This article on the human sense of smell is provided for reference.
Amphioxus (Branchiostoma floridae) has orthologs ... (PubMed) http://www.ncbi.nlm.nih.gov/pubmed/19804645?ordinalpos=1&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_SingleItemSupl.Pubmed_Discovery_PMC&linkpos=1&log$=citedinpmcarticles&logdbfrom=pubmed
"A common feature of chemosensory systems is the involvement of G protein-coupled receptors (GPCRs) in the detection of environmental stimuli. Several lineages of GPCRs are involved in vertebrate olfaction, including trace amine-associated receptors, type 1 and 2 vomeronasal receptors and odorant receptors (ORs). ...
We have identified 50 full-length and 11 partial ORs in Branchiostoma floridae. No ORs were identified in Ciona intestinalis. Phylogenetic analysis places the B. floridae OR genes in a monophyletic clade with the vertebrate ORs. ...
Here, we show that amphioxus has orthologs of vertebrate ORs. This conclusion demonstrates that the receptors, and perhaps other components of vertebrate olfaction, evolved at least 550 million years ago."
Exploring developmental, functional, and evolutionary aspects of amphioxus sensory cells (Wiki)
"The G protein-coupled receptor (GPCR) family in vertebrates contains multiple copies of gustatory and olfactory receptor (OR) genes ..." "Each vertebrate olfactory sensory neuron selectively chooses only one allele of the receptor gene from a vast array of genes, and vertebrate ORs predominantly lack introns. Gentles and Karlin  proposed that the mRNA of an ancestral olfactory receptor gene was reverse-transcribed and incorporated in the genome, resulting in the early template of the intronless ORs." "An intronless GPCR gene found in the amphioxus genome showed weak sequence similarity to the vertebrate ORs. "
"The expression of the amphioxus GPCR gene was seen in the bipolar neurons embedded within the rostral epithelium (Fig. 2)."
" Figure 2
Schematic representation showing expression of Amphi-GPCR1 (red) in relation to peripheral nerves, dorsal nerve 2 (n2), the so-called anterodorsal nerve (adn) and dorsal nerve 1 (n1), the rostral nerve (rn). The anterodorsal nerve is composed of two major branches, a dorsal branch (db) and ventral branch (vb) . bv; brain vesicle, not; notochord.(Click on the image to enlarge.)"
"Taking neuroanatomical data into consideration, neurites of the GPCR-positive epithelial sensory cells seem to constitute a nerve fiber called the anterodorsal nerve (n2)." "Furthermore, there is another nerve located rostrally in relation to the anterodorsal nerve, the so-called rostral nerve (n1). The rostral nerve terminates at the rostral tip, a GPCR-negative region."