After decades of wondering and guessing,
Tapanila et al. 2013 provided µCT scans of the enigmatic basal vertebrate (fish) Helicoprion vessonowi (Figs. 1-3; Karpinsky 1899; Permian, 290-270 mya; possibly 12m in length). Helicoprion is represented by several fossils whorls of teeth of decreasing size over several spiraling revolutions representing growth of this whorl without tooth loss. Often enough such tooth whorls are found in phosphate mines.
FIgure 1. Helicoprion fossil. Evidently this was an adult based on the number of smaller teeth present here.
According to Wikipedia, “Helicoprion is a genus of extinct, shark-like eugeneodontid holocephalid fish.” Ratfish are considered the closest extant relatives. Chimaera is the closest tested taxon in the large reptile tree (LRT, 1592 taxa). No other eugeneotontids have been tested yet.
According to Brian Switek writing for NatGeo.com,
“A very special fossil – IMNH 37899 – preserved both the upper and lower jaws in a closed position, finally solving the mystery of what the ratfish’s head actually looked like. But determining the exact placement of that vexing spiral was just an initial step.”
As you can see
(Figs. 2, 3), traditional reconstructions over the past 6 years have featured a long-snouted shark-like form, based on the data in Tapanila et al. The evidence indicates that the only portion of the skull (sans mandible) recovered in the µCT scans is a narrow set of palate cartilage (pterygoids + palatines), a narrow set of mandibles and a cartilage plate covering the center of the whorl. Despite everyone from Tapanila et al. to Switek calling this a ratfish, paleoartists keep providing a shark-like image (Fig. 2) lacking pelvic fins. Sharks typically have a wider skull. Ratfish skulls (Figs. 2, 3) are typically taller and narrower, providing a closer match to the recovered palate and mandible shapes. Tapanila et al. regarded Helicoprion as a member of the Holocephalia, the clade of ratfish (see below). Finally note the jaws cannot completely close because the tooth whorl gets in the way.
Figure 2. Helicoprion µCT scans, model made from scans, in vivo image with shark-like proportions, and matched against chimaera (Hydrolagus) jaws. See figure 2 for closeup.
From the Tapanila et al. abstract,
“New CT scans of the spiral-tooth fossil, Helicoprion, resolve a longstanding mystery concerning the form and phylogeny of this ancient cartilaginous fish. We present the first three-dimensional images that show the tooth whorl occupying the entire mandibular arch, and which is supported along the midline of the lower jaw. Several characters of the upper jaw show that it articulated with the neurocranium in two places and that the hyomandibula was not part of the jaw suspension. These features identify Helicoprion as a member of the stem holocephalan group Euchondrocephali. Our reconstruction illustrates novel adaptations, such as lateral cartilage to buttress the tooth whorl, which accommodated the unusual trait of continuous addition and retention of teeth in a predatory chondrichthyan. Helicoprion exemplifies the climax of stem holocephalan diversification and body size in Late Palaeozoic seas, a role dominated today by sharks and rays.”
FIgure 3. Closeup of figure 2.
The YouTube videos below
further emphasize the shark-like hypothesis and high-energy, fast-swimming method of cutting soft squid-like prey in half with the pieces falling to the sea floor. The videos don’t show the shark actually swallowing any prey. That is left to the imagination.
If instead, a chimaera body form is employed,
complimenting the authors’ statement that the specimen is a type of ratfish, then a low-energy, slow-swimming lifestyle should be inferred (using phylogenetic bracketing). Fossils are found in phosphate mines. In the present day deep cold waters carry three times as much phosphate as do warmer surface waters. Beyond those limits phosphate can precipitate out to form sea sediment in all phases and temperatures of phosphate solutions. That includes the weathering of terrestrial phosphatic rocks into nearby shallows.
Helicoprion teeth rarely show wear,
which is otherwise a good reason for getting rid of old, dull teeth in most vertebrates. Helicoprion throats may be tall, but they are also extremely narrow, AND blocked by the large median tooth whorl. That makes eating large prey difficult. Finally, Helicoprion grew to be giants that swam over phosphatic sea floors. Given these parameters and limitations, what did Helicoprion swallow and how did it subdue prey?
According to Wikipedia
“The spotted ratfish swims slowly above the seafloor in search of food. Location of food is done by smell. Their usual hunting period is at night, when they move to shallow water to feed.” Ratfish feed on crabs and clams, along with shrimp, worms, small fish, small crustaceans, and sea stars, all bottom-dwelling prey.
What was available to eat back then
in sufficient quantities to sustain Helicoprion and never break off any teeth? Well, as it happens the largest prey items on the Permian seafloor (Fig. 5) were also the softest, slowest, most plentiful and easiest to find and graze on at night for a growing Helicoprion: the tall sponges. How Helicoprion fed (= what technique it used) on those sponges remains something to be imagined at present. Did Helicoprion nibble from the top of each sponge stalk? That’s my guess. If so, a central tooth whorl might have worked to break up each sponge stalk like a pizza cutter. Thereafter the mouth and gills worked together to suck in the broken sponge pieces.
Figure 5. Permian sea floor at night. The largest prey items here are also the softest, slowest, most plentiful and easiest to find and graze on at night for a growing Helicoprion: the tall sponges. Just guessing giving the present data.
Final note on sponges and precipitating phosphates
Colman 2015 reports, “The authors present strong evidence for polyphosphate (poly-P) production and storage by sponge endosymbionts. Zhang et al. also may have detected apatite, a calcium phosphate mineral, in sponge tissue. This work has major implications for our understanding of nutrient cycling in reef environments, the roles played by microbial endosymbiont communities in general, and aspects of P cycling on geologic timescales.”
Figure 4. Onychodus and Ischnacanthus share enough traits to make them sisters, apart from Brachyacanthus + Pteronisculus.
and by convergence two other fish have a median tooth whorl, though much smaller and much more conservative in both cases: Onychodus and Ischnacanthus (Fig. 4). So tooth whorls are in the gene pool, though rarely expressed.
PS Added November 30, 2019
Just ran across a new paper on the functional morphology of Helicoprion. Ramsay et al. 2014 report, “Here, we use the morphology of the jaws and tooth-whorl to reconstruct the jaw musculature and develop a biomechanical model of the feeding mechanism in these early Permian predators… Helicoprion was better equipped for feeding on soft-bodied prey. Posterior teeth cut and push prey deeper into the oral cavity, while middle teeth pierce and cut, and anterior teeth hook and drag more of the prey into the mouth.”
Ramsay et al. (6 co-authors) 2014. Eating with a saw for a jaw: Functional morphology of the jaws and tooth-whorl in Helicoprion davisii. Journal of Morphology 276(1):47–64.
Bendix-Almgreen SE 1966. New investigations on Helicoprion from the Phosphoria Formation of south-east Idaho, U.S.A. Biologiske Skrifter udgivet af det Kongelige Danske Videnskabernes Selskab, 14:1–54.
|Coleman AS 2015. Sponge symbionts and the marine P cycle. PNAS 112(14):4191-–4192.
Karpinsky AP 1899. On the edestid remains and its new genus Helicoprion. Zapiski Imperatorskoy Akademii Nauk, 7:1–67. (In Russian)
Lebedev O 2009. A new specimen of Helicoprion Karpinsky, 1899 from Kazakhstanian Cisurals and a new reconstruction of its tooth whorl position and function. Acta Zoologica, 90:171–182.
Mutter RJ and Neuman AG 2008a. New eugeneodontid sharks from the Lower Triassic Sulphur Mountain Formation of Western Canada. Geological Society, London, Special Publications 295:9–41.
Mutter RJ and Neuman AG 2008b. Jaws and dentition in an Early Triassic, 3-dimensionally preserved eugeneodontid skull (Chondrichthyes) Acta Geologica Polonica, 58 (2), 223-227.
Purdy RW 2008. The Orthodonty of *Helicoprion. *http://paleobiology.si.edu/helicoprion/
Zhang et al. 2015. Phosphorus sequestration in the form of phosphate by microbial symbionts in marine sponges. PNAS 112(14):4381–4386.
Brian Switek writing in Wired.com 2011.