Pinworms as a natural group
Within the Nemata, the Oxyurida is the only order with representative species occurring as parasites in both vertebrates and invertebrates (Figure 4). They are found in arthropods ,, fishes, birds, lizards, amphibians, and mammals . Whatever their host group, pinworms feed on endosymbiotic bacteria and protists living in the digestive tract of their hosts and these parasites only occur in animals that digest relatively large quantities of cellulose. One question then arises: Did the Oxyurida have a single common origin or is the group polyphyletic, having originated more than once from ancestral nematode species. Several lines of evidence provide support of the Oxyurida as a natural or monophyletic group.
Recent studies using molecular data to define the phylogenetic relationships of the main groups of the phylum Nemata all agree in the placement of species of Oxyurida in a monophyletic group or clade - (Figure 4). All Oxyurida thus far investigated have a haplodiploid mode of reproduction: Unfertilized eggs give rise to haploid males and fertilized eggs always produce diploid females . There is currently no evidence showing a reversal of this mode of reproduction in any taxa of the Nemata or other groups of organisms that manifest a haplodiploid mode of reproduction; this list includes, but is not restricted to, the Hymenoptera (ants, wasps, honey bees), Homoptera, and some mites (Acari: Mesostigmata). Data available from the phylogenetic databases of the Nemata, and the groups mentioned above, show clearly that once a species has made the transition from a diploid to a haplodiploid mode of reproduction, reversal back to a diploid system of sexual recombination is unlikely. We therefore interpret haplodiploidy in the Nemata as an ancient life-history character that probably originated simultaneously with the speciation event that gave rise to the Oxyurida .
Host specificity and coevolution
Host specificity gauges the degree to which a parasite species is restricted to a particular host species without reference to phylogeny . Pedersen et al. showed that parasites using close contact transmission are primarily specific to single host species. Pinworms certainly belong to this category. In addition to having a direct life-cycle without involving an intermediate host, observations demonstrate that different behaviors of the hosts of pinworms facilitate direct-transmission of the parasite eggs thus enabling successive self-infections ,. Common behaviors of host animals that may increase the probability of repetitive self-infections are: coprophagy (feeding on excrements) and caecotrophy (re-ingestion of fecal pellets naturally produced by digestion from the cecum).
Parasites with direct life-cycles occurring in hosts that manifest behaviors that increase the probability of self-infection appear to be able to maintain this association for long periods of time. At the same time these characteristics of both the host and parasite appear to decrease the probability that the parasite would switch to a new species of host. One evolutionary consequence of host specificity is a long-standing association between hosts and parasites through geological time. Darwin  first discussed this idea and afterwards, many authors have subsequently suggested that the phylogenetic relationships of highly host-specific parasites could provide valuable information on the evolutionary history of their hosts ,-. This defines the fields of Historical Ecology and Cophylogeny: The study of two or more associated groups of organisms sharing a common history of speciation, making possible phylogenetic conclusions. Accordingly, numerous studies have produced evidence for extreme host specificity and patterns of cophylogeny among mammals and their oxyurid parasites ,.
Identification of the host
The Cynodontia, are therapsid vertebrates that first appeared in the Late Permian (approximately 260 Mya). The group includes modern mammals as well as their extinct ancestors and close relatives. Non-mammalian cynodonts spread throughout southern Gondwana and are represented by fossils from South America, Africa, India, and Antarctica. The non-mammalian cynodont fossils collected from the site from where the coprolite was identified belong to the species Massetognathus ochagaviae Barberena, ; Chiniquodon theotonicus Huene, ; Traversodon stahleckeri Huene, ; Luangwa sudamericana Abdala and Sá-Teixeira, ; and Protheriodon estudianti Bonaparte et al. . Massetognathus and Traversodon were most abundant in the cenozone of the site ,.
Species of Chiniquodon and Protheriodon were carnivorous cynodonts, which lived during the early Late Triassic in South America. Traversodon, Luangwa, and Massetognathus all are members of family Traversodontidae, which includes fairly advanced, plant-eating, non-mammalian therapsids . For instance, Massetognathus had the distinctive long snout of its cynodont relatives, with nipping incisors and fang-like canines, but its cheek teeth were flat- topped and covered with low ridges, which made them good for grinding stems, roots and other plant materials. Thus, the fauna associated with the coprolite is dominated by herbivorous or semi-herbivorous animals.
Species of animals (including humans and other primates) infected by pinworms always consume large quantities of cellulose, with no known exceptions, regardless of their phylogenetic affinities: vertebrate or arthropod. This feeding behavior may be considered necessary for the Oxyurida to be able to infect and survive in a host. Thus, the cynodonts that were herbivorous and were found at the collection site appear to be the most plausible sources of the coprolite from which we found the pinworm: i.e. one of the species identified in the Traversodontidae (Figure 4).
Species assigned to the Traversodontidae appear to have had a primarily Gondwanan distribution, with many species known from Africa and South America . Battail  has hypothesized that they originated in what is now South America with subsequent diversification east into Africa and north into what is now Europe and eastern North America. We therefore consider representatives of the family Traversodontidae identified at the site of discovery of the coprolite, as the hosts of the new pinworm species described in this work. We also consider that the Traversodontidae are the primitive hosts of the Heteroxynematidae with a Gondwanan origin for both families (Figure 4).
Did the dinosaurs have pinworms?
The discovery of an egg of a nematode of the order Oxyurida in the Cynodontia shows that this host-pinworm parasitic association existed as far back as the transition point of the reptile-mammal phylogenetic divergence, approximately 240 million years ago. This finding shows that parasitism as an ecological trait originated at least as early as this group of cynodont infecting nematodes and that this group of nematodes has successfully been infecting herbivorous mammals (and birds to a lesser extent) to the present time. Most animal groups that contain herbivorous representatives are parasitized by pinworms (Figures 4 and 5). This begs the question of the existence of pinworms in extinct animal lineages having similar feeding behaviors: as all herbivorous vertebrate groups, including birds, are infected with pinworms, the probability for the extinct herbivorous dinosaurs to be unaffected may be considered to be low; especially considering the extreme variety and wide geographical distribution of the herbivorous lineages within the Dinosauria.