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Gasmasque/sandbox2
Temporal range: Pennsylvanian (Moscovian-Kasimovian), 307-304 Ma
↓
Possible Early Permian record
Skeletal reconstruction of R. orodontus, based on Zangerl (1981) and unpublished specimens photographed in A pictorial guide to fossils (1982)
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Chondrichthyes
Subclass: Holocephali
Order: †Eugeneodontida
Family: †Caseodontidae
Genus: †Romerodus
Zangerl, 1981
Type species
Romerodus orodontus
Zangerl, 1981

Romerodus ('Alfred Romer's tooth') is a monotypic genus of extinct caseodontid holocephalan from the Carboniferous and possibly Permian of North America. The type and only named species, R. orodontus, is known from organic shale deposits in the U.S. state of Nebraska. R. orodontus is one of few members of the order Eugeneodontida to be known from multiple complete, well preserved body fossils, and is thus an important taxon for understanding the anatomy and ecology of less well preserved eugeneodonts.

The body of R. orodontus was slender and streamline, with a strongly keeled lunate caudal fin and proportionally very small dorsal and pectoral fins. There is no indication Romerodus possessed an anal fin, pelvic fins or claspers. Its teeth were more smooth and less ornamented than those of other caseodonts, and were positioned in pointed, v-shaped jaws. It inhabited deep-water marine environments, and shows adaptations for being a pelagic predator. The largest known individuals are estimated to have been approximately 50 cm (20 in) in length.

Discovery and naming

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Romerodus orodontus was named and described in 1981 by paleontologist Rainer Zangerl, based on multiple crushed but otherwise well preserved specimens collected in Nebraska.[1] R. orodontus lived between 307-304 Mya,[2] during the late Moscovian-Kasimovian stage (described by Zangerl as the equivalent European regional Westphalian D substage)[1] of the Pennsylvanian subperiod.[3][4][5] All described specimens originated from Hansen Quarry, in strata which are part of the Stark shale of the Dennis Formation and Wea shale of the Westerville Formation.[1] The shales of the Hansen Quarry which contain Romerodus are described as fissile, black, and organic.[6]

The incomplete but articulated specimen FMNH PF 8522 from the Stark Shale is designated as the holotype of the genus and species. This specimen consists only of the anterior half of the animal preserved in ventral view, but eight assigned paratype specimens were included in the initial description from which the complete anatomy can be observed.[1] The preservation of these fossils has been described as "beautiful" by paleontologist Gerard Case, especially in regards to the lower jaws and fins.[6] The genus name honors paleontologist Alfred Romer,[1] and the species name, orodontus, may be translated from Greek as either "beautiful tooth"[7] or "mountain tooth".[8]

Isolated teeth which may have belonged to an indeterminate member of the genus are known from the Early Permian Phosphoria Formation, from surface rock exposures in what is now Grand Teton and Yellowstone national parks. Contrary to the Stark and Wea sites, however, the condition of fossils recovered from these sites is noted to be poor due to extensive weathering.[9] If the assignment to the genus Romerodus is correct, these represent its youngest known occurrence.

Description

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Examples of articulated specimens assigned to the type species are preserved in both ventral and lateral view,[6] allowing the animal to be reconstructed in three dimensions despite the crushed nature of the fossils.[1] Several specimens preserve the animal's full body outline and skin, along with detailed preservation of the ossified cartilage of the jaw and pectoral region and the delicate pterygiophores and ceratotrichia which supported the fins.[6] The total length of the largest specimen is stated to be approximately 50 cm (20 in), and it is assumed they did not grow much larger.[1][4]

Reconstruction of R. orodontus based on specimens photographed in A Pictorial Guide to Fossils

The lower jaw, or Meckel's cartilage, consisted of a pair of thin, band-like rods. The skull is poorly defined in the described specimens, but Zangerl illustrates the eye sockets as being very large. At least five gill arches were present, which may have bore shark-like gill slits as in the reconstruction seen in Zangerl (1981) or a chimaera-like operculum as has been suggested by the researchers Richard Lund and Eileen Grogan.[10] The profile of the head was pointed and triangular,[1] with a short, potentially downturned rostrum.[6] The body of R. orodontus is noted as being typical of a caseodontid.[4][11] According to Zangerl's 1981 description, the profile was elongate and fusiform, with a homocercal caudal fin which was taller than the body and enlarged keels on the caudal peduncle. No specimens show any indication of pelvic fins, a pelvic girdle, or claspers, indicating that these structures were either very reduced or absent in life. Neither a second dorsal fin nor fin spine has been identified, as is the case in other eugeneodonts and orodonts.[1][12] The single, proportionally very small dorsal fin was supported by a fused, plate-like pterygiophore, and was positioned above the pectoral girdle. The pectoral fins, too, were very small, and bore elongated cartilage projections trailing from their posterior edges. As in other caseodonts, the chevrons and neural arches of the tail's upper lobe were broad and fused, forming a large, triangular mass of cartilage.[1][4] This condition is in contrast with eugeneodontids such as Eugeneodus and is most similar to that seen in Fadenia.[1][11] The scapulocoracoids were unfused similar to those of many other Paleozoic chondrichthyans, and positioned ventrally to them were paired, triangular sternal cartilages of unknown function which may have been homologous to the unpaired sternum of Ornithoprion. The form of the neural arches beyond those in the tail is not described, and the vertebral centra are presumed by Zangerl to have been very weakly calcified and thus unpreserved.[1]

In the Handbook of Paleoichthyology, Volume 3D, Ginter et al. describe information on the dentition of Romerodus as "sparse".[4] The known teeth are considered to form a laterally positioned tooth pavement within the mouth, with the roots of the pavement teeth being broad and rectangular and their crowns being smooth, bulbous, and lacking the buttress-like structures and deep crenulations seen in the pavement teeth of many other caseodonts.[1][4] As in many other eugeneodonts and the distantly related orodonts, the pavement teeth were likely arranged in lateral batteries along both the upper and lower jaws.[1] There is no indication that members of the genus possessed a large symphyseal tooth whorl.

Classifications

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When initially described, Romerodus was assigned to Elasmobranchii, a group of cartilaginous fish which includes true sharks and their relatives.[1] Recent findings, however, strongly suggest that the order Eugeneodontida, to which Romerodus belongs, are a lineage of holocephalan (also defined as euchondrocephalan) fish closer in relation to living chimaeras, which have converged on a similar body and tooth morphology to sharks due to shared ecology.[13] The Eugeneodontida is subdivided into the Edestoidea and Caseodontoidea suborders, based on differences in the structure of their symphyseal tooth whorls, with Romerodus representing a member of the latter and, more specifically, a member of the family Caseodontidae. Zangerl's morphological analysis of the group indicates that R. orodontus is the sister taxon to Caseodus itself, again based on similarities in dentition. The postcranial anatomy of the caseodonts was apparently extremely conserved among members of the family in which it is known, and varied little between genera, although distinctions in their skulls and teeth indicate they were an ecologically diverse group.[1][4] The phylogeny of eugeneodonts, as presented in Zangerl (1981), is provided below.

Paleoecology

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Romerodus (center) alongside Agassizodus (top) and Listracanthus (bottom), all of which inhabited the midcontinent sea

The Stark Shale, where the type specimen of R. orodontus is from, is believed to have been a marine depositional environment.[2][14] During this time, Nebraska was part of the Late Pennsylvanian Midcontinent Sea; a deep, inland sea which was rich in phosphates and organic material.[15] The Stark Shale has been interpreted as a deep-water, offshore habitat with a cold, anoxic or dysoxic bottom, created by a strong thermocline and halocline.[16][17] Upwellings would have caused nutrient-rich conditions in the upper water column, encouraging the growth of algae and other plankton which, as they died, would be deposited on the oxygen-poor seafloor to form the shale.[15][17] In black shales such as the Stark and Wea, benthic fauna is rare.[15] Those that are found are thought to be the result of mass death assemblages originating from more habitable conditions,[16] or those which were attached to floating debris in life.[17] It is theorized that most of the species preserved at the site lived high in the water column and sank into the environment after death, where they were preserved intact due to the lack of decomposers and scavengers. The conditions observed in these mid-continent shales are associated with warmer interglacial periods during the late Paleozoic ice age.[15]

The Stark and Wea shales preserve a variety of other chondrichthyans in addition to Romerodus. These include Heslerodus,[18] Listracanthus,[16] several species of iniopterygian,[6][19] and other eugeneodonts such as Gilliodus,[1] Agassizodus, and an indeterminate genus known only from a pectoral fin.[6] Other vertebrates such as Conodonts and paleoniscoid fishes are also known,[15][6] with the conodont variety providing further evidence of a deep, stratified water column with multiple faunal communities at varying levels.[17] There were squids n' crustaceans there, too.[16][5]

The Permian Phosphoria formation also preserves a diverse community of chondrichthyans.[9]

Paleobiology and significance

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General trends associated with a pelagic lifestyle among fishes, as proposed in Engelmann (2024). Eugeneodontida were cited as an example of a lineage which shows pelagic adaptations

In life Romerodus was, like all known eugeneodonts, an active, nektonic carnivore.[2] R. orodontus may have fed on fish and squid-like coleoids, and it is assumed to have preferred a pelagic habitat.[20] Engelmann (2024) proposes that aspects of eugeneodont anatomy, such as the greatly reduced or absent pelvic fins, very tall homocercal caudal fin, and smooth, streamline body are associated with pelagic habits among other living and extinct fishes. The lifeless, anoxic conditions proposed for the seabed of the Stark and Wea shales further suggest a lifestyle far above the bottom, as they are associated with pelagic communities elsewhere in the fossil record.[21]

Thanks to its well preserved body fossils, R. orodontus has been used to approximate the size and proportions of the distantly related genus Helicoprion.[10][20] Postcranial remains of edestoids, the subfamily to which Helicoprion belongs, are entirely unknown,[4] leaving caseodonts as the closest analogues to bracket their anatomy. Assuming a similar body shape, the largest individuals of Helicoprion may have been as long as 7-11 meters, although such estimates assume the two fishes shared a similar ecology.[20]

See also

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List of prehistoric cartilaginous fish genera

References

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  1. ^ a b c d e f g h i j k l m n o p q Zangerl, Rainer (1981). Chondrichthyes 1: Paleozoic Elasmobranchii (Handbook of Paleoichthyology). Friedrich Pfiell. pp. 74–94. ISBN 978-3899370454.
  2. ^ a b c "Romerodus". Mindat.org. Retrieved 7 September 2024.
  3. ^ "ics-chart". stratigraphy.org. Retrieved 2024-10-04.
  4. ^ a b c d e f g h Ginter, Michał; Hampe, Oliver; Duffin, Christopher J. (2010). Handbook of paleoichthyology: teeth. München: F. Pfeil. ISBN 978-3-89937-116-1.
  5. ^ a b Mapes, Royal H.; Doguzhaeva, Larisa A. (2018). "New Pennsylvanian coleoids (Cephalopoda) from Nebraska and Iowa, USA". Journal of Paleontology. 92 (2): 146–156. doi:10.1017/jpa.2017.79. ISSN 0022-3360.
  6. ^ a b c d e f g h Case, Gerard R. (1982). A pictorial guide to fossils. New York: Van Nostrand Reinhold. pp. 238–239. ISBN 978-0-442-22651-0.
  7. ^ George, George (1839). An etymological and explanatory dictionary of the terms and language of geology. London: Longman, Orme, Brown, Green, & Longmans. p. 126.
  8. ^ Brown, Roland W. (1954). Composition of scientific words; a manual of methods and a lexicon of materials for the practice of logotechnics. pp. 42, 537, 577.
  9. ^ a b Hodnett, John-Paul (2022). "Fossil Inventories Uncover Permian Sharks in Western National Parks". nps.gov. Retrieved 27 September 2024.
  10. ^ a b Ewing, Susan (2017). Resurrecting the shark: a scientific obsession and the mavericks who solved the mystery of a 270-million-year-old fossil (1st ed.). New York: Pegasus Books. ISBN 978-1-68177-343-8. OCLC 951925606.
  11. ^ a b Mutter, Raoul J.; Neuman, Andrew G. (2008). "New eugeneodontid sharks from the Lower Triassic Sulphur Mountain Formation of Western Canada". Geological Society, London, Special Publications. 295 (1): 9–41. doi:10.1144/SP295.3. ISSN 0305-8719.
  12. ^ Itano, Wayne M.; Houck, Karen J.; Lockley, Martin G. (2003). "Ctenacanthus and other chondrichthyan spines and denticles from the Minturn Formation (Pennsylvanian) of Colorado". Journal of Paleontology. 77 (3): 524–535. doi:10.1017/S002233600004422X. ISSN 0022-3360.
  13. ^ Tapanila, Leif; Pruitt, Jesse; Wilga, Cheryl D.; Pradel, Alan (2020). "Saws, Scissors, and Sharks: Late Paleozoic Experimentation with Symphyseal Dentition". The Anatomical Record. 303 (2): 363–376. doi:10.1002/ar.24046. ISSN 1932-8486.
  14. ^ "PBDB Taxon". Retrieved 7 September 2024.
  15. ^ a b c d e Heckel, Philip H. (2008), "Pennsylvanian cyclothems in Midcontinent North America as far-field effects of waxing and waning of Gondwana ice sheets", Special Paper 441: Resolving the Late Paleozoic Ice Age in Time and Space, vol. 441, Geological Society of America, pp. 275–289, doi:10.1130/2008.2441(19), ISBN 978-0-8137-2441-6, retrieved 2024-10-01
  16. ^ a b c d Schram, Fredrick R. (1 January 1984). "Frederick R. Schram; Upper Pennsylvanian arthropods from black shales of Iowa and Nebraska". Journal of Paleontology. 1: 197–209 – via JSTOR.
  17. ^ a b c d Heckel, Phillip H. (1977). "Origin of Phosphatic Black Shale Facies in Pennsylvanian Cyclothems of Mid-Continent North America". AAPG Bulletin. 61. doi:10.1306/C1EA43C4-16C9-11D7-8645000102C1865D. ISSN 0149-1423.
  18. ^ "Collection: Hansen Quarry, Quarry 6, Stark Shale, Papillion Sarpy County, Nebraska, USA". mindat.org. Retrieved 1 October 2024.
  19. ^ Zangerl, Rainer; Case, Gerard R. (1973). Iniopterygia : a new order of Chondrichthyan fishes from the Pennsylvanian of North America. Fieldiana. Chicago: Field Museum of Natural History.
  20. ^ a b c Gayford, Joel H.; Engelman, Russell K.; Sternes, Phillip C.; Itano, Wayne M.; Bazzi, Mohamad; Collareta, Alberto; Salas‐Gismondi, Rodolfo; Shimada, Kenshu (3 September 2024). "Cautionary tales on the use of proxies to estimate body size and form of extinct animals". Ecology and Evolution. 14 (9). doi:10.1002/ece3.70218. ISSN 2045-7758. PMC 11368419. PMID 39224151.{{cite journal}}: CS1 maint: PMC format (link)
  21. ^ Engelman, Russell K. (2024-09-07). "Reconstructing Dunkleosteus terrelli (Placodermi: Arthrodira): A new look for an iconic Devonian predator". Palaeontologia Electronica. 27 (3): 1–79. doi:10.26879/1343. ISSN 1094-8074.
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