Mawsoniids is a well-defined family of Mesozoic coelacanths ranging from the Late Triassic to the Late Cretaceous. They dwelled in marine and freshwater environments, and most species are characterized by the presence of ossified ribs, coarse rugosities of the dermatocranium and cheek bones, spiracular and suboperculum usually absent, and reduction or loss of the descending process of the supratemporal, as well as by a large body size. Although relatively abundant in some localities in South America and Africa, the phylogenetic relationships among the species are still poorly understood. Here, we propose for the first time a species-level phylogeny of the mawsoniids, and we discuss its implications on the evolutionary history of the clade. This evolutionary history can be divided into two main episodes: a Triassic episode that occurred mostly in North America and a Western Gondwanan early Cretaceous episode that occurred mostly on Western Gondwana with a Late Cretaceous European extension. The Jurassic has yielded few mawsoniid remains, except the marine Trachymetopon, whose place in this evolutionary history remains to be understood. Other problematic taxa in this scheme, either for their age or for their phylogenetic relationships, are Parnaibaia and “Mawsonia” lavocati. Lualabaea is closely related, or possibly co-generic with Axelrodichthys. This analysis highlights the rich evolutionary history of this clade, and proposes some biogeographic patterns composed of both vicariant and dispersal events.
Three genera and four species of coelacanths, which are Piveteauia madagascariensis LEHMAN, Rhabdoderma madagascariensis (WOODWARD), Whiteia woodwardi MOY-THOMAS, W. tuberculata MOY-THOMAS have been described from the Lower Triassic of Madagascar. In the present study, we examined a coelacanth fossil from Madagascar deposited in Aquamarine Fukushima and determined it as a new species of the genus Whiteia of the family Whiteiidae. This new species, W. uyenoteruyai sp. nov. is distinguished from other species of the genus in having the combination of the following characters: 20 or more rows of fine long and short ridges separated from each other on scales, ridged ornamentation of lateral line scales on the sensory canal, eight rays on the first dorsal fin, no denticle on fin rays of the first dorsal fin and caudal fins (the third dorsal fin and the second anal fin) and ridges on the posteroventral portion of the operculum. The whiteiid coelacanths are most abundant in the Early Triassic and the origin is probably during the Early Triassic in shallow water along the northern coast of Pangea. Guizhoucoelacanthus guanlingensis LIU et al. from the Middle and Upper Triassic of China is probably derived from the Early Triassic Whiteia, which is most diversified among the family and survived with one species, W. oishii from the Late Triassic of West Timor (237–207 Ma), Indonesia. Atacamaia solitaris from the Late Jurassic of Chile is probably derived from Axelia or Wimania.
The evolutionary history of coelacanths is long and complex with indications of adaptations to different aquatic environments. Fossil coelacanths are registered in shallow brackish, fresh or marine waters, while extant coelacanths inhabit moderate deep marine environments. Here we review some evidences of this long-time adaptation to moderate deep waters, focusing mainly on the hypothesis of bimodal respiration (gas exchange through gills and lungs) in most fossil coelacanths and on the aquatic gas exchange and the presence of a non-functional lung in the extant coelacanth Latimeria chalumnae. Another morphological evidence is the high rate of lipid accumulation in the whole body of L. chalumnae.
The present overview of the histological studies on Latimeria mineralized tissues, since the discovery of the first living coelacanth in 1938, allows some anatomical and evolutionary considerations. It enlightens: i) a drastic reduction of cartilage to bone transformation processes during evolution; ii) the persistence of large volume of cartilage in the endoskeleton at adult stage; iii) the bony nature of the plates that surround the lung diverticle; iv) the presence of a developed process of spheritic mineralization in various skeletal organs: in teeth, in odontodes of the tegumentary skeleton (scales, fin rays), in scales at the interface between the external layer and the basal plate, as well as in lung bony plates.
Habitats of the Indonesian coelacanth, Latimeria menadoensis, were investigated by Remotely Operated Vehicles (ROVs) surveys in the northern coast of Sulawesi Island and southern coast of Biak Island by collaboration of Aquamarine Fukushima (Japan), and Indonesian Institute of Sciences and Sam Ratulangi University (Indonesia) from 2005 to 2015. The Remotely Operated Vehicles operations were conducted 1173 times and coelacanths were encountered 30 times. A total of 30 different individuals were observed at a depth range from 115.6 m to 218.9 m deep. The water temperature was between 12.4 to 21.5 ºC. Most of the individuals were found alone, however, schools of two, three and six individuals were also observed. The Indonesian coelacanth, L. menadoensis, was observed at a similar depth of the African coelacanth, Latimeria chalumnae, and the temperature range also seems to be very similar. However, Latimeria menadoensis was sometimes observed besides big rocks or the steep wall. It seems to be less sensitive to daylight than L. chalumnae. Here we report also, for the first time in the world, a juvenile coelacanth was observed in a small crack at 165 to 171 m depth during these surveys.
The juvenile of Indonesian coelacanth, Latimeria menadoensis is here described for the first time in detail with comparison to embryos of Latimeria chalumnae. The juvenile was found in free swimming at 164.6 m depth off Manado, Indonesia on the 6th October in 2009. Because the total length of the juvenile is 31.5 cm, which is smaller than the embryos of L. chalumnae, it is speculated that not much time has passed from its birth. The depth at which the juvenile was found is within the range of the depth where adult L. menadoensis were observed, hidden in a narrow and long overhang where large predators could not enter. The juvenile has a more slender body, smaller orbit, shorter and deeper posterior part of the body (caudal peduncle) between the second dorsal and the anal fins and anterior ends of the dorsal and ventral lobes of the caudal fin (the third dorsal and second anal fins), longer dorsal and ventral lobes of the caudal fin (the third dorsal and second anal fins), broader peduncles of broader lobed fins, larger first dorsal fin and longer supplementary lobe of the caudal fin (caudal fin) than embryos of L. chalumnae. This indicates clear differences in the first ontogenetic stages of the two species, although adults have almost the same morphological features. Latimeria menadoensis appears to reproduce in a rather confined area, because both the juvenile and adults have been found within the same area inside of Manado Bay.
Seventh specimen of Indonesian coelacanth, Latimeria menadoensis POUYAUD et al., 1999 was caught off Sulawesi, Indonesia on November 5, 2014. This specimen is a female, 1276 mm in standard length. Counts and measurements on the external morphology of the specimen was carried out by a team of Japanese and Indonesian researchers, providing the most comprehensive data on the morphology of this species. All available data on morphology of L. menadonesis is compiled together to provide insights into the biology and taxonomy of extant coelacanths.