Romundina is a small, heavily armored extinct genus of acanthothoracid placoderms which lived in shallow marine environments in the early Devonian (Lochkovian). The name Romundina honors Canadian geologist and paleontologist Dr. Rómundur (Raymond) Thorsteinsson of Calgary, Alberta, Canada. Romundina are believed to have lived on Earth between 400 and 419 million years ago. The closest known relative to Romundina is the acanthothoracid Radotina. The type and only described species is R. stellina. The first specimen of Romundina was originally discovered by Swedish paleontologist Tor Ørvig in 1975 on Prince of Wales Island (Nunavut) in a formation that geologically dates back to the Gedinnian. Only one known species of Romundina has been discovered which was named Romundina stellina by Ørvig. The species name stellina refers to stellate (derived from Latin word stella meaning star) tubercles that the Placoderm has ornamenting its dermal skeleton. Romundina stellina is one of the earliest known acanthothoraciforms discovered to date. Recently, Romundina stellina has been heavily researched as it shows signs of having extremely primitive teeth, which structurally share characteristics of both dermal tubercles and the teeth of modern and fossil gnathostomes. Although Romundina are known to be relatively small especially when compared to some large predatory placoderms of the time, it is still thought to be carnivorous due to the tooth-like structures on its gnathal and supragnathal plates. Romundina shares many characteristics of both cyclostomes and Gnathostomes to varying degrees. This reinforces the hypothesis that placoderms are a grade as they have some features more closely related to cyclostomes then other placoderms while other features are much more closely related to crown group Gnathostomes.
Description and paleobiology
Dentition and the evolution of teeth Romundina is currently understood to be the first placoderm to develop primitive tooth-like structures. Originally, teeth were thought to arise following Placodermi but prior to crown Gnathostomata; however, recent research suggests that teeth may have arisen in derived placoderms such as Romundina. There is currently much debate surrounding whether these tooth-like structures as well as the gnathal plates they sit on can be seen as the true primitive condition of Gnathostome dentition. Some researchers see the tooth-like structures of extensions of a primitive crushing plate, which is common in Placoderms. These primitive tooth-like structures lie on supragnathal plates and have a multi-cuspid appearance suggesting some degree of tooth-to-tooth occlusion occurred. Additionally, synchrotron radiation X-ray tomographic microscopy reveals that these tooth-like tubercle structures likely consist of a dentine based core and an enameloid cap common to many extant fish groups; however, they lack internal vascularization. The presence of enameloid cap suggests that Romundina is either closely related to crown Gnathostomata or that this feature arose through convergent evolution and was later lost. Romundina's dental tubercles lack any apparent organization which is a more primitive feature found in Cyclostomes suggesting that organized tooth rows evolved just prior to the evolution of the first Gnathostomes. The supragnathal plates which these tooth-like tubercles sit on are oval-shaped, flat, and relatively symmetrical. This suggests that tubercles were added radially and episodically to the margins of these gnathal plates around a central large pioneer tooth. As tubercles were added the gnathal plates were thickened leading to the presence of growth rest lines. The gnathal plates consist of three layers: the most superficial containing the marginal tooth-like tubercles, the medial layer containing structures containing vascularization, and the basal lamellar layer. Furthermore, Romundina's tubercle structures on the gnathal plates suggest that teeth evolved from an external epithelial structure that moved internal rather than from a non-skeletal antecedent organ system, most likely coming from the stellate tubercles on the dermal skeleton.
Skull The skull of Romundina consists of thin proportional dermal bones, which are heavily ornamented by mesodentine rich stellate (star shaped) tubercles. These stellate tubercles likely grew on top of each other with between two and three generations. Furthermore, these tubercles exhibit rough ridges radiation from an apex, which is in contrast to other species, which had smooth ridged stellate tubercles such as Radotina. These stellate tubercles were likely used as a form of protection from predators Additionally, Romundina stellina only has one dermal cheek element (suborbital plate) while most other species of Placoderms such as Radotina have multiple dermal cheek elements. This suggests that Romundina lost the submarginal plates and postsuborbital seen in earlier vertebrates. An additional feature that distinguishes Romundina from previous Placoderms is the absence of tesserae (a type of mineralized tile) between the dermal bones on the roof of the skull as the dermal bones in the roof of Romundina's skull are fused together. The anatomy of the Romundina nasal capsule combines characteristics of both jawless Cyclostomata and jawed Gnathostomes. Previous to the discovery of Romundina there appeared to be a large jump in the cranial features between the two extant taxa with no apparent bridge. The most prominent of these "bridge" features is the positioning of the nasal capsule dorsally between the eyes, a feature only present in early to mid Placoderms. Cyclostomes lack a nasal capsule instead having a nasohypophysial opening while Gnathostomes have an anterior nasal capsule. Romundina has been shown to unambiguously have a jaw, however the cranial anatomy and proportions of the brain are more closely aligned with those of jawless vertebrates. Analysis of the cranial structure reveals that Romundina had a large precerebral region, broad suborbital shelves, and a small (or potentially non existent) telencephalon. Furthermore, Romundina have a large protruding bony upper "lip", which begins anteriorly to the nasal capsule.
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