Phyllolepis

''Phyllolepis'' (from the Greek roots for ‘leaf’ and ‘scale’)Murphy, Dennis C. 4..20.2016. “Phyllolepis rossimontia (placoderm).” ''Devonian'' Times. <http://www.devoniantimes.org/who/pages/phyllolepis.html#credits> is the type genus of Phyllolepida, an extinct taxon of arthrodire placoderm fish from the middle to late Devonian.“The life and times of Phyllolepis.” ''Phyllolepida''. Palaeos.com. 2005 <http://palaeos.com/vertebrates/placodermi/phyllolepida.html> The species of ''Phyllolepis'', themselves, are restricted to the Famennian-aged freshwater strata of the Late Devonian, around 360 million years ago. Fossils of this genus have been found primarily in Europe and North America. The end of the Devonian saw them disappear in a mass extinction.

''Phyllolepis'' lived in freshwater environments, possibly rivers and streams. As with all other known phyllolepids, ''Phyllolepis'' were presumed to have been blind, bottom-dwelling predators that detected prey through sensory organs in the surface grooves of their armor plates (which gave their plates a distinctive "wooden surface" appearance).

Description and paleobiology

''Phyllolepis'', as was typical of Phyllolepida, tended to be 30 to 40 cm in length and very flat. ''Phyllolepis'' have extensive armor made of full, flat plates, rather than scales, with both a wide jaw and mouth. They are characterized by a broad, enlarged nuchal plate paired with four small plates around the upper jaw. They have short and broad anterior ventrolateral plates, as well as paranuchal plates with postnuchal process. They also feature long, narrow anterior dorsolateral plates and a dermal ornamentation of smooth, concentric rings upon their plates.

As a placoderm, ''Phyllolepis'' was a primitive jawed fish, with their jaws likely having been evolved from the first set of their gill arches. Elongated basiptergia have been found on some phyllolepid placoderms, such as ''Austrophyllolepis'' and '' Cowralepis'', suggesting ''Phyllolepis'' may have had them as well.Long, J.A. 1984. New phyllolepids from Victoria and the relationships of the group. ''Proceedings of the Linnean Society of New South Wales'' 107: 263–308.

As with other members of Phyllolepida, ''Phyllolepis'' have eyes on either side of their heads, unlike modern flounders, as well as an absent sclerotic ring. This suggests that the eyes of ''Phyllolepis'' may have been much reduced, vestigial, or completely absent. The pattern of raised ridges on its head and trunk plates are a defining trait, setting them apart from other placoderms. The highly developed system of lateral ridges are suggested to have been used for electroreception, in order to sense their surroundings in absence of functional eyes.

Synapomorphies of ''Phyllolepis'' include the sub triangular dermal portion of the marginal plate. This genus can be distinguished from ''Cowralepis'' because of their deeply concave anterior margin, where it meets the postorbital plate, as well as the exit of the main lateral line canal in the middle of the plate (rather than in its posterior third division). ''Cowralepis'' also features a distinct posterior dorsolateral plate, which ''Phyllolepis'' lack entirely.

However, because many defining traits for ''Phyllolepis'' must be used in conjunction with other traits (for example, the genera ''Phyllolepis'', '' Placolepis'', and '' Cobandrahlepis'' all lack a posterior median ventral plate), it is difficult to identify phyllolepid placoderms unless a full fossil assemblage is known. Due to the isolated phyllolepid plates found for species such as ''P. concentrica, konicki, undulata'', and '' delicatula'', it has been suggested by J. Long and E. Daeschler that they be recategorized as Phyllolepidae gen. et sp. Indeterminate. ''P.'' ''woodwardi'''', orvini, neilseni'', and '' rossimontina'' are all species with complete and valid sets of plates.

Ecology

These fishes were likely ambush predators, lying in wait in the substrate as bottom-dwellers. Their long tails would be used for rapid propulsion to catch their prey. This is typical of many flattened placoderms.

One of the sites where various ''Phyllolepis'' specimens have been discovered, Red Hill in Clinton County, Pennsylvania, shows signs of having been a freshwater subhabitat, similar to current Midwestern lakes. However, dig sites in EuropeLane, J. A. and R.J. Cuffey. (2005) .Phyllolepis rossimontina sp. nov. (Placodermi) from the Uppermost Devonian at Red Hill, North-Central Pennsylvania. Revista Brasileira de Paleontologia 8(2): 117-126- and even locations still in the Pennsylvania Catskill formationLong, J., & Daeschler, E. (2013). First articulated phyllolepid placoderm from North America, with comments on phyllolepid systematics. ''Proceedings of the Academy of Natural Sciences of Philadelphia,'' ''162'': 33-46. Retrieved from http://www.jstor.org/stable/42751947- may have been home to coastal-marine ''Phyllolepis'' species, based on the composition of siltstones and sediment deposits. At least one phyllolepid specimen has been found surrounded by plant material and greenish siltstones, suggesting a low energy environment, such as a floodplain pond. This divide raises some questions about how the genus lived in both places. It is possible there were land connections intercontinentally, or the fishes were euryhaline or marine-tolerant. The fish may have even evolved a freshwater habit after arriving in North America by marine waterways.

All of the phyllolepids found in the Northern Hemisphere have been found in rock dated to the later Devonian Period (Famennian), however Australian and Antarctic samples date earlier, from the late middle Devonian up through the late Famennian. The oldest confirmed Phyllolepid is from the middle Devonian. The genus ''Phyllolepis'' has so far only been found in Famennian age strata.

Species

History of categorization

''Phyllolepis'' was the first genus to be recognized in the order Phyllolepidae until 1984. ''Phyllolepis concentrica'' was the first species discovered of the genus, by Louis Agassiz in 1844 at Clashbennie, Scotland. This specimen caused Woodward to incorrectly classify them as a distinct group belonging to the jawless fishes in 1915, along with '' Drepanaspidae''. Sensiö continued, with several publications in the 1930s, to show that phyllolepids had jaws, and belonged with placoderms. He also proclaimed them part of Phyllolepida, based on his studies of ''P. orvini, P. soederberghi,'' and ''P. neilseni''.

From the 1930s to 50s, there were several isolated dermal plates of phyllolepids found in Australia, attributed by Hills to be ''Phyllolepis'' but specific species left undefined. In the 1970s many complete, articulated Phyllolepid specimens were found from the middle Devonian at the Mt. Howitt site in Australia. This material was gradually defined as genus ''Austrophyllolepis'', based on plate proportions, but also solidified the assignment of the group to Placodermi and as a clade inside Arthrodira.

The more recent introduction of ''P. rossimontina'' as a species in 2005 demonstrated, in part, a wide range of variability within the genus ''Phyllolepis'', causing some scientists to begin a revision of the genus.

The low number of complete ''Phyllolepis'' specimens found combined with key identifying features only visible on specific plates has made it difficult to confidently identify species, causing some paleontologists to call for redesignations of the current named species of ''Phyllolepis''.

References

* ''Wildlife of Gondwana: Dinosaurs and Other Vertebrates from the Ancient Supercontinent'' (Life of the Past) by Pat Vickers Rich, Thomas Hewitt Rich, Francesco Coffa, and Steven Morton
* ''The Rise of Fishes: 500 Million Years of Evolution'' by John A. Long

External links

Phyllolepida
at Palaeos.com

{{Taxonbar, from=Q7188831

Phyllolepids
Placoderms of Europe
Placoderms of North America
Fossil taxa described in 1844
Famennian life
Famennian genus first appearances
Famennian genus extinctions