Lonchodomus volborthi


In the figure above, the order Trinucleida is seen as an offshoot of the problematic "Ptychopariida"
originating in the Middle Cambrian, and extending to the late Silurian via the genus Raphiophorus


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(revised per Bignon et al 2019)    
last revised 08 July 2020 by S. M. Gon III

Introduction: Recognized as an order in 2019, order Trinucleida was elevated from out of the Order Asaphida, where it was considered a Superfamily, Trinucleioidea.   
Cephalon: Short eyes (often secondarily lost) connected to glabella by eyeridge without palpebral lobe. Glabella convex or pyriform. Three or fewer short glabellar furrows. Tubercle, when present, on glabella or occipital ring. Free cheeks anteriorly fused. Facial sutures marginal or opistoparian. Perforated fringe present in more derived forms. Genal spines typically long and narrow at base; hypostome conterminent or impendent.
Thorax: usually 5 8 segments, but only 2-3 segments in progenetic Raphiophoridae, and up to 30 in Seleneceme (Alsataspididae), with long, narrow adaxial pleurae.
Pygidium: triangular or semi-circular, smaller than cephalon, wider (tr.) than long (sag.), with a curved anterior border. Pygidial axis narrow, may extend onto posterior border.
Other: Usually with small ovoid-globular (commutavi) protaspides.
Occurrence: Middle-Upper Cambrian boundary to upper Ordovician-lower Silurian.
Suborders: None.
Superfamilies: None.




Trinucleioidea Guide

Families: Alsataspididae (including Orometopidae), Dionididae, Liostracinidae, Raphiophoridae, Trinucleidae, 
Genera: Alsataspididae: Ajrikina, Alataupleura, Araiopleura, Calycinoidia, Caputrotundum, Clavatellus, Falanaspis, Hapalopleura, Huamiaocephalus, Jegorovaia (=Hermosella), Jiangxiaspis, Orometopus, Pagometopus, Palquiella, Paracalymenemene (/Paracalymene LIU), Plesioparabolina, Pyrimetopus, Rhadinopleura, Seleneceme (=Alsataspis), Sibiriopleura, Skljarella (=Proaraiopleura), Spirantyx, Trigocephalus, Yumenaspis, Zacompsus.

Dionididae: Aethedionide, Digrypos, Dionide (/Dione; /Polytomurus; =Dionidepyga; =Trigrypos), Dionideina, Dionidella, Huangnigangia, Paradionide, Tongxinaspis, Trinucleoides.
Liostracinidae: Aplexura, Doremataspis, Liostracina, Lynaspis.
Raphiophoridae: Ampyx (/Brachyampyx), Ampyxella, Ampyxina, Ampyxinella, Ampyxoides, Anisonotella, Bulbaspis, Caganaspis, Carinocranium, Cerampyx, Cnemidopyge, Collis, Edmundsonia, Ellsaspis, Endymionia (/Endymion), Globampyx, Jiuxiella (=Miboshania), Kanlingia, Lonchodomas, Malinaspis, Malongullia (=Ampyxinops), Mendolaspis, Metalonchodomas, Miaopopsis, Nambeetella, Nanshanaspis, Parabulbaspis, Parampyx, Pseudampyxina, Pytine, Raphioampyx, Raphiophorus, Raymondella (/Reedaspis), Rhombampyx, Salteria, Sinampyxina, Sinoluia, Taklamakania (=Xinjiangia).
Trinucleidae: Anebolithus, Australomyttonia, Bancroftolithus, Bergamia (=Bohemaspis; =Brandysops; =Cochliorrhoe), Bettonolithus, Botrioides, Broeggerolithus (=Ulricholithus), Costonia, Cryptolithoides, Cryptolithus, Deanaspis, Declivolithus, Decordinaspis, Eirelithus, Famatinolithus, Furcalithus, Guandacolithus, Gymnostomix, Hanchungolithus (=Ichangolithus; =Yinjiangolithus), Huenickenolithus, Incaia, Jianxilithus, Lloydolithus, Lordshillia, Marekolithus, Marrolithoides, Marrolithus, ?Microdiscus, Myinda, Myindella, Myttonia, Nankinolithus, Ningkianolithus (=Cerato1ithus; =Hexianolithus), Novaspis, Onnia, Paratretaspis, Paratrinucleus, Parkesolithus, Pragolithus, Protoincaia, Protolloydolithus, Reedolithus, Reuscholithus, Salterolithus (=Smeathenia), Stapeleyelta, Telaeomarrolithus, Tetrapsetlium, Tretaspis, Trinucleus (/Edgellia), Whittardolithus, Xiushuilithus, Yinpanolithus.

Bignon et al (2019) provided an extensive review of evidence arguing that the Trinucleioidea within the order Asaphida should be elevated to a distinct Order Trinucleida, as described above. They argued that the ventral median suture did not evolved only one time, rebutting the argument uniting all members bearing that feature as a monophyletic group.  
The ontogeny of the advanced members of the Asaphida (sensu Fortey 1990 e.g., Asaphoidea, Trinucleioidea, Cyclopygoidea) are characterized by  an effaced, globular protaspis. However, there are strong variations in the protaspid form among the globular types, and the hypothesis that the globular protaspis evolved only once was rejected by Bignon et al (2019), who distiguished the globular protaspides of trinucleids as "commutavi," and split out the Trinucleioidea from Asaphida, re-erecting the order Trinucleida, which is here accepted.  

Bignon, A., B. G. Waisfeld, N. E. Vaccari, & B.D.E. Chatterton. 2019. Reassessment of the Order Trinucleida (Trilobita). J. Syst. Paleontol. 18(13): 1061-77. 
Fortey, R. A. 1990. Ontogeny, hypostome attachment, and trilobite classification. J. of Paleontology. 33:529-76.
Fortey, R. A. 2001. Trilobite systematics: the last 75 years. J. of Paleontology. 75(6):1141-51.
Fortey, R. A. and B. D. E. Chatterton. 1988. Classification of the Trilobite Suborder Asaphina. Palaeontology 31(1):165-222
Jell, P.A. & J.M. Adrain. 2003. Available generic names for trilobites. Memoirs of the Queensland Museum 48(2):331-553.


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Walking Trilobite animation 2000 by S. M. Gon III