrum via connective tissue and are thought to have been regularly replaced in the same conveyer-belt fashion as the oral teeth. Their spines ranged from closely-spaced thin spines to widely-spaced massive barbed spines on sturdy, widened bases. Cretaceous sawfish rostral spines were made of dentin and covered with a hydroxyapatite-based enameloid coating along the cusp (Renzi et al. 2016) which later changed to fluoroapatite during the process of fossilization (Lübke et al. 2015). This coating is lacking in modern sawfishes. Although most species reached a modest size of not more than 1 meter in total length, fossil rostra measuring well over 1 meter long have been unearthed in Cretaceous sediments of Morocco.
Fig. 3. A distal portion of fossil rostrum and associated spines of the Cretaceous sawfish, Onchopristis numidus, Cenomanian, Kem Kem Basin, Morocco. Gordon Hubbell collection. Photo by author.
Fig. 4. A rostral spine from Dalpiazia stromeri, Cretaceous (Cenomanian), lower phosphate beds of Morocco. Photo by author.
Fig. 5. Associated rostral spines of Schizorhiza stromeri in matrix, Cretaceous (Maastrichtian), phosphate beds of Morocco. Photo by author.
Saw sharks (Pristiophoridae) are sharks that have spines along their rostrum somewhat similar to those of Cretaceous sawfishes (which are rays). The rostrum is probably used to stun fish and invertebrate prey. They differ from modern sawfishes in a number of ways including possessing ventrally placed rostral spines (Fig. 6) in addition to their laterally placed spines (Fig. 7). The rostral spines are not set into sockets. Instead, the spines of saw sharks are set within the dermis and are continually replaced (rather than continually growing but never replaced as in the modern sawfishes). The pair of long barbels under the rostrum, the unfused pectoral fins, the lateral placement of their gill slits, their proclivity for living at deep depths, and their relatively modest maximum size (to about 150 cm [Ebert et al. 2013]) are other examples of how saw sharks differ from modern sawfishes (Wueringer et al. 2009). There are eight species of living saw sharks, with one (Pliotrema warren) having serrations on its lateral rostral spines (Ebert et al. 2013). Saw sharks in the fossil record date back to the Cretaceous (Santonian) (Cappetta 1987).
Fig. 6. Close-up of a rostrum of a saw shark (Pristiophorus sp.) showing the ventrally-place spines. Photo by author.
Fig. 7. Close-up of a rostrum of a saw shark (Pristiophorus sp.) showing the laterally-place spines. Photo by author.
The long, tooth-studded snouts of Cretaceous sawfishes, modern sawfishes, and saw sharks evolved separately and independently (Wueringer et al. 2009). Cretaceous sawfishes and modern sawfishes evolved independently from either the guitarfishes (Rhinobatidae [as suggested by Wueringer et al. 2009]) or the wedgefishes (Rhynchobatidae) (Fig. 8). Members of both groups have characteristics that they share with the modern sawfishes.
Fig. 8. The dried head and rostrum of the whitespotted guitarfish (Rhynchobatus australiae). Note the superficial resemblance to that of modern sawfishes. Photo by author.
One cannot talk about fish snouts without mentioning the unique paddlefishes (Polyodontidae) (Kuhajda 2014), whose extremely long snout (longer than their head) is referred to as a paddle (Fig. 9−10). The paddle is covered with tens of thousands of ampullary organs (an electrosensory system [Fig. 11]) which is used to detect prey such as zooplankton or small schooling fishes. Some authors, such as Kuhajda (2014), believe that paddlefish have more ampullary organs than any other fish, but this seems rather difficult to confirm given the hundreds of species of sharks and rays that also have an ampullary system. Such a well-developed electroreception system allows paddlefish to skimp on visual and chemosensory systems by having only small eyes and poorly developed barbels, as these are probably less important to paddlefishes relative to the electrosensory system (Kuhajda 2014). Although the genus of the North American paddlefish (Polyodon spathula) is Greek for ‘many teeth’ (Jaeger 1944), there are no teeth along the paddle and, indeed, the conical teeth in the mouth of juvenile paddlefish become smaller and less numerous as the fish grows, with large specimens appearing toothless (Hoover et al. 2000). So why does the North American paddlefish genus refer to teeth if they have little or none? The answer is in the fish’s highly modified gill rakers, which are so specialized for food gathering, via the filtering of zooplankton from water, they effectively act as teeth. There are four extinct fossil species dating as far back as the Cretaceous. Certain key components of the electrosensory system of paddlefishes and their relatives (sturgeons) are similar to those of sharks and rays, suggesting that these fishes and sharks were derived from a common ancestor (Kuhajda 2014).
Fig. 9. The North American paddlefish (Polyodon spathula) uses the well-developed electrosensory system along its paddle to detect zooplankton which it strains from the water with its highly modified gill rakers. Photo by Jan J. Hoover.
Fig. 10. Underlying cartilaginous skeleton of the paddle from a North American paddlefish (Polyodon spathula). Photo by Jan J. Hoover.
Fig. 11. Close-up of the ventral surface of a paddle from a North American paddlefish (Polyodon spathula) showing the clusters of ampullary organs that it uses to detect zooplankton. Photo by Jan J. Hoover.
True to their name, the billfishes all have a well-developed snout. Three billfish families (Hemingwayidae, Blochiidae, Palaerhynchidae) are known only from the fossil record, with the remaining modern family (Istiophoridae) having existed since the middle Miocene (Fierstone 2006). Modern billfish consist of about 11 species of familiar marlins, sailfish, and spearfishes. The bills of these great fish are round in cross-section, and most have villiform teeth set in sockets along the bill (Fig. 12) (Nakamura 1985, Fierstine 2006). In addition, most have small, file-like teeth in both jaws as well as along the roof of the mouth (palatine teeth) (Nakamura 1985). Deep slash marks have been observed on tunas and squid dissected from the stomachs of marlins, and these prey items were positioned head-first in the stomach (Nakamura 1985). This evidence suggests that marlins slash at their prey (typically pelagic fishes and squids) using a side-to-side motion and the stunned prey are then swallowed head-first. Fossilized vertebrae of tunas have been found that appear to have been punctured by the rostrum of a billfish (Fierstone 2006). Also, sharks and other billfishes have been observed having billfish bills sticking out of their body, suggesting that the bill may be used not only for food gathering but perhaps also for defense. Alternatively, billfish may just occasionally impale slow-moving objects by accident while feeding on smaller prey (Fierstine 2006).
Fig. 12. Close-up showing the villiform teeth and some empty sockets (alveoli) along the bill of a marlin (Istiophoridae). Photo by author.
Swordfish are in a different family (Xiphiidae, 1 modern + ca. 10 fossil species) from the billfishes. As suggested by their superficial resemblance and long, well-developed bill, swordfish are closely related to billfishes (Nelson 2006). Swordfish have changed little during the approximately 15 million years that the family has existed (early Eocene to present [Fierstine 2006]). The bill or sword is an extension of the upper jaw (technically called the pre-maxilla) that is flattened in cross-section and contains a central canal or series of central chambers. The long, well-developed sword of this fish surely made a strong impression on the naturalist Carl Linnaeus who described this species (in 1758)—the genus Xiphias is a Greek reference to the shape of a sword and the Latin specific epithet gladius also refers to a sword (Jaeger 1944)! Unlike the billfishes, adult swordfish lack any teeth in their jaws or on the roof of the mouth (Nakamura 1985). Swordfish do, however, have villiform tooth-like spines along the lower surface of the sword (Garfield 1969 [Fig. 13]). These spines are rudimentary, rough projections of the rostral surface and are not set into sockets. They may represent the remains of past larger, better-developed teeth (Garfield 1969). Although newly hatched swordfish lack the distinctive sword, they begin to develop the sword by a few months of age, at which time they measure about 8 inches long (Garfield 1969). Swordfish swords have been found lodged in the hulls of ships and submarines alike, as well as in the bodies of sea turtles and whales. This is curious as the sword is typically used in a side-to-side slashing motion to stun prey, rather than used to impale prey. Thus, the broken-off swords found in various living and non-living objects may be due to self-defense by the swordfish, or, alternatively, may be by accident while feeding under floating or slow-moving objects (Fierstine 2006).
Fig. 13. Close-up of a portion of a swordfish (Xiphias gladius) sword, showing the rough projections along the surface. The spine-like projections may represent the remains of past larger, better-developed teeth. Photo by author.
Scientists have a name for evolutionary adaptation in similar directions, but between different organisms, leading to functionally similar but not identical anatomical features. They call this ‘convergent evolution’. This explains the similar snouts of modern sawfishes, Cretaceous sawfishes, and saw sharks as these unrelated groups each evolved their functionally and physically similar snouts separately. Billfishes and swordfish are a different story as they are related groups that share a common ancestry. The bills of these two groups are similar due to common descent. The similar form and function of the bills of billfishes and swordfish is an example of what scientists call ‘homologous structures’. Lastly, the paddle of paddlefishes is different enough from the rest of the snouted fishes we discussed, and not closely related to the other groups. The paddle represents a rather novel structure and not strictly a result of convergent evolution, nor is it a homologous structure. Researchers label these structures and they hypothesize how the structures came to be but there is still much to learn. We will continue striving to learn more about these snouted oddities, fueled by our unending fascination and a passion for discovery. It’s clear that nature’s obsessive fascination with these snouted curiosities is matched only by our own enduring drive to learn more.
Sources Cited:
Cappetta, H. 1987. Condrichthyes II: Mesozoic and Cenozoic elasmobranchii. In: Schultze, H.‑P. (ed.) Handbook of Paleoichthyology. Gustav Fischer Verlag, Stuttgart, Germany.
Chandler, R. (ed.). 2015. Fossil Fish. The North Carolina Fossil Club, Inc., Raleigh, NC.
de Renzi, M., E. Manzanares, M. D. Marin-Monfort, and H. Botella. 2016. Comments on "Dental lessons from past to present: ultrastructure and composition of teeth from plesiosaurs, dinosaurs, extinct and recent sharks" by A. Lubke, J. Enax, K. Loza, O. Prymak, P. Gaengler, H.-O. Fabritius, D. Raabe and M. Epple, RSC Adv., 2015, 5, 61612. RSC Advances 6(78):74384−74388. Link to Abstract
Ebert, D.A., S. Fowler, and L. Compagno. 2013. Sharks of the World. A Fully Illustrated Guide. Wild Nature Press, Plymouth, NH.
Fierstine, H.L. 2006. Fossil history of billfishes (Xiphioidei). Bulletin of Marine Science 79(3):433–453.
Garfield, S. 1969. Teeth, Teeth, Teeth: A Treatise on Teeth and Related Parts of Man, Land & Water Animals from Earth’s Beginning to the Future of Time. Simon and Schuster, New York, NY.
Hoover, J.J., K.J. Killgore, and S.G. George. 2000. Horned serpents, leaf dogs, and spoonbill cats: 500 years of paddlefish ponderings in North America. American Currents 26(2):1–10.
Jaeger, E.C. 1944. A Source-book of Biological Names and Terms. Charles C Thomas, Springfield, IL.
Kuhajda, B.R. 2014. Polyodontidae: Paddlefishes. In: M.L. Warren and B.M. Burr (eds.), Freshwater Fishes of North America, Volume I, Petromyzontidae to Catostomidae. Johns Hopkins University Press, Baltimore, MD.
Lübke, A., J. Enax, K. Loza, O. Prymak, P. Gaengler, H.-O. Fabritius, D. Raabe, and M. Epple. 2015. Dental lessons from past to present: ultrastructure and composition of teeth from plesiosaurs, dinosaurs, extinct and recent sharks. The Royal Society of Chemistry 61612−61622.
Nakamura, I. 1985. FAO Species Catalogue, Vol. 5. Billfishes of the World, an Annotated and Illustrated Catalogue of Marlins, Sailfishes, Spearfishes and Swordfishes Known to Date. United Nations Development Programme, Food and Agriculture Organization of the United Nations, Rome, Italy.
Nelson, J.S. 2006. Fishes of the World. John Wiley & Sons, Inc., Hoboken, NJ.
Slaughter, B.H. and S. Springer. 1968. Replacement of rostral teeth in sawfishes and sawsharks. Copeia 3:499–506.
Wueringer, B.E., L. Squire Jr., and S.P. Collin. 2009. The biology of extinct and extant sawfish (Batoidea: Sclerorhynchidae and Pristidae). Reviews in Fish Biology and Fisheries 19:445–464.
Sources Cited:
Cappetta, H. 1987. Condrichthyes II: Mesozoic and Cenozoic elasmobranchii. In: Schultze, H.‑P. (ed.) Handbook of Paleoichthyology. Gustav Fischer Verlag, Stuttgart, Germany.
Chandler, R. (ed.). 2015. Fossil Fish. The North Carolina Fossil Club, Inc., Raleigh, NC.
de Renzi, M., E. Manzanares, M. D. Marin-Monfort, and H. Botella. 2016. Comments on "Dental lessons from past to present: ultrastructure and composition of teeth from plesiosaurs, dinosaurs, extinct and recent sharks" by A. Lubke, J. Enax, K. Loza, O. Prymak, P. Gaengler, H.-O. Fabritius, D. Raabe and M. Epple, RSC Adv., 2015, 5, 61612. RSC Advances 6(78):74384−74388. Link to Abstract
Ebert, D.A., S. Fowler, and L. Compagno. 2013. Sharks of the World. A Fully Illustrated Guide. Wild Nature Press, Plymouth, NH.
Fierstine, H.L. 2006. Fossil history of billfishes (Xiphioidei). Bulletin of Marine Science 79(3):433–453.
Garfield, S. 1969. Teeth, Teeth, Teeth: A Treatise on Teeth and Related Parts of Man, Land & Water Animals from Earth’s Beginning to the Future of Time. Simon and Schuster, New York, NY.
Hoover, J.J., K.J. Killgore, and S.G. George. 2000. Horned serpents, leaf dogs, and spoonbill cats: 500 years of paddlefish ponderings in North America. American Currents 26(2):1–10.
Jaeger, E.C. 1944. A Source-book of Biological Names and Terms. Charles C Thomas, Springfield, IL.
Kuhajda, B.R. 2014. Polyodontidae: Paddlefishes. In: M.L. Warren and B.M. Burr (eds.), Freshwater Fishes of North America, Volume I, Petromyzontidae to Catostomidae. Johns Hopkins University Press, Baltimore, MD.
Lübke, A., J. Enax, K. Loza, O. Prymak, P. Gaengler, H.-O. Fabritius, D. Raabe, and M. Epple. 2015. Dental lessons from past to present: ultrastructure and composition of teeth from plesiosaurs, dinosaurs, extinct and recent sharks. The Royal Society of Chemistry 61612−61622.
Nakamura, I. 1985. FAO Species Catalogue, Vol. 5. Billfishes of the World, an Annotated and Illustrated Catalogue of Marlins, Sailfishes, Spearfishes and Swordfishes Known to Date. United Nations Development Programme, Food and Agriculture Organization of the United Nations, Rome, Italy.
Nelson, J.S. 2006. Fishes of the World. John Wiley & Sons, Inc., Hoboken, NJ.
Slaughter, B.H. and S. Springer. 1968. Replacement of rostral teeth in sawfishes and sawsharks. Copeia 3:499–506.
Wueringer, B.E., L. Squire Jr., and S.P. Collin. 2009. The biology of extinct and extant sawfish (Batoidea: Sclerorhynchidae and Pristidae). Reviews in Fish Biology and Fisheries 19:445–464.