Wednesday, October 25, 2017

[PaleoIchthyology • 2017] Foreyia maxkuhni • Heterochronic Evolution Explains Novel Body Shape in A Triassic Coelacanth from Switzerland


Foreyia maxkuhni 
Cavin, Mennecart, Obrist, Costeur & Furrer, 2017


Abstract

A bizarre latimeriid coelacanth fish from the Middle Triassic of Switzerland shows skeletal features deviating from the uniform anatomy of coelacanths. The new form is closely related to a modern-looking coelacanth found in the same locality and differences between both are attributed to heterochronic evolution. Most of the modified osteological structures in the new coelacanth have their developmental origin in the skull/trunk interface region in the embryo. Change in the expression of developmental patterning genes, specifically the Pax1/9 genes, may explain a rapid evolution at the origin of the new coelacanth. This species broadens the morphological disparity range within the lineage of these ‘living fossils’ and exemplifies a case of rapid heterochronic evolution likely trigged by minor changes in gene expression.


Figure 1: Skeleton of the new coelacanth Foreyia maxkuhni gen. et sp. nov.
(A) Photo and (B) outline of the holotype (PIMUZ A/I 4620). (C) Reconstruction of the whole skeleton.


Sarcopterygii Romer, 1955

Actinistia Cope, 1891

Latimeriidae Berg, 1940 sensu Dutel et al., 20125

Foreyia gen. nov.

Diagnosis: Latimeriid coelacanth with dermal bones covered with numerous large tubercles; hypertrophied otico-occipital portion of skull; fusion of postparietal, supratemporal and extrascapular in postparietal shield, which forms a dome in occipital region; supraorbital sensory canal running in a wide groove; short and curved mandible; pterygopalatine deeper than long with enlarged autopalatine; lachrymojugal and squamosal fused; hypertrophied clavicle; few abdominal vertebrae (seventeen); expanded dorsal and caudal fins; and atrophied pectoral fins.

Foreyia maxkuhni gen. et sp. nov.

Etymology: The generic name honors late Peter L. Forey for his contribution on the study of coelacanth fishes. The specific epithet refers to Max Kuhn, who kindly supported for 12 years the preparation and study of fossils from the Middle Triassic of Graubünden and especially the specimens described here.

Type locality and horizon: Site DF 4 near the Ducanfurgga (Graubünden, Switzerland), upper part of the Prosanto Formation, Middle Triassic (early Ladinian, 240.91 million years ago).


Figure 2: Osteological details of the new coelacanth  Foreyia maxkuhni gen. et sp. nov. (A) Photo and (B) surface CT reconstruction of the skull of the paratype (PIMUZ A/I 4372). (C) Tubercles and denticles in the Holotype (PIMUZ A/I 4620) and (D) in the paratype (PIMUZ A/I 4372).

 1, tubercles on the skull roof. 2, large spine-like tubercles on the posterior margin of the otico-occipital shield. 3, denticles on the fin rays of the first dorsal fin. 4, scales with denticles from the ventral margin of the caudal peduncle. 5, scales with denticles from the anal region. 6, scales with denticles from the belly region. 7, toothed coronoid bones. 8, scales with denticles from the flank. 9, supplementary caudal fin lobe with spiny scales. 10, Scales with denticles from the lobe of the anal fin.

Figure 3: Phylogenetic relationships of  Foreyia maxkuhni gen. et sp. nov. and developmental origin of the derived characters.
(A) Strict consensus trees of the 259 most parsimonious trees of 317 steps (CI = 0.3817, RI = 0.6766) with some of the uniquely derived characters present in Foreyia maxkuhni on the left, and reconstructions of genera with atypical general morphology. (B and C) Shared features of Ticinepomis peyeri and Foreyia maxkuhni (in orange) not included in the cladistics analysis (see main text for numbers). (D) Reconstruction of a coelacanth embryo with localization of embryonic tissues that give rise the derived skeletal features present in Foreyia. It is hypothesized that changes in the expression of Pax9 may have altered the derived characters shown in blue on the reconstruction (E).
All the drawings were made by LC.  Abbreviation: Boc, basioccipital; Cla, clavicle; Exo, exoccipital; lat. Meso., lateral mesoderm; neur. cr., neural crest; pect. f., pectoral fin; S (numbered), somite.

Figure 4: Reconstruction of the living coelacanth  Foreyia maxkuhni gen. et sp. nov.
 Artwork by Alain Bénéteau.

Lionel Cavin, Bastien Mennecart, Christian Obrist, Loïc Costeur and Heinz Furrer. 2017. Heterochronic Evolution Explains Novel Body Shape in A Triassic Coelacanth from Switzerland.  Scientific Reports. 7,  13695.  DOI:  10.1038/s41598-017-13796-0


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