Tuesday, 5 July 2022

Cokotherium jiufotangensis: A new species of Eutherian Mammal from the Early Cretaceous Jehol Biota of China.

Eutherian Mammals (or Placental Mammals) represent the largest Mammal group alive today. Molecular clock analysis suggests that thy diverged from the Marsupials during the Jurassic, which is supported by the oldest known Eutherian Mammal fossil, Juramaia sinensis, coming from the Late Jurassic Tiaojishan Formation of Liaoning Province in China. However, the fossil record of early Eutherians is sparse, with only Juramaia sinensis known from the Jurassic, and a only a few examples, such as Eomaia, Acristatherium, Ambolestes, Sasayamamylos, Sinodelphys, and Prokennalestes coming from the Early Cretaceous. This makes it hard to make solid predictions about the relationships between these early Eutherian species, with the discovery of new species often leading to the reassessment of the importance of features used to define the group (for example Sinodelphys was considered the earliest known Marsupial until 2018, when a re-evaluation of the group following the discovery of Ambolestes zhoui lead to it being classified as a Eutherian.

In a paper published in the Philosophical Transactions of the Royal Society Series B: Biological Sciences on 7 February 2022, Hai-Bing Wang of the Key Laboratory of Vertebrate Evolution at the  Institute ofVertebrate Paleontology and Paleoanthropology, the Centre for Excellence in Life and Paleoenvironment of the Chinese Academy of Sciences, and the Key Laboratory of Palaeobiology and Stratigraphy at the Nanjing Institute of Geology and PalaeontologySimone Hoffmann of the  Department of Anatomy at the New York Institute of TechnologyDian-Can Wang of the Department of Oral and Maxillofacial Surgery at Peking University, and Yuan-Qing Wang, also of the Key Laboratory of Vertebrate Evolution at the  Institute of Vertebrate Paleontology and Paleoanthropology, and the Centre for Excellence in Life and Paleoenvironment of the Chinese Academy of Sciences, and of the College of Earth and Planetary Sciences at the University of Chinese Academy of Sciences, describe a new species of Eutherian Mammal from the Early Cretaceous Jehol Biota of Liaoning Province, China.

The new species is described from a single specimen collected from the Jiufotang Formation at the Sihedang Site in Lingyuan City, and has an estimated age of 120 million years. It is named Cokotherium jiufotangensis, where 'Cokotherium' is intended to honour the late Chuan-Kui Li for his  contributions to our understanding of the evolution of early Mammals, and 'jiufotangensis' means 'from Jiufotang'.

Holotype specimen of Cokotherium jiufotangensis (IVPP V23387). (a) Skeleton of Cokotherium jiufotangensis; (b) skull in ventrolateral view; (c) forelimb mainly in lateral view; (d) virtual reconstruction of the skull in dorsolateral view; (e) virtual reconstruction of the skull in ventrolateral view. Right side indicated by (r), left side indicated by (l). (d) and (e) at same scale. ap, angular process; as, alisphenoid; bh, basihyal; c, lower canine; C, upper canine; C2, axis (cervical vertebra 2); ca, capitate; ci, crista interfenestralis; cl, clavicle; cot, coronoid tubercle; cp, coronoid process; ct, centrale; cv, cervical vertebrae; d, dentary; eh, epihyal; fc, fenestra cochleae; fr, frontal; fv, fenestra vestibuli; gf, glenoid fossa; h, humerus; ha, hamate; if, infraorbital foramen; ju, jugual; lc, lacrimal; lcf, lacrimal foramen; lu, lunate; mac, mandibular condyle; maf, masseteric fossa; max, maxilla; mc, metacarpals; na, nasal; oc, occipital condyle; omc, ossified Meckelian cartilage; pa, parietal; pgf, postglenoid fossa; pgp, postglenoid process; ph, phalanges; pi, pisiform; pmx, premaxilla; po, postorbital process; pr, promontorium; ptf, posttemporal foramen; r, radius; s, scapular; sc, scaphoid; sh, stylohyal; sf, stapedius fossa; sq, squamosal; st, sternum; T, thoracic vertebrae; td, trapezoid; th, thyrohyal; tm, trapezium; tq, triquetrum; u, ulnar. Wang et al. (2022).

The specimen is a partial skeleton with a complete skull, forelimbs and part of the trunk and hindlimbs, which is preserved on a single slab of material. The dorsal and right lateral portions of the skull are obscured by the rock-matrix, but could be observed by computerised tomographic imaging.

Cokotherium jiufotangensis has an ossified Meckelian cartilage, something seen in modern Eutherians, but not previous Early Cretaceous examples such as Eomaia, Prokennalestes, Hovurlestes or Ambolestes. This had led to speculation that early members of the group retained a cartilaginous Meckelian sulcus into adult life, as was the case in the contemporary Eutriconodontan and Zhangheotheriid Mammals. 

Furthermore, this cartilage is reduced in size, which likely indicates that the middle ear bones have become detached from it, a key development of the ear in modern Mammals, which is not seen in the Eutriconodontans. The ear bones are separated from the Mecklian cartilage in some Zhangheotheriids, although only by a small gap, and this may also be the case in Cokotherium jiufotangensis.

Jaws and dentition of Cokotherium jiufotangensis (IVPP V23387). (a) Right upper jaw in lateral view; (b) right upper jaw in occlusal view; (c) left P5-M3 in occlusal view; (d) left p5-m3 in occlusal view; (e) right mandible with the ossified Meckelian cartilage (yellow) and hyoid bones (blue) in medial view; (f) right mandible with the ossified Meckelian cartilage (yellow) and hyoid bones (blue) in lateral view; (g) right mandible in dorsal view; (h) left mandible in dorsal view; (i) left mandible in lateral view showing unerupted canine. Right side indicated by (r), left side indicated by (l). (e)-(i) at same scale. ap, angular process; bh, basihyal; c, lower canine; C, upper canine; cot, coronoid tubercle; cp, coronoid process; DP, deciduous upper premolar; eh, epihyal; end, entoconid; hcd, hypoconid; hcld, hypoconulid; i, lower incisor; I, upper incisor; m, lower molar; M, upper molar; mac, mandibular condyle; maf, masseteric fossa; mdf, mandibular foramen; mec, metacone; med, metaconid; mef, mental foramen; omc, ossified Meckelian cartilage; p, lower premolar; P, upper premolar; pac, paracone; pacl, paraconule; pad, paraconid; pas, parastyle; pmc, post-metacrista cusp; pps, preparastyle; prc, protocone; prd, protoconid; sh, stylohyal; stc, stylocone; th, thyrohyal; uc, unerupted lower left canine. Wang et al. (2022).

Wang et al. were also able to reconstruct the inner ear of Cokotherium jiufotangensis in three dimensions using computerised tomographic scanning, the first time this has been done for an Early Cretaceous Eutherian Mammal (although the morphology of part of the inner ear has been described in Prokennalestes, a Eutherian from the Early Cretaceous of Inner Mongolia).

The  cochlear canal of Cokotherium jiufotangensis comprises a single coil (i.e. 360°). This is similar to the state in most later Cretaceous Eutherian Mammals, with greater coiling seen in most modern Mammals as well as some Cretaceous Zhelestids. Many other features of the ear, including  a secondary crus commune, the base of a secondary osseous lamina, the primary osseous lamina and a bony cribriform plate, are similar to those in both later Cretaceous and modern Eutherian Mammals, confirming these arose early in the history of the group.

Inner ear of Cokotherium jiufotangensis (IVPP V23387). (a) Position of inner ear (green), veins (blue), and nerves (yellow) in ventral view of cranium; (b) endocast of right and left inner ear (green), veins (blue), and nerves (yellow) in same ventral view; (c) cross section through left inner ear showing internal structures of cochlear canal; (a-c) at same scale. Endocast of inner ear (grey), cochlear and vestibular nerves (yellow) in (d) ventral, (e) dorsal, (f) medial and (g) lateral views, anterior is down, all at same scale. am, ampulla; asc, anterior semicircular canal; ca, cochlear aqueduct; cc, crus commune; ci, crista interfenestralis; cn, cochlear nerve; co, cochlear canal; cp, cribriform plate; crp, crista parotica; fc, fenestra cochleae; fn, facial nerve; fv, fenestra vestibuli; gf, glenoid fossa; gg, geniculate ganglion; hf, hiatus Fallopii; ips, inferior petrosal sinus; jf, jugular fossa; lhv, lateral head vein; lsc, lateral semicircular canal; pl, primary osseous lamina; pr, promontorium; psc, posterior semicircular canal; scc, secondary crus commune; sff, secondary facial foramen; slhv, sulcus for lateral head vein; vca, vein of cochlear aqueduct; vn, vestibular nerve. Wang et al. (2022).

The earliest Eutherian Mammals generally have a larger number of teeth than modern members of the group, and consequently these teeth are closely packed together. Cokotherium jiufotangensis has a reduced number of both incisors and molars, and consequently a less densely packed dentition, a trait otherwise recorded in Eutherian Mammals from the Late Cretaceous onwards. It still has four premolars on each side of both the upper and lower jaws, but the hindmost premolars are starting to show signs of molarisation, another trait previously known only from later Eutherian species.

Despite these apparently derived traits, a phylogenetic analysis carried out by Wang et al. suggests that Cokotherium jiufotangensis is one of the most basal Eutherians known, and possibly outside the Theria (Eutherians plus Marsupials) altogether. However, it is possible that this placement is an artefact of the small number of early Eutherians known, and our subsequent poor understanding of the relationships between these early Mammals.

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