Saturday 31 March 2012

New Tetrapodomorph Fish from the Devonian of Nevada.

Tetrapods are four-limbed terrestrial Vertebrates, plus animals which can be shown to have descended from four-limbed terrestrial Vertebrates, such as snakes or whales. Tetrapodomorph is a term which is used to refer to Tetrapods plus those fossil fish more closely related to them than to Lungfish. A Tetrapodomorph Fish is therefore any Tetrapodomorph that is not actually a Tetrapod, an imprecise taxonomic definition (since such definitions should not rely on simply excluding groups for convenience) but a useful one. The term 'fish' has little taxonomic significance (contrary to what tends to appear in school textbooks) since at its most precise it simply means any Vertebrate which is not a Tetrapod.

The study of Tetrapodomorph Fish is therefore important to our understanding of the colonization of land by Vertebrates, generally considered one of the most important events in the history of life on Earth.

In a paper published in the journal PLoS One on 20 March 2012, Brian Swartz of the Department of Integrative Biology and Museum of Paleontology at the University of California, Berkeley, describes a new species of Tetrapodomorph Fish from the Middle Devonian Red Hill I Beds in the Simpson Park Mountains in Eureka County, Nevada.

Tinirau clackae. Top, photograph. Middle, interpretive drawing based upon photograph. Abreviations: ba.a, anal basal;, basal articulation of the basipterygoid process; Basb1, basibranchial #1; bas.f, basicranial fenestra; Ch, ceratohyal; Clth, cleithrum; Clv, clavicle; Co3, posterior coronoid; de.f, dentary fang; Dpt, dermopalatine; Enpt, entopterygoid; Ept, ectopterygoid; Exsc.l, lateral extrascapular; Exsc.m, median extrascapular; Fe, femur; fib, fibula; fib.p, posterior process of the fibula; gle, glenoid fossa; Gu, lateral gular; Hh, hypohyal; Hu, humerus; Hyo, hyomandibular;, hyomandibular articulation; Ic, intercentrum; Int, intermedium; It, intertemporal; Ju, jugal; La, lacrimal; Mk, Meckelian bone; mk.grv, Meckelian groove; Mx, maxilla; na.a, anterior naris; nc.c, notochordal canal; ns, neural spine; Op, operculum;, parietal pitline; Part, prearticular; pin.f, pineal foramen; Plv, pelvis; Pmx, premaxilla; Po, postorbital; Pop, preoperculum; Pp, postparietal; Pq, palatoquadrate; Psph, parasphenoid; Qj, quadratojugal; Quad, quadrate; R, radius; ra, radial; ra.a, anal radial; ra.c, caudal radial; Ri, rib; Sang, surangular; Sbm, submandibular; Sca, scale; Sco, scapulocoracoid; Spl, splenial; St, supratemporal; st.f, subtemporal fossa; So.p, posterior supraorbital; Sq, squamosal; Tab, tabular; Te.a/Ro.l, anterior tectal+lateral rostral; tib, tibia; U, ulna; ul, ulnare; Uh, urohyal; Vo, vomer. (l) or (r) refers to left or right when displaced from natural side. Scale bar 10 cm. Bottom, reconstruction. Known elements in black, theorized elements in grey. Swartz (2012).

Swartz names the new species as Tinirau clackae, after Tinirau, a fish-man from Polynesian legend and Jenny Clack, a palaeontologist at the University of Cambridge noted for her expertise on evolution of early Tetrapods. Conodont fossils from the same sediments place Tinirau clackae in the Klapperina disparilis biozone, i.e. in the Late Givetian Stage of the Middle Devonian, a little over 385 million years ago. The Red Hill I Beds comprise lakers of limestone and calcarious mudstones, and contain fossil Conularids (solitary corals) which are thought to have been exclusively marine in origin and trace fossils associated with near-shore environments.

Left, Devonian outcrops in Eureka County, black. Yellow star labeled UCMP V74080 marks the site where Tinirau clackae was discovered. Inset, position of Eureka County in Nevada. Right, stratigraphic column showing where Tinirau clackae (yellow stars) was found vertically and chronologically. Swartz (2012).

Tinirau clackae is described from six specimens originally excavated by a University of California, Berkeley team lead by Joseph Gregory in the 1970s. These were extracted from the limey matrix using washes of dilute acid, then washed under running water for a month and air dried. The material had previously been studied by a graduate student at Berkeley in the 1980s, but the results never published.

While Tinirau clackae does not appear to be a direct ancestor of modern Tetrapods, it does reveal quite a lot about the evolution of the group, since its unique set of features required a re-assessment of relationships within the group, with a number of implications, most notably that Eusthenopteron, often thought of as being close to the lineage from which Tetrapods descended, is probably more distantly related than previously thought.

Evolutionary relationships within the Tetrapodomorph Fish, as implied from the glenoid fossa (A) and pelvic limbs (B) of Tinirau clackae. Swartz (2012).