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A left angular (PRC 160) Mawsoniidae indeterminate in Cavin, Tong, Buffetaut, Wongko, Suteethorn & Deesri, 2023. |
Abstract
Mawsoniidae is a family of coelacanths restricted to the Mesozoic. During the Cretaceous, mawsoniids were mainly represented by the Mawsonia/Axelrodichthy complex, long known to be from western Gondwana only (South America and Africa). This apparent biogeographical distribution then faded following the discovery of representatives in the Late Cretaceous of Laurasia (Europe and North America). We report here the presence, in the Lower Cretaceous site of Kham Phok, NE Thailand, of an angular bone referred to the Mawsonia/Axelrodichthys complex. A comparison with angulars referring to both genera found in various regions of the world between the Late Jurassic and the Late Cretaceous indicated that the distinctions between these genera, and even more so between their constituent species, are unclear. This discovery is further confirmation of the very slow morphological evolution within this lineage, which may explain why their evolutionary history appears to be disconnected, at least in part, from their geographical distribution over time.
Keywords: Actinistia; Mawsoniidae; paleobiogeography; angular; Khorat Plateau; Early Cretaceous; Axelrodichthys; Mawsonia
Systematic Paleontology
Actinistia Cope, 1871
Latimerioidei Schultze, 1993
Mawsoniidae Schultze, 1993
Mawsoniidae indeterminate
Referred material: A left angular associated with a dermal skull bone (supraorbital?) (PRC 160) from the Kham Phok fossil site.
Locality and horizon: Kham Phok, Khamcha-i district, Mukdahan Province, upper part of the Phu Kradung Formation, basal Cretaceous.
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| Comparison in labial (a–k) and lingual (a’–k’) views between left angular PRC 160, Kham Phok locality ((g), framed in red), and other angulars referred to the Mawsonia/Axelrodichthys complex, discovered in various parts of the world dated between the Late Jurassic and the Late Cretaceous (red spots). Remark: Specimens i and j, figured by Cupello et al. [2016] and Batista et al. [2019] are both from a formation called Missão Velha and Brejo Santo, respectively, with an uncertain age of the Late Jurassic or Late Cretaceous. (a) A. megadromos, Southern France, terminal Cretaceous (inverted); (b) M. sp., USA, Woodbine Fm. (inverted); (c), A. lavocati, Northern Africa, ‘Continental Intercalaire’; (d) A. araripensis, Brazil, Santana Fm. (inverted); (e) M. gigas, Brazil, Marfim Fm.; (f) M. gigas, Brazil, Sanfranciscana Fm. (inverted); (h) mawsoniid indet., UK, Kimmeridge Clay; (i) mawsoniid indet., Brazil, MissãoVelha/Brejo Santo Fm.; (j) M. gigas, Brazil, MissãoVelha/Brejo Santo Fm. (inverted); (k) M. gigas, Brazil, Taruarembó Fm. Data from Cavin et al. (2021). Orange spots indicate other mawsoniid remains, not detailed here. The main anatomical structures are figured with colored areas. Abbreviations: a.f, adductor fossa (blue); con.Part, contact surface with prearticular (red); gr.VII.m.ext groove for external mandibular ramus of VII (green); l.f, longitudinal fossa (orange); m.s.c, mandibular sensory canal (yellow); sut.p.Co, sutural contact surface with principal coronoid (purple); sut.De: sutural surface for dentary (pink); sut.Spl: sutural surface for splenial (grey). |
Conclusions:
The discovery of a mawsoniid coelacanth in the Lower Cretaceous of Thailand is an important new addition to the already rich vertebrate assemblages of the Phu Kradung Formation and, more generally, the Jurassic–Early Cretaceous assemblages of the Khorat Group. On a global scale and on the scale of tens of millions of years, this new occurrence blurs the palaeobiogeographical model previously proposed for this clade. It questions both the vicariance and dispersal events previously proposed to explain the observed distribution. The recognition of the coelacanths as forming a slowly evolving clade, in particular the mawsoniid clade, can explain the disconnection between the phylogenetic pattern and the paleobiogeographical framework, whereas such a connection is normally expected in biogeographical studies. One way to test this scenario is (1) to attempt to build a stronger phylogeny based on a re-study of known and hopefully new material and (2) to better decipher the Cenozoic evolutionary history of the extant coelacanth, Latimeria, which shows a split of species between 30 and 40 million years ago associated with almost no morphological differentiation and therefore represents a good model to better understand the Cretaceous mawsoniid case.
Lionel Cavin, Haiyan Tong, Eric Buffetaut, Kamonlak Wongko, Varavudh Suteethorn and Uthumporn Deesri. 2023. The First Fossil Coelacanth from Thailand. Diversity. 15(2), 286. DOI: 10.3390/d15020286 (This article belongs to the Special Issue Evolution and Diversity of Fishes in Deep Time)