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Review of Pathologies on MOR 693: An Allosaurus from the Late Jurassic of Wyoming and Implications for Understanding Allosaur Immune Systems

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Palaeopathology is important as it provides remarkable insights into the lifestyles of dinosaurs and other prehistoric animals. The Late Jurassic Allosaurus known as Big Al (MOR 693) from Big Horn County, Wyoming preserves at least 19 injuries. About 2% of all bones showed abnormalities, including osteomyelitis on the right foot, on the first phalanx of the third toe, which may have contributed to the animal's death. There would likely have been many more pathologies that did not make it into the paleontological record due do the lack of soft-tissue preservation. Analysis of MOR 693's immune response to bone infections and comparing it to other theropods, we can confidently say that dinosaurs possessed an immune system that isolated and localized infections like extant Aves. Institution Abbreviations MOR, Museum of the Rockies; UUVP, University of Utah Vertebrate Paleontology
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Review of Pathologies on MOR 693: An Allosaurus
from the Late Jurassic of Wyoming and Implications for
Understanding Allosaur Immune Systems
Jack Thomas Rhodes Wilkin 1
1 Camborne School of Mines, University of Exeter, Penryn, TR10 9FE, United Kingdom.
Keywords: Paleopathology, Dinosauria, Theropoda, Allosaurus, MOR 693.
Abstract
Palaeopathology is important as it provides remarkable insights into the lifestyles of dinosaurs
and other prehistoric animals. The Late Jurassic Allosaurus known as Big Al (MOR 693) from
Big Horn County, Wyoming preserves at least 19 injuries. About 2% of all bones showed
abnormalities, including osteomyelitis on the right foot, on the first phalanx of the third toe,
which may have contributed to the animal's death. There would likely have been many more
pathologies that did not make it into the paleontological record due do the lack of soft-tissue
preservation. Analysis of MOR 693’s immune response to bone infections and comparing it to
other theropods, we can confidently say that dinosaurs possessed an immune system that
isolated and localized infections like extant Aves.
Institution Abbreviations
MOR, Museum of the Rockies; UUVP, University of Utah Vertebrate Paleontology
1. Introduction
Dinosaur fossils often display evidence of injuries that can be attributed to accidents, old age,
predation attempts, intraspecific combat, or metabolic disorders (e.g. Carpenter, 2000; Currie,
2000; Wolff et al., 2009; Rothschild and Depalma, 2013; Foth et al., 2015; Gonzalez et al.,
2017;). The specimen being reviewed is a 150 million-year-old Allosaurus fragilis, MOR 693,
from the Upper Jurassic Morrison Formation of Big Horn County, Wyoming. The fossil,
nicknamed Big Al, is one of the most complete large theropods recovered to date, with 95% of
a partially articulated skeleton recovered (Laws, 1996; Hanna, 2002; Bates et al., 2009). Due
to the specimen’s completeness, much is known about the animal’s life and death. Arguably
the most important, and scientifically compelling, aspect of MOR 693 is the pathologies. MOR
693 had at least 19 injuries that are visible on the skeleton (Laws, 1996; Hanna, 2002; Figure
1; Table 1). Differential diagnosis is possible, especially when, and this is undoubtedly true for
MOR 693, most of the skeleton is present.
Figure 1: Skeleton of MOR 693 showing locations of the 19 pathologies. Photograph courtesy
of Laura Vietta.
Table 1: Descriptions of pathological bones from MOR 693. Adapted from Hanna (2002).
Skeletal abnormalities have three origins 1) trauma-induced, 2) infectious, 3) developmental.
Trauma induced pathologies are characterized by boney growths with a rugose texture called a
callus (Mann and Murphy, 1990; Marsell and Einhorn, 2012). Infectious pathologies are
indicated by an exostosis, growths that appear on the surface of the bone (Meyerding, 1927).
Penetrating lesions, cloaca, for pus drainage may be present. In an advanced stage of infection,
an exostosis, termed involucrum, surrounds the original bone (Ortner and Putschar, 1981).
Developmental disorders occur during bone growth and may be caused by genetic defects
(Kornak and Mundlos, 2003). MOR 693 has pathologies of all three origins, making it a useful
case study.
2. Systematic Description of Pathologies
2.1. Right manual phalanx II-I.
The right manual phalanx II-I displays evidence of a healed fracture that was surrounded by
substantial bone growth (Figure 2). Thin section examination of the bone under a scanning
electron microscope revealed an oblique longitudinal healed fracture line through the bone.
Rugose bone growth surrounds the pathology (Laws, 1996; Hanna, 2002). The fracture was
likely caused by twisting of the digit.
Figure 2: Manual phalanx II, shown by marker, showing exostosis. Photograph courtesy of
Laura Vietti.
The texture and extent of the rugose suggest it was an infection-induced pathology:
osteomyelitis (Gross et al., 1993). Small lesions on the growth may be cloaca allowing for pus
drainage (Hanna, 2002), a feature consistent with chronic osteomyelitis. Osteomyelitis is the
progressive infection of bone and bone marrow by micro-organisms, resulting in inflammatory
destruction of bone, bone necrosis and new bone formation (Ikpeme et al., 2010) However,
other chronic infectious diseases, for example tuberculosis (Mannepalli et al., 2010) can yield
similar traces to osteomyelitis making unequivocal diagnosis difficult. The features that argue
against this differential diagnosis are: (1) the sheer amount of bone regeneration, and (2)
amount of sequestrum and diaphyseal involvement (Ortner and Putschar, 1981; Rothschild and
Martin, 1993; Aufderheide and Rodriguez-Martin, 1998).
Chronic superlative osteomyelitis diagnosis seems the more likely of the two (Hanna, 2002).
The cause of the osteomyelitis is also contentious. Osteomyelitis originates from either trauma-
induced (Roesgen et al., 1989; Mann and Murphy, 1990) or as a blood-born infection (Mann
and Murphy, 1990). If trauma-induced, then the longitudinal fracture must have been open to
the elements allowing microbes to enter (Birt et al., 2017) and if blood-borne the infection
would have arisen within the intramedullary space (Calhoun et al., 2009). It appears that the
fracture preceded osteomyelitis as bone healing is observed and the fracture line is confined to
the original bone and does not extend into the exostosis (Hanna, 2002). The above point that
of bone healing is important as it suggests that some time had passed from fracture to infection
because avascular necrosis, bone death caused by the lack of blood supply, occurs in
osteomyelitis which inhibits new bone formation (Fondi and Franchi, 2007). Thus, the manus
phalanx II-I can be best described as a post-traumatic, hematogenous chronic superlative
osteomyelitis (Hanna, 2002).
2.2. Second caudal vertebra.
The chevron of the second caudal vertebra was extensively remodeled following a trauma-
induced transverse fracture on the left diaphysis. Exostosis is present on the second caudal
vertebra and third dorsal vertebra. However, when dealing with bone abnormalities, one does
not presume that the bone remodeling followed an injury, as the third left dorsal rib shows no
sign of trauma (Hanna, 2002).
2.3. Dorsal ribs
The third and fifth right dorsal ribs have a rugose callus and the fourth dorsal rib has a smooth
callus on the medial shaft. The third, fourth and fifth right dorsal ribs have traumatic healed,
misaligned fractures with callus formation which resulted from a violent impact from the right
side (Hanna, 2002). The exact cause of this impact is unknown. The third rib also shows
evidence of misalignment during healing. The fifth rib shows a putative cloaca for pus drainage
due to suppurative osteomyelitis, inflammation of the bone and marrow caused by bacterial
infection (Figure 3). Traumatic, healed fractures in the form of calluses are also present on
other Allosaurus ribs in the Cleveland-Lloyd Dinosaur Quarry collection (UUVP 2753; UUVP
4946; UUVP 5660; UUVP5661; Hanna, 2002). The bone infection may have been formed as
a result of complications during healing (Rega, 2012). `
Figure 3: Pathological right dorsal ribs. Pathologies indicated by triangles. Fifth dorsal rib
(left), fourth dorsal rib (center), and third dorsal rib with lesions (insert). Photograph courtesy
of Laura Vietti.
The sixth dorsal rib does not appear to have any fractures (Laws, 1996). Although it has a 6.5
cm long spicule on the lateral side. Additionally, the fourteenth right dorsal rib has two bone
spicules. The origins for these pathologies are unclear. The spicule on the sixth rib may be a
developmental defect due to lack of evidence of bone regrowth or fractures. The spicule on the
fourteenth rib may be either traumatic, developmental or isopathic (Hanna, 2002).
2.4. Lateral blade, right scapular.
There is a depression on the lateral blade of the right scapula. Similar pathologies speculated
to have been subperiosteal abscesses formed during osteomyelitis with periosteal stripping
(Ikpeme et al., 2010) are observed in UUVP 1528 and UUVP 5599. However, in MOR 693,
the origin of the pathology is speculative as the depression in the bone would have resulted
from early-stage infection that would have begun only a few days before death (Laws, 1996).
Instead other possibilities include traumatic tendon avulsion, an unidentified disease process
(idiopathic; Hanna, 2002), or aberrancy (Laws, 1996).
2.5. Right pes phalanx III-I.
The most devastating injury which was a contributing factor to the individual's death was a
bone infection, osteomyelitis, on the right pes phalanx III-I. Much of the shaft is surrounded
by a large exostosis with lesions for pus drainage (Figure 4; Hanna, 2002). The exostosis and
lesions are infection-induced caused possibly by a joint infection as these are good locations
for microbial entrance. The infection-induced lesions resulted in osteomyelitis and later
developed into an involucrum (Laws, 1996). The involucrum resulted from the stripping of the
periosteum by the accumulation of pus within the bone, and new bone growing from the
periosteum (Moser and Gilbert, 2014). Thus, this pathology can be best described as pyogenic
osteomyelitis. The infection itself was long-lived with the infected phalanx rubbing against the
other two toes causing significant discomfort (Hanna, 2002).
2.6. Dorsal neural spines.
The dorsal neural spines of MOR 693 shows irregular-shaped exostoses which Hanna (2002)
diagnosed as an idiopathic pathological ossification of interspinous ligaments. However, such
a feature may not be pathological as ossified tendons are known from other dinosaurs to stiffen
the axial skeleton (Organ, 2006), and such structures are not interpreted as pathologic (Foth et
al., 2015).
2.7. Left iliac blade.
A thickened area of bone occurs on the dorsal-most margin of the left iliac blade. The region,
dorsal of the ischiac peduncle, has a medial-lateral thickness of 4.1 cm, compared to 2.1 on the
right iliac blade, and extends for 10 cm along the length of the bone. The origin of the pathology
is idiopathic. The thickening may have been trauma-induced. If this were the case, the ilium
would have received an avulsion fracture. However, an avulsion fracture would have affected
a larger area and would not have been as localized (Hanna, 2002)
Figure 4: Involucrum on right pes phalanx III-I. Image courtesy of Laura Vietti.
3. Implications for Allosaur Immune Response
The infections in MOR 693 reveals information about the allosaur immune system. Infections
observed in right pes phalanx III-1, metatarsal V, and manus phalanx I-1 of MOR 693, and two
pes phalanges III-1 (UUVP 1657 and 6768), pes phalanx IV-1 (UUVP 1851), metatarsal IV
(UUVP 30-783), and two scapulae (UUVP 1528 and 5599) are all localized and some are even
chronic demonstrating Allosaurus processed an immune response that localized infections
preventing them from spreading allowing the animal to live with microbial infection in their
bones for prolonged periods (Hanna, 2002).
It is possible that the high number of infected foot bones from allosaurs resulted from allosaurs
standing on rotten carcasses which, presumably, would have been teeming with bacteria (Laws,
1996).
The ability for theropods to localize chronic infections is also observed in tyrannosaurs
suffering from trichomoniasis and suggests an immune response like modern birds. Symptoms
of trichomoniasis include swelling and holes in the back of the lower jaw. The disease is
prevented from infecting the entire interior of the bone by an innate immune response that
localized infections because of the actions of unique avian white blood cells called heterophils
(Wolff et al., 2009).
Heterophils are the primary polymorphonuclear leukocytes in birds and are the avian
equivalent to mammalian neutrophils (Scanes, 2015). The heterophilic inflammatory response
in Aves more closely resembles the reptilian response than the mammalian response to
infections (Montaili, 1988).
4. Conclusion
Although none of the pathologies are directly responsible for the death of MOR 693, they did
reduce the animal’s ability to obtain food. The infection on pes phalanx III-1 would have
rubbed against the adjacent toes causing considerable discomfort.
Sedimentological evidence suggests that the allosaur died in a dried-up riverbed, perhaps
driven there by desperation. There is little evidence of vertebrate scavenging on MOR 693.
However, the bones do show evidence of beetle burrows, suggesting that the carcass was the
dining spot of hundreds to thousands of dermestid beetle larvae (Breithaupt, 2001). Both adult
and juvenile dermestids thrive on dried soft tissues, consistent with the paleoenvironmental
interpretation of the final resting place being a dried-up riverbed. The pits on the bones are
marks left by pupal chambers excavated by the beetles who used the bones as a substrate.
Roughly 12% of the skeleton was damaged by beetle activity (Poiner and Poinar, 2008). The
beetle damage was caused after post-mortem exposure of the body for a few months, but before
the bones were covered by a series of flooding events (Rega, 2012).
MOR 693 provides paleontologists with a remarkable insight into the lifestyles of large
theropods. Thanks not only to the completeness of the specimen but also the pathologies, it is
possible to reconstruct critical moments in the animal’s life. The ability to localize chronic
infections shows an avian-style immune system.
Acknowledgements.
The author would like to thank Laura Vietta (University of Wyoming) for providing the
photographs back in 2015.
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